Popular Science Monthly/Volume 72/April 1908/The Utilization of Auxiliary Entomophagous Insects in the Struggle Against Insects Injurious to Agriculture I
|THE UTILIZATION OF AUXILIARY ENTOMOPHAGOUS INSECTS IN THE STRUGGLE AGAINST INSECTS INJURIOUS TO AGRICULTURE|
By Professor PAUL MARCHAL
THE NATIONAL AGRONOMICAL INSTITUTE, PARIS
I. The Rôle of Entomophagous Insects in Nature
IF phytophagous insects could develop and multiply without hindrance in proportion to their natural reproductive power, in a short time they would cause all species of terrestrial vegetation to disappear. The multiplication of these destructive forms is very fortunately kept within limits compatible with the existence of plants by the presence of other insects, predatory or parasitic, which places a check on their propagation.
The capacity for prolification of entomophagous insects is itself very considerable: their eggs may often be counted by hundreds or even by thousands; moreover, as several of the species of parasites often attack a single species of plant-feeding insect, it is certain that the latter would in its turn become very rapidly annihilated if the parasites were not themselves held in check by hyperparasites, and if they were not repressed in their spread by all the obstacles that render their struggle for existence more difficult than can be imagined.
The role of entomophagous insects is of the first importance, whether from the point of view of the economy of nature or from the point of view of utility to man.
Some, like the Carabidæ and Coccinellidæ, are predatory. They destroy for food the insects which they attack, and the benefit derived from their action is immediate.
The others, which are represented both by the hymenopterous and dipterous parasites, lay their eggs in the interior of developing insects, or in their near neighborhood, and the larvæ which hatch from these eggs nourish themselves at the expense of their hosts, accomplishing their death at a more or less advanced stage of their evolution. In this case the benefit brought about is sometimes immediate, as in certain species of minute Hymenoptera (Teleas, Tetrasticcus, etc.) which lay their eggs in the interior of eggs of other insects and complete their development within the eggs. The phytophagous insect is then killed within the egg, and the plant thus completely escapes its depredations.
Much more frequently it does not immediately stop the growth of its host; it introduces its egg into the phytophagous insect and in the more or less advanced stages of its development, either during the embryonic period (Encyrtus fuscicollis, divers Platygasters), or frequently during the larval or nymphal period (Ichneumonids, Braconids, etc.). The phytophagous insect which carries in its interior the larva of the parasite, continues to grow and to feed upon vegetation, and is killed by the parasitic insect only when the latter has reached its full development, and when the host has done all the damage it is capable of doing during its existence. The benefit accomplished by the parasite is manifest only in the following generation, and consists in the suppression of the descendants that would have been mothered by the phytophagous larvæ, if they had been able to develop until their transformation into adult insects.
Whether they belong to one or another of these catagories, the predatory and parasitic insects play a regulating role that is useful and remarkable. When, on account of cultural conditions or climatic circumstances or other influences, the phytophagous species tends to increase beyond the average, it thus furnishes conditions eminently favorable to the multiplication of the parasitic species, and that in its turn causes the phytophagous form to decrease.
In a very interesting, but insufficiently known work, Bellevoye and Laurent (1897) have shown that it is not necessary that the parasite should have a greater fecundity than the phytophagous species in order to bring the latter back to its normal condition when it has exceeded it. As paradoxical as is this assertion, with a fecundity simply equal and even inferior, it may rapidly reach the point of annihilation, if other factors and other conditions do not interfere to interrupt this action. All other things being equal, nothing prevents the development of the parasites, so that by their work a greater and greater quantity of the plant-feeding species are destroyed each year. In order to state this fact precisely let us, with the authors just cited, take as simple an example as possible, that of an invasion of the caterpillars of Bombyx. Suppose that at a given period the proportion of parasitized caterpillars is one fourth, and that the parasites have placed a single egg in each caterpillar. Of 8 chrysalids, 6 will give out Bombyx and 2, parasites. We will suppose that of the 6 moths there are 3 males and 3 females; and of the 2 parasites, 1 male and 1 female. Let us suppose that the fecundity of the parasitic species is equal to that of the host species, and that the number of eggs laid by a female of each of the two species is 100.
We will have for the first year 300 caterpillars, of which 100 will be parasitized. That will give, as eventually issuing, 200 moths (100 females) and 100 parasites (50 females).
The second year there will be 100 times 100 = 10,000 caterpillars of which 50 times 100 = 5,000 will be parasitized. That will be 5,000 issuing moths and 5,000 issuing parasites with 2,500 females of each species.
The third year the number of caterpillars will be 2,500 times 100 = 250,000 and all these will be attacked by parasites so that there will be no moths issuing.
This theoretical example shows very well how an injurious species, after having increased in threatening progression during several years, immediately after having reached its maximum can suddenly disappear in a short time under the influences of the parasite. The diminution of food also contributes to limit the propagation of the plant-feeding species, and may hasten the inevitable triumph of the useful species. Every one who is interested in agricultural entomology knows that incidents like this are frequently observed in nature.
When an insect has been very injurious for two or three years, and has multiplied to the point of taking the proportions of a veritable plague, it disappears, usually in a sudden manner at the moment when the alarm which it has provoked has reached its highest degree. Experience has shown that it is almost always to the work of parasites that these rapid retrocessions of injurious species must be attributed.
The damage of the Hyponomeutas to fruit trees is almost always stopped at the end of two or three years by Tachinids, or other parasites. The same thing occurs with the Bombycids which devastate coniferous forests.
A remarkable example of the same phenomena is shown with the Cecidomyiids of cereal plants.
After the destruction caused in Vendee and in Poitou by the Hessian fly and the oat midge, in 1895, these insects disappeared almost completely, and the farmers had no further cause to complain of their presence.
Now I ascertained that an enormous majority of the pupæ which should have been in the grain at the end of 1894, or the beginning of 1895, were parasitized. So much so that it was difficult for me to find specimens with which to carry on certain studies in which I was engaged at that time. Having collected in March, 1895, in the suburbs of Poitiers, some stubble from the harvest of 1894, remaining through the winter and containing an enormous quantity of the pupæ of the flies, I obtained in the jars in which I had this stubble enclosed, only a cloud of parasites which came out during the months of April and May.
If one would take the trouble to observe, one could multiply similar examples without difficulty.
It must be remarked that in the case of which we have just spoken, the injurious species are very injurious only from time to time in a rather periodical manner. Several years will occur in a given region when they are not mentioned. Then, under the influences of certain conditions, they multiply for two or three years in an excessive way, giving rise to terrible invasions, until the parasites favored by this great development of the host species become themselves sufficiently multiplied to bring about the retrocession. This repressive and regulating action of the parasite, having for its object the limiting of the increasing abundance of the plant-feeding species, moves then in a successive periodical manner, which recalls a little the action of the siphon of an intermittent fountain. This type of injurious species, with great invasions more or less separated and presenting a periodical character, is met with especially with those species which attack plants cultivated upon a very large scale, and corrects an unstable equilibrium which man himself has provoked by the establishment of great homogeneous cultures. Exception, however, should be made in regard to certain migratory and omnivorous species, such as the grasshoppers and crickets, whose invasions seem to exist during all time and without any correlation with cultural conditions.
In other cases which more nearly approach the general and primitive law of nature, the injurious species maintains always about the same rank, and the fluctuations which it presents are only of secondary importance. The parasites act as a moderating check to the continued increase, and prevent the injurious species from multiplying in an excessive manner. They are themselves present in almost constant number from one year to the other. Their role is not only to bring back an injurious species to a small number of indidivuals when it has passed the mean, but to hold it constantly at a numerical point much below that which it would reach without their presence.
It is very certain, however, that in nature all the intermediate stages between these two types just mentioned are to be found, and these two types themselves, as we admit, are more theoretical than real. The conditions and factors which control the relations of beings among themselves are so numerous and so complex that to interpret them and render them intelligible one is forced to speak more or less theoretically in considering only certain of the causes and in momentarily omitting the others.
In fact, the regulation can in no case be considered as the exclusive result of the action of any given parasitic species, of which the fecundity will be proportioned to that of the host species and in such relation that it maintains a constant numerical tax.
The fecundity of the parasitic species is only one of the factors which determines this equilibrium. If it is true that it is of prime importance, that fact should not prevent us from taking account of the others. There are a number of these, as follows:
First.—The hyperparasites, or secondary parasites, living at the expense of the primary parasite, and having themselves tertiary parasites.
Second.—The coparasites, that is to say, other species living in the same host.
Third.—Other plant-feeding species occurring with the host species.
Fourth.—The enemies of all insects (insectivorous birds, etc.), attacking both the plant-feeding species and the parasitic species.
Fifth.—Climatic conditions influencing in a favorable or in an unfavorable way either the host species or the parasites, the hyper-parasites, or the enemies of all kinds liable to attack the insect.
Sixth.—The rapidity with which the generations are developed,—of the host species, on the one side, and the parasitic on the other.
Seventh.—The tendency in the plant-feeding species to retard the development of certain individuals of a given generation for a longer or shorter time. Eighth.—A faculty similar to the preceding which the parasite may possess.
There exists an intimate relation between all these conditions, and this relation binds in a particularly striking manner the plant-feeding insect and parasites which live at the expense of the latter. The harmony which results from the mutual adaptation of these beings should not be surprising, since it is the condition sine qua non of the existence of the species. From the reciprocal actions which they exercise upon one another, results the equilibrium in which they are maintained.
II. Perturbations brought by Man in the Natural Equilibrium
The intervention of man in disturbing the laws of nature is capable of breaking this natural equilibrium, and of bringing about in the existing order a perturbation from which he is perhaps the first sufferer of serious consequence. This rupture can be occasioned by two principal causes: (1) by new conditions created for insects by cultures; (2) by the accidental carriage of certain species from one country to another.
1. Perturbations provoked by New Conditions created by Cultures; Methods bringing about the reestablishment of the Equilibrium
Man, in planting over a vast extent of country certain plants to the exclusion of others, offers to the insects which live at the expense of these plants conditions eminently favorable to their excessive multiplication; for he diminishes in their favor the difficulties of their struggle for existence and often favors their alimentary specialization, while the food-plant, in the conditions which it finds itself, is not always capable of reacting by defensive adaptations of sufficient compensating value.
In this case, man, in order to regain the equilibrium favorable to his own interests, should have recourse to a regular rotation of crops, destined to interrupt the life cycle of the injurious species, and to all methods possible to increase the resistance of the plant. But also the beneficial insects whose useful role is incomparable should be watched. It is necessary to aid or at least to start their work; and, finally, in any circumstances it is necessary to know them in order to protect them in a judicious way, and above all not to destroy them by inopportune cultural practises.
Protection of Beneficial Insects.—Apropos to the Hessian fly, we
have elsewhere insisted upon the fact that one of the measures most often recommended—the destruction of the stubble remaining in the field after the harvest—may have unfortunate consequences, for doing this in a tardy manner one risks intervening at a moment when all of the flies have emerged and have abandoned the stubble, exposing to destruction only the parasites whose part would have been to stop the invasion the following year.
Kieffer has pointed out a remarkable analogous fact for a Cecidomyiid, namely, Diplosis tritici, attacking not the stubble, but the grains of wheat, and has shown that one of the measures which has been advised—burning the debris after the threshing—has only an injurious effect, for while it is true that this debris contains pupæ of the midges, it should be remembered that the healthy and nonparasitized larvæ of these flies transform in the ground, while those which remain in the heads are, on the contrary, parasitized.
In the cases which we have just mentioned, the protection to be accorded to the parasites consists solely in abstaining from inopportune measures capable of bringing about their destruction without any advantage whatever. In other cases it is an active protection which has been advised, and which comprises operations destined to insure the survival of the parasites.
It is in this way, for example, that Decaux, struck by the multitude of ichneumon flies, or Braconids, which came out of the buds of apple attacked by Anthonomus, advised, in place of immediately burning these buds as was generally done, preserving them in boxes covered with gauze, raising the latter from time to time during the period of issuing of the parasties so as to permit them to escape. In 1880, he put this method into practise and collected in Picardy buds reddened by the Anthonomus from 800 apple trees, amounting to 5 hectoliters; and thus accomplished the destruction of more than a million Anthonomi, and set at liberty about 250,000 parasites which the following year were aids in the destruction of the weevils. The orchards treated being isolated in the middle of cultivated fields, it sufficed to repeat the same operation the following year in order to stop all serious damage during ten years.
This plan started by Decaux has been perfected by Berlese (1902) in order to protect the parasites of the Cochylis. This author recommends the use of boxes with the cover pierced by a window, being also covered by a metal plate perforated with holes 2 mm. in width. In the autumn there is placed in the box nearly full-grown larvæ with the leaves necessary for pupating. In the springtime the parasites will issue through the openings, while the moths perish in the box. There is no reason why this method should not be adopted on a large scale, replacing the box by a room in which the window is closed by means of a wire screen. A similar method could be employed with a number of other insects, and quite recently (1907) Silvestri has advised it for the olive fly in arranging a plan of defense against this insect, based both upon the protection to parasites and on cultural methods.
Against the injurious scale insects of the Aspidiotus group it will be particularly easy to put into practise a similar method, and it will give good results. It will not be necessary to burn during the winter time the branches taken from attacked trees, but to collect them in the neighborhood of the infested trees. Aspidiotus, which can live only upon living plants and are incapable of traveling, will thus perish by inanition. Parasites living at their expense can easily, upon issuing, gain the fruit trees. In the same way, if one is using insecticidal measures, it will only be necessary to remove the cut-off branches or cut-down trees to sufficient distances so that they will not be touched by the insecticides. It is necessary to have seen the parasites of scale insects at work; it is necessary to have observed that the immense majority of those which cover a tree are often eaten and perforated by one or several regular, round holes through which the parasites emerge, in order to understand how such measures are justified.
Johnson, in 1899, having put some fragments of twigs covered with the San Jose scale in a series of tubes, obtained more than a thousand parasites (Aphelinus fuscipennis Howard) in each of them. Struck by this observation, he recommended the protection of this Aphelinus by the application of such measures as we have mentioned.
Berlese, in 1902, suggested the same method in the struggle against Diaspis pentagona, one of the most dangerous enemies of the mulberry, in Italy.
There still exists a method entirely different from that which precedes, but whose end is also to protect parasites and to assist in their multiplication. It consists in encouraging, or cultivating in the neighborhood of the plantations, wild plants which harbor them.
Thus, for example, parasites of the olive fly do not live exclusively upon that insect, but also upon certain gall insects of the oak and the briar rose. Therefore, it has been recommended to preserve in the neighborhood of the olive groves, bushes or hedges of these plants, or even to transport galls into the olive groves.
Some authors, struck by the eminently useful part played by parasites in certain invasions of insects, have actually advised the abstaining from destructive measures in fear of killing at the same time parasites which they harbor, or the predaceous insects which destroy them. This way of looking at the question is very exaggerated. It is only in case where the parasites constitute restricted and very localized centers of contamination that this idea can hold for these determined points, admitting that it will still be possible to utilize such centers of propagation. In the great majority of cases, on the contrary, it must be said that however useful parasites may be, the fear of destroying them ought never to prevent the undertaking of all measures having for an end the direct destruction of the injurious insect. Parasites act, in fact, only at a more or less long maturity, and admitting that with an invasion of caterpillars the majority or even all harbor in their interior larval parasites, they will none the less accomplish the greater part of their depredations in a manner quite as complete as if they were not parasitized. Should we, then, allow them to devastate a field or orchard in order that the parasites can, the following year, accomplish their beneficent work? An intervention with destructive methods, far from being dangerous, will permit us, on the contrary, always to obtain a double result: first, it will immediately stop the damage and save in a more or less complete manner the products of that year; and, second, it is not likely in the great majority of cases that the caterpillars will be more abundantly parasitized in that particular spot than in any other portion of the country; and in destroying a certain number of non-parasitized caterpillars, one will diminish for the whole region the number of possible adults which would assure the generation of the following year, and that without changing the existing proportion between the parasites and the representatives of the injurious species.
The assertion that insectivorous birds can cause more harm than good by attacking either the useful species or larvæ parasitized by them, does not appear to us well founded and seems to us to be refuted by analogous arguments. In spite of the thesis formerly proposed by Perris, and ably defended of recent days by Berlese and Severin, the protection of insectivorous birds appears to us not at all as susceptible of thwarting the beneficent action of useful insects.
Utilization of Indigenous Insects in the Fight against Indigenous Injurious Species.—Aside from the intelligent protection which should be given to beneficial insects and which, as we have just shown, can be based only upon exact knowledge of their biology and the relations which they have to other organized beings, can man assist in artificially multiplying them, and making of them a forced subject to his will which will serve him at will in the struggle against indigenous enemies of cultivated plants—those which for centuries have devastated our prairies, fields, orchards and forests?
The fungous parasites and microbes have already been brought into our arsenal, from which we draw against the enemies of agriculture. Can we bring in entomophagous insects in their turn?
While they habitually accompany injurious insects wherever they are found, it may happen in a restricted region and an isolated plantation that the beneficial forms are absent, and there will be undoubtedly a benefit in introducing them. It often happens that coniferous forests are ravaged by insects without any of their most important enemies, such as Calosoma sycophanta. Will it not be opportune in such a case to transport a lot of these beetles from the region where they exist and acclimatize them in the devastated forests, where they have not appeared naturally?
Then also with sedentary insects, such as the scale insects, which develop often in closely circumscribed localities, it will be possible, when one chances to find a colony particularly invaded by parasites, to cut off certain branches and carry them into other orchards infested by scale insects and less favored from the point of view of the presence of parasites.
In 1871-72, Le Baron, in the United States, made some experiments in the transportation of the small hymenopterous parasite, Aphelinus mytilaspidis, from one locality to another, attaching the branches covered by parasitized scale insects to infested trees which were found in a region where the Chalcidid parasite did not exist. At the end of the year it was stated that the parasite had become domiciled in that locality.
Johnson has noticed that another parasite very close to the preceding, Aphelinus fuscicollis, may be extremely abundant in certain localities invaded by the San Jose scale, and be totally absent, on the contrary, in others, and he succeeded in propagating this insect by suspending upon a tree, at small distances, small baskets containing twigs covered with parasitized scale insects.
In France, Decaux was the promoter of the same method, and in 1872 had the honor of attracting attention to the question, making experiments in the transportation of parasites from one locality to another. However justifiable such practises may be in certain determined cases, one can not deny that they have not the certainty which they should have in order to be perfectly convincing. In fact, with an indigenous species, it is very difficult to say that it is, at a given moment, really absent from a locality. If it is absent to-day there is a great chance that it will appear to-morrow, coming from a neighboring region. The experimenter will find himself also exposed to possible criticism, not without reason, that he has attributed a result to his own work when nature would have perfectly accomplished the same thing without his intervention.
A more profound study of parasites and predaceous insects—of their development, their migrations, their geographical distribution—will show us without doubt and in a more precise way, the real value of the consistent method to be used in transporting indigenous parasites, and thus assisting in their spread. In any event it suffices to say that actually it appears applicable in a rather limited number of cases.
2. Perturbations brought about by Accidental Importations. Reestablishment of the Equilibrium by the Introduction of Predaceous Insects and Parasites
The point of view becomes quite different if, in place of considering the perturbations which man has provoked by the substitution of homogeneous cultures for the primitive vegetation of the soil, we look at what he has accomplished in accidentally introducing, by commerce, a plant-feeding insect into a country where it had not previously existed and where it finds conditions favoring its development. It is readily understood that the chances are great that this species will be introduced without the procession of parasites and predatory species which limit its propagation in its original home. It can notably be imported without the parasites which are especially adapted to live at its expense, or often, indeed, without a single one of its natural enemies, and then finding itself unhampered in its multiplication, the injurious species takes prodigious strides and becomes a scourge infinitely more redoubtable than in its own country.
In such a case everything indicates the value of an endeavor to reestablish the equilibrium by introducing into the invaded country all the auxiliaries capable of checking the plague.
In the United States it has been ascertained that nearly one half of the injurious insects of the first importance are of exotic origin and have been accidentally imported into the country, so it is not astonishing that it is America which has started the method which consists in fighting the enemies of agriculture by means of their parasites and that in that country it has taken on a prime importance. After several fruitless efforts with different insects, Riley, in 1883, succeeded in bringing about the first true acclimatization of a beneficial insect, in importing from England into the United States a small hymenopteron of the family Braconidæ, Apanteles glomeratus, which is a parasite of the larvæ of the cabbage butterfly (Pieris brassicæ). This experiment, however, was only against an enemy of secondary importance, and in order to popularize the method a striking success was necessary—an unprecedented triumph against one of the most redoubtable enemies of cultivated plants. This occurred with a small Coccinellid, Novius cardinalis, which brought about this decisive victory. The history of this insect and the work which it has accomplished in the country to which it was introduced is of such importance that we will give a somewhat detailed recital.
The Use of Novius Cardinalis against Icerya
Icerya purchasi and Novius cardinalis in America.—Icerya purchasi is a scale insect living upon different trees and particularly upon citrus trees. It is originally from Australia, and was accidentally introduced, about 1868, into California where it did enormous damage, and threatened to ruin the cultivation of oranges and lemons. All attempts to fight this Australian insect with different insecticides were vain. It continued to spread in a progressive manner from the orchards that had already been annihilated or were in bad condition.
Riley, then director of the Division of Entomology of the Department of Agriculture, at Washington, thought of utilizing the natural enemies of the scale insect. Ascertaining that in Australia, its original home, it did not seem to be seriously injurious, and to be without importance from the economic point of view, he was led to think that it probably was held in check there by parasites. Investigations which he made on this question having confirmed his ideas, he made every effort to accomplish the desired end, namely, the acclimatization of the natural enemies of Icerya in California. Finally, after numerous appeals to the government, he was able to arrange for a sending of two agents of the Division of Entomology to Australia on the occasion of the exposition at Melbourne, in 1888, with a credit of $2,000. One of these agents, Mr. Koebele, was especially instructed to search for parasites of Icerya.
On his return he brought a collection of the natural enemies of the Australian scale insect. Among these there were a hundred living specimens of Novius cardinalis. It multiplied so rapidly that in the following year, 1889, they could distribute to the fruit growers of California 10,000 specimens. A year and a half after its introduction it had relieved the region from Icerya, and had reduced their number to a practically negligible quantity. According to witnesses this deliverance possessed for the inhabitants of the country an almost miraculous character. Immense groves of oranges bearing no fruit, covered with a horrible, white leprosy composed of the Iceryas, and which seemed irremediably lost, suddenly took on a new vigor and furnished abundant crops. Now the only natural means necessary to hold Icerya in check consist in sending a small number of Novius cardinalis to start colonies in the district where the scale insect shows a tendency to regain its foothold. In this way reserves of Novius are constantly kept on hand for exportation, either to the different districts of the State of California or to foreign countries, and the State Board of Horticulture of California has constructed small boxes of glass and wire gauze of octagonal form, 16 feet in diameter and 18 feet high, allowing the Iceryas and the Novius to live upon the trees surrounded in this way.
In 1894, at the opposite extremity of the United States, in the State of Florida, a new invasion of Icerya purchasi was produced, and the scale insect was introduced, this time under conditions which show very well the risks attending such attempts to acclimatize useful insects when they are made by incompetent persons. A nurseryman in Hillsboro county, Florida, having heard of the extraordinary services rendered by the Australian ladybird, being ignorant of the fact that this insect will not attack other insects than Icerya, sent to California for Novius cardinalis to fight the Aspidiotus, or other scale insects, feeding upon their plants. The Novius was naturally sent with some Iceryas which would serve as food for it on the journey, and they were all placed together by the nurseryman upon the trees which he desired to protect. No one knows what became of the Novius, but the Icerya multiplied and was not slow in covering the trees upon which it had been placed. Radical measures were almost immediately taken; all the trees attacked were burned; and for four years nothing was heard of the insect. It was believed to have been entirely exterminated, but in 1898 the presence of specimens of Icerya was discovered. The formidable scale insect was found at this time at several points near the locality where it had been first imported, and it had invaded one or two orange groves. It was necessary to give up all hope of exterminating the species, and an immediate effort was made to introduce Novius cardinalis.
Mr. Gossard, state entomologist of Florida, and Mr. Al. Craw, entomologist of the State Board of Horticulture of California, directed the work. The Novius was at first colonized with success in two of the infested orchards, and in 1900 had become sufficiently well established to enable them to distribute it in good number in infested localities. It seems to be certain that Icerya finds in Florida conditions less favorable to its development than in California. It has been stated that it is attacked there by a fungous disease. In any event it can not be doubted that with the assistance of Novius cardinalis, it will never play in Florida the injurious rôle which it has played on the Pacific coast.
America is not the only country which has suffered from the introduction of Icerya purchasi. This insect has been imported, or was imported at almost the same time, into South Africa; and more recently it has made its appearance in the Sandwich Islands, in Portugal and in Italy. Novius cardinalis has in these cases been sent for, and the success has been as complete as that obtained in California. The history of these successive invasions and of the efforts which have been made to combat them conveys useful instruction and is worthy of our attention.
Icerya purchasi and Novius cardinalis at the Cape.—About 1890, Icerya purchasi having been at the Cape already for some years a great subject of alarm, the Secretary of Agriculture, at Capetown, made an effort to secure Novius cardinalis from Australia and from New Zealand, but the correspondents to whom he wrote had not been able to collect a sufficient number to make a sending, and a demand was made upon the Department of Agriculture of the United States. Following the year 1891 an ample provision of larvæ and pupæ was sent from California to the Cape. But on account of the length of the voyage, no living specimens arrived. At the end of the same year, Mr. Thomas Low, member of the Legislative Assembly of the Cape of Good Hope, went to the United States, charged by his government with a mission connected with different agricultural questions, and notably to secure the sending of Novius cardinalis. He procured three boxes full of this insect, and left New York the twenty-third of December, 1891. One of these boxes was placed in the ice-box of the steamer. He kept the two others in his cabin, feeding the Novius regularly during the journey with Icerya. The three lots, including those preserved in the ice-box, arrived in perfect condition, and on the twenty-ninth of January were placed in the hands of the Secretary of Agriculture of the Cape.
The insects were then utilized in the following way: a small number were placed in the open air upon an infested tree in the botanical garden at Capetown; but the majority were used for rearings in captivity. Some were placed upon an infested orange tree which was surrounded by a great wire-gauze cage, while others taken to a different locality were placed in a sort of glass house constructed around the orange tree, and similar to those used in California for the same purpose.
The efforts destined to naturalize Novius cardinalis in South Africa were reinforced about the end of 1892 by a new sending coming from Australia, and sent by Koebele, who was then on a mission to that country.
To-day the Novius is perfectly naturalized at the Cape. In some spots which are particularly exposed to cold and where the winter is very vigorous, they succumb to the low temperature, and the Entomological Service is obliged to frequently renew the colonies. This, however, is exceptional, and almost everywhere the Novius, perfectly acclimatized, holds the Icerya in check so efficaciously that since several years they have not worried about it.
Icerya ægyptiaca and Novius cardinalis, in Egypt.—About the same period several attempts were made to introduce Novius cardinalis from California into Egypt, not to fight Icerya purchasi, but an allied species, Icerya ægyptiaca, which is of unknown origin and for several years had been found in the gardens of Alexandria, where it did great damage to the oranges, lemons and figs. The first attempts failed on account of the length of the voyage, but a new attempt made about the beginning of the year 1892 was crowned with success. Six adult insects and several of the larvæ arrived in living condition at Alexandria. They were set at liberty upon an orange tree infested with Icerya ægyptiaca, and accommodated themselves so well to this new food that in a short time they had become so numerous as to cause an almost complete disappearance of the Icerya. But later the Icerya again began to increase. Happily, however, the Novius had not entirely died out and it also recommenced to multiply, and, thanks to the successive see-saw movements between the two species, the Icerya is held in check in a definite way.
Icerya purchasi and Novius cardinalis in the Hawaiian Islands.—In the Hawaiian Islands, the alarm provoked by the invasion of Icerya purchasi was of short duration. The injurious insect was discovered in 1889 in the suburbs of Honolulu, and multiplied there with rapidity. In 1890, Novius cardinalis was in its turn introduced from California, and a year afterwards the trouble was entirely stopped.
Icerya purchasi and Novius cardinalis in Portugal.—In 1897, the presence of Icerya purchasi was discovered in the orange groves around Lisbon, and the agricultural population began to be alarmed. This insect had multiplied already for several years along the banks of the Tagus River, and it seemed that the first infested plants had been brought from the Azores, where for a long time the Australian scale insect had existed.
In 1897, almost all the gardens of Lisbon and its suburbs were infested with Icerya, and the insect was known to occur in 32 localities. Before attempting the introduction of the natural enemies of the Australian insect, they tried insecticides which were found insufficient to stop the scourge, but which, nevertheless, were of much service from time to time, when it was deemed desirable to introduce Novius cardinalis. Messrs. de Silva and Le Cocq were particularly interested in this latter matter. In spite of a hostile press and the opposition of the greater part of the administrative authorities, they placed themselves in relations with Mr. Howard, the learned director of the Bureau of Entomology, of the Department of Agriculture of the United States, and he wrote to San Francisco, to the State Board of Horticulture of California, and procured from Mr. Alex. Craw sixty Novius cardinalis in the adult condition, as well as a certain number of larvæ in different stages of development.
As soon as they arrived in Washington, in October, these insects were placed in boxes with moss with an ample supply of Iceryas for food, and were then sent on to Lisbon. The greater part of the Novius perished on the voyage. Five only, coming from larvæ which transformed on the journey, arrived alive at their destination. On their arrival they were placed in breeding cages at the Experimental Agricultural Station of Lisbon, and were cared for in such a fortunate way that in the month of December they had already a numerous progeny.
On the twenty-second of November a second colony of Novius was sent to Lisbon. The journey this time, on account of accidental delays, was particularly long, and from San Francisco to Lisbon it lasted not less than 44 days. Five females and one male still lived on arrival, and these received the same care as the others, and the success was such that in June, 1898, these six samples received in December, 1897, had several thousands of descendants. On account of the danger to which we are exposed of some time seeing the orange groves of the south of France and the north of Africa invaded by Icerya, we believe that it will be useful to give some details as to the methods used in breeding, under the direction of M. Le Cocq. [See pp. 32-37, Bulletin 18, New Series, U. S. Dept. Agriculture, Division of Entomology.]
This method of work, which permits the handling of the Novius without touching them, has been practised in Portugal on a large scale, and thanks to this method, they were able to obtain an immense multiplication of the Australian ladybirds, but in order to facilitate the rearing still more and to obtain as great a production as possible, they constructed a large wire-gauze cage after the model already used in the United States.
In 1898, thirty-eight centers of dispersion, in Lisbon and in the suburbs, had been thus established and were in active operation. In the month of August, ninety colonies existed; in September, four hundred and eighty-seven, without counting the secondary colonies started by the orchardists themselves, who had given one another specimens of these precious insects.
The gardens and orchards, which were completely infested and almost ruined, were cleaned of the scourge as if by enchantment. The number of Icerya became practically negligible, and all treatment with insecticides was from that time entirely superfluous. In a letter addressed at this time to Mr. Howard, Mr. Le Cocq wrote as follows:
However happy these results, it should not be forgotten that the Novius, in Portugal as well as elsewhere, has not been able to completely exterminate the Icerya. It keeps it from reaching the condition of a pest, but it is not able to prevent its dissemination or its slight increase. Mr. Duarte d'Oliveira, of Oporto, to whose kindness I owe certain interesting documents upon the history of Icerya in Portugal, has written me that he has recently found several colonies of this insect in the north of the kingdom, in the province of Traz-os-Montes, where it had previously made its appearance.
Icerya purchasi and Novius cardinalis in Italy.—Icerya purchasi was introduced accidentally into Italy at the end of the year 1899 or in the spring of the year 1900, without any indication of the origin of the infection. But it was observed for the first time, in the month of May, 1900, at Portici, near Naples. It was there found in a little garden upon orange trees, and was represented only by a rather small number of individuals. The proprietor of the garden, ignorant of the character of the insect and of the danger arising from its presence, took no trouble to destroy it, and the scale insect, able to develop freely without enemies, spread so rapidly that in the autumn it already covered the bark and leaves of the oranges upon which it had first been observed. Startled at this sudden invasion, but not yet deciding to apply to competent persons, the owner of the garden tried to stop the trouble by cutting down the most infested trees without bothering himself with those that were not badly infested, so the infestation continued. The eighteenth of November, the same year, the entomological laboratory was notified of the occurrence; and Professor Berlese, the director of the laboratory, recognized the species as Icerya purchasi. The insect was still very localized, and it was found at this time only in the adult condition in the little garden which was the center of the infection, and upon a large bay tree which occurred in another neighboring garden, occupying a high position and whose branches overhung the infested garden. This bay tree, on account of the strong winds which blow at that time of the year, constituted a center especially favorable to the dispersion of the larvæ into the surrounding trees, and, in fact, the larvæ were found in all the little gardens about. An examination was made and it was found that the infested area did not exceed one hectare.
On account of the small spread of the insect they tried to exterminate it by an energetic application of insecticides. The first thing done was to cut down the bay tree, which was the principal center of diffusion, and to burn it after having cut off and disinfected all the branches bearing leaves. In the infested gardens all the affected trees were cut back and all the branches were disinfected with a solution of rubina. The low-growing plants were treated with sprayings of the same insecticide. They hoped to rid these of the insects in this way, but in the following spring living larvæ, still very numerous, were found crawling over the plants treated, and then, despairing of destroying them with insecticides, they had recourse to Novius cardinalis. At Professor Berlese's request successive sendings were forwarded from Portugal by Mr. A. Le Cocq, director of agriculture, and by Mr. L. O. Howard, director of the Bureau of Entomology, of Washington. With the assistance of the material thus obtained it was possible to make at the entomological laboratory, at Portici, a methodical rearing of the beetles. Different methods were employed and on June 8 the first distribution of Novius, of both sexes, was made in the garden which had been the center of infection, the Icerya having made rapid progress and the garden being again infested by legions of Icerya.
June 28 other similar distributions were made in the other neighboring gardens.
The insects prospered marvelously, rapidly seeking the Iceryas wherever they could find them. It should be remarked that Novius, once acclimatized in a region, knows very well how to find trees attacked by Iceryas, even when they are some distance away. Therefore it is not absolutely necessary to distribute them to all points. In July the results were already evident. One could hardly find patches of Icerya which did not show the work of Novius, and at the end of the month it was difficult to find adult Iceryas with which to continue its breeding in the laboratory to afford food for Novius. By the end of autumn there was only here and there a rare individual that had escaped the massacre. In 1902 the intensity of the invasion was entirely minimized, but under the influence of the winds the area of dispersion extended to about a kilometer. Very fortunately the Novius, which had become very rare, reappeared. According to information very obligingly sent to me by Mr. Leonardi, of the laboratory at Portici, they still continue to-day to fill in a marvelous way the role which devolves upon them, and their naturalization can be considered an accomplished fact. This single fact alone indicates, without any need of further evidence, that they have not exterminated the Iceryas.
If they have reduced the multiplication of the scale insect to the point of rendering it practically negligible, it is none the less true that the original infestation persists, and that the area of distribution of the scale insect is slowly enlarging.
The Icerya is met with to-day not only at Portici, but in all the little towns around Vesuvius, and all the gardens of Naples have it in greater or less quantity. It is probable that the area will always exist about the first locality. If the beneficent ladybird did not exist by the side of the scale insect, the culture of oranges and lemons would be seriously interrupted, and in a few years throughout the whole Mediterranean region.
(To be concluded.)
- Translated from "The Annals of the National Agronomical Institute" (Superior School of Agriculture), Second Series, Vol. VI, Part II, Paris, 1907, pp. 281-354.
- Aside from parasitic insects, bacterial or fungous epidemics may intervene in a similar manner, but the consideration of these does not fall within the province of this article.
- The condition by which the fraction of parasitized insects remains constant from year to year is represented by the equation: ; c representing the number of eggs laid by an individual of the parasitic species, b the number of eggs laid by an individual of the plant-feeding species, and 1/a the proportion of parasitized insects. (Bellevoye & Laurent, loc. cit.) In other terms, if a quarter of the insects are parasitized, it would be necessary, in order that this proportion should remain constant from year to year, that the fecundity of the parasite should be to that of the host as three is to four.
- Factor No. 7, taken alone or combined with the preceding factor, has a prime importance in preserving the host species from destruction by the parasitic species. Three examples will serve to illustrate this:
First Example.—Encyrtus (Ageniaspis) fuscicollis is a hymenopterous parasite whose power of prolification is immense, since, as I have shown in an earlier memoir, it presents the very exceptional phenomenon of polyembryony, that is to say, that a single one of its eggs can give birth to more than 100 individuals, capable of multiplying in this way.
Now this Encyrtus lays its eggs in the eggs of a moth of the genus Hyponomeuta, which has only a single generation each year, as has the Encyrtus itself. Under these conditions it may be asked how the Hyponomeutas, instead of being promptly annihilated, are, on the contrary, capable of multiplying in certain years to the point of being destructive to fruit trees during their larval stage, and this in spite of a number of other parasites, particularly Tachinids. The reasons are certainly numerous; but the one to which we wish to call attention is, that the time of the swarming of the Encyrtus is notably shorter than the egg-laying period of the Hyponomeutas. However immense may be the number of Encyrtids that appear in a season, one may be certain that all of the eggs of the Hyponomeutas are never parasitized, and the adult generation of the Encyrtus will already have disappeared while the Hyponomeutas still continue to lay eggs, which will thus escape the parasite.
Second Example.—With the Hessian fly, a small dipterous insect whose larva is extremely injurious to wheat, the generations succeed one another the whole year, and under particularly favorable conditions there can be five or six generations in a single year. However, the time necessary for an individual to perfect its development is extremely variable, according to conditions in which the pupæ find themselves, and especially in regard to their position on the plant—whether on the green part or near the earth or even dry stubble. Some can complete their development in two weeks, while others, finding the conditions of dryness exceptionally long, wait even as long as two years before issuing. The hymenopterous insects, living at the expense of the insect, which may appear in innumerable quantities, have, on the contrary, only two generations each year at the maximum, and appear only at a definite time, and during a lapse of time of usually short duration. Now, since the parasites never attack more than one of the developmental stages of the insect—egg or larva, according to parasitic species—it results, from what precedes, that there will always be, at the period of egg-laying, existing individuals of the species which will escape them because they are in a developmental condition in which they are not pierced; and when the generation of parasites has passed, these individuals will remain unharmed and constitute the indispensable reserve for the perpetuation of the species. It is not necessary that this reserve should exist throughout the range of the plant-feeding species. On the contrary, they may be annihilated in certain localities by a combination of climatic conditions or factors of some other nature having an unfavorable influence, and it is in this way that the local disappearance of certain species, of which this one is an example, is to be explained.
Third Example.—In the preceding case the average conditions, and in particular the relative dryness, play a part of the first rank in the determination of retarded development, and the adaptation of the plant-feeding species enables it to react in a more or less energetic manner to external influences. In other cases the plant-feeding species has acquired a great variability in the time necessary for the development of individuals. This variability, which appears to be independent of average conditions, consists in reality in the fact that different individuals present a variable power of reaction to identical external influences.
It is thus that, according to Boisduval ("Essay on Horticultural Entomology," Paris, 1867, p. 15), the chrysalids of Bombyx everia and lanestris, which cause great damage in Germany, issue in a very irregular manner. "One sees," says he, "moths of these Bombycids issue in September after three months of metamorphosis, others in the spring of the following year; but what is more astonishing is, that from the same egg laying, from the same brood of caterpillars, reared under the same conditions, the moths have issued during seven years in April and September—a wise foresight on the part of nature, which does not wish to expose the species to sudden destruction."
- This factor, which favors the parasitic species, is in conflict with factor No. 7. Some very remarkable examples of this have been given me by M. Künckel d'Herculais concerning egg-feeding parasites of locusts (Cantharids, Clerids and Bombyliids), the development of which in certain cases can be retarded for several years, thanks to a condition of torpidity which the larvæ may enter into. "The retarded development of these parasitic larvæ," says Künckel, "enables the successive issuing of adult insects during several years, and is evidently in close correlation with the appearance of the locusts; the latter, decimated, fly from their enemies to reproduce far away; the former awaiting their return to insure the well-being of their progeny; thus is established a regular balance between the multiplication of the locusts and that of their egg-feeding parasites, which assures the perpetuity of both species."
- If it is desired to preserve the parasites for study, the perforated plate is covered with a bell glass, in which the parasites accumulate without ever reentering the dark box.
- Berlese is of the opinion that winter treatments for scale insects should be discontinued, on account of the great number of parasites destroyed by them. We can not adopt this opinion, for reasons which we shall give later on.
- Novius cardinalis has also rendered great service against Icerya in New Zealand.
- Rubina is a mixture of equal parts of soda and tar, recommended by Berlese, and very much employed in Italy.