Popular Science Monthly/Volume 72/May 1908/The Utilization of Auxiliary Entomophagous Insects in the Struggle Against Insects Injurious to Agriculture II
| THE UTILIZATION OF AUXILIARY ENTOMOPHAGOUS INSECTS IN THE STRUGGLE AGAINST INSECTS INJURIOUS TO AGRICULTURE |
II
By Professor PAUL MARCHAL
THE NATIONAL AGRONOMICAL INSTITUTE, PARIS
Generalization of this Method. Different Applications
THE striking success brought about in the struggle against Icerya by the use of Novius cardinalis gave rise to great enthusiasm in favor of the method of fighting injurious insects by their parasites.
First in California, then in Australia and the Hawaiian Islands, under the auspices of Alex. Craw and his adepts, the application of this method became popular and enjoyed extreme favor. From the facts just given, they generalized to excess and imagined that in collecting beneficial insects and naturalizing them in the country where they proposed to use them, they would be able to check completely the plagues of agriculture.
Fight against the Fruit Fly. Compere's Mission.—No example appears to us to show better the belief inspired by this new method, the exaggerated hopes to which it gives birth, and the zeal with which it fills its promoters, than the incredible Odyssey around the world of Mr. Compere, charged at the beginning of 1903 by the government of West Australia with a mission having for its end the search for the home of the fruit fly (Ceratitis capitata), and of finding the parasites which in its original home should limit its propagation. This fly, which is a great plague to fruit culture in South Africa, and which has also invaded western Australia, has also for us an interest, since it is abundant in all the Mediterranean regions where it is particularly injurious to oranges in Algeria and Tunis, and which several years ago even made its appearance in the suburbs of Paris, where it attacks peaches and apricots. The Ceratitis has for a long time existed in Mediterranean countries, and it is from this region that it was probably transported to the Cape of Good Hope and to Australia.
It would seem, then, natural to direct one's observations first to this locality. Nevertheless, the damage accomplished by this insect in the Mediterranean region appears to be too large to warrant the conclusion that this is its original home. Spain having received the famous fly from one of its colonies, one naturally thinks of the Philippines; and these islands not being very far from Australia, Compere started then for that archipelago. He only passed through there, and then visited China and Japan without meeting the Ceratitis. From Japan he went to the United States, where the fly in question does not exist, but where in the collections and the laboratories he thought to gain facts which should throw some light upon the problem of its origin. From there he went to Spain, and there tried to learn from what region this country received the fruit fly. He was not able, however, to get any knowledge of this kind, but a large number of fruit growers told him that they remembered the time when oranges, peaches and other fruit were not damaged by the larva of this insect. Was that not sufficient to confirm his opinion that the Ceratitis was a fly not indigenous to Spain?
After having traversed the south of France and Italy, he went back again to Australia, and shortly afterwards departed for the Indies. In the following month of September he landed at Bombay, traveling through Hindostan, visiting the great markets as well as the orchards and the principal fruit regions around Bombay, Calcutta, Madras and Ceylon, studying the flies of the different species of fruits, as well as the parasites living at their expense, wherever he could find them. Then, always without having found the Ceratitis and finding nothing which could serve him in the struggle against this insect, he returned to Australia persuaded from certain indications he had collected in the United States, that the original country of the celebrated dipterous insect was Brazil. The seventh of January, 1904, he sailed, then, for South America. Arrived in Brazil, he quickly ascertained that Ceratitis capitata exists there in company with other flies injurious to fruit, and at the same time he observed some Ichneumonids and some beetles of the family Staphylinidæ, which were carrying on a war against the flies. If the Ceratitis causes, in general, only slight damage in Brazil, it can be only owing to the presence of all these natural enemies which hold it in check. And it, therefore, resulted that Brazil was the promised land for the fervent entomological traveler, the original home of the Mediterranean fruit fly! Arranging as ample a provision as possible for the parasites and predatory enemies of this insect, and arranging for their food for the time necessary for their journey, he then returned to Australia.
On his arrival, the Staphylinids were set at liberty in one of the gardens of Perth, where the conditions seemed particularly favorable to insure their subsistence. The pupæ parasitized by the Hymenoptera to the number of about 200 were placed in breeding jars, and as the parasites emerged they were liberated in the orchards most infested by the fruit fly.
If we have told with some details this story of the journey around the world of a practitioner, launched by his government into the search for the parasites of a fly, it is to show the vogue which the use of beneficial insects in the struggle against injurious insects enjoys in certain countries.
What will be the result of this experience? It is premature to express an opinion on the subject. The time necessary for the definite acclimatization of a parasite, and above all for it to multiply sufficiently to restrain the insect which it is its mission to attack, must be extremely variable, according to the species and also according to the extent of spread of the plague. It is only necessary for some months to elapse in the case of Novius cardinalis, but years may be necessary for such parasites as the Ichneumonids.
Utilization of Beneficial Insects in the Hawaiian Islands.—Journeys of Koebele.—The method of which we are speaking has nowhere been applied in a more extensive way than in Hawaii. These islands, ever fertile, present, as is well known, a climate extremely favorable for large number of tropical and subtropical crops. At the beginning of the American colonization, the only plants of economic importance were yams and cocoanuts, but since that time enormous numbers of useful plants, coming from all parts of the world, have been acclimatized in this rich country, and with them also have unfortunately been imported a large number of their natural enemies, among them, and the most important, scale insects and plant lice. It has been stated that the Hawaiian Islands are the paradise of these insects, since they are represented by numerous species coming from all parts of the world, which prosper there and flourish.
After having seen, in 1890, their orange and lemon trees relieved from Icerya by Novius cardinalis, the planters directed their efforts to this method in order to combat other agricultural plagues, and particularly the enemies of coffee and sugar cane.
They, therefore, gave Mr. Albert Koebele a commission to undertake this work. This entomologist, celebrated for his discovery of Novius cardinalis, and already employed at the same time by the State Board of Horticulture of California upon a similar mission, commenced by sending from California the Coccinellids which seemed most desirable for Hawaii, notably: Hyperaspis undulata Say, Scymnus debilis Lec, Chilocorus bivulnerus Muls., Rhizobius ventralis Er., and R. lophantæ Blaisd. The two latter became naturalized and constituted a useful resource for the country.
In 1893 he visited the islands and left immediately for Australia. From 1894 to 1896 he journeyed through Australia, China, Ceylon and Japan, and made during this journey numerous sendings of this insect to Hawaii and California. Among the best of these must be mentioned, in the first place, Cryptolæmus montrouzieri Muls., originally from Australia, where it had rendered inestimable service in destroying mealy-bugs (Dactylopius, Eriococcus, etc.), and which, introduced into the Hawaiian Islands, developed with a surprising rapidity, comparable to that of Icerya in California. This ladybird is there considered as one of the most important enemies of the scale insects of the coffee plantations, and particularly Pulvinaria psidii, one of the greatest enemies of this crop. Coccinella repanda is also naturalized and is to-day one of the most common ladybirds, and most efficacious among those which attack orange plant-lice and the plant-lice of Hibiscus and sugar cane.
In 1899, Koebele left for Australia and the Fiji Islands and made numerous sendings of ladybirds and different parasites to the Hawaiian Islands, notably to combat Ceroplastes rubens Mask.
At the end of 1902 the attention of planters was particularly directed to an injurious leaf-hopper on the sugar cane, Perkinsiella saccharicida Kirk. It was introduced from Australia in about 1897 and has since that time increased and spread and become a perfect plague for this important crop. The attempts made to introduce living parasites in California at the expense of similar leaf-hoppers having given unsatisfactory results, Koebele and Perkins left in the spring of 1904 for Australia, and during the course of that year sent to Hawaii a great number of insects parasitic or predaceous, and among them a considerable quantity of enemies of Perkinsiella.
Considering that the money appropriated by the government was insufficient, the Hawaiian Sugar Planters' Association did not hesitate to advance important sums in order to further the study of the question, and created itself a section of entomology in its experimental station, and has started a series of investigations exclusively upon the enemies of sugar cane and their parasites. Some remarkable monographs are now in course of publication.
The method of utilizing beneficial insects has been, during fifteen years, used with such activity that the list of useful insects which have been imported from one country to another, in order to combat the plagues of agriculture, is already very long.
We have already spoken of some of them in giving a resumé of Koebele and Compere. Omitting those which up to the present time have given only uncertain results, or which have not succeeded in naturalizing themselves, we will limit ourselves to a mention of some interesting species, either because they have fully justified the hopes founded upon their introduction, or because they appear likely to soon play an important role in the struggle against the enemies of agriculture.
Rhizobius in California.—Among the ladybirds must at once be mentioned Rhizobius ventralis Erichs. This little ladybird, of a specially dark color, very useful in Australia for the destruction of various scale insects, was introduced by Koebele into California while on his second mission to Australia in 1893. It naturalized itself and plays an important role in fighting the black scale of the olive. A hundred thousand of these insects were distributed in different districts. In different localities they multiplied in a prodigious way, and proved to be particularly efficacious in the moist climate of the seashore. Mr. Cooper, president of the State Board of Horticulture, had such confidence in the efficacy of this Rhizobius and other ladybirds that, yielding to a perhaps exaggerated enthusiasm, he renounced for a time all other intervention, and, in order to allow them to multiply at their ease, he suspended all treatment. According to him, to spray trees upon which there is Rhizobius is a crime and should be severely punished.[1]
Attempts to Fight the San Jose Scale by Means of Ladybirds.—The disasters caused by an insect commonly known as the San Jose scale in the United States are well known. The damage done to the fruit trees can only be compared in intensity to that done in our country by the Phylloxera, and about 1898, the fear that it would be introduced into Europe occasioned prohibitory, special legislation on the part of European states. Since it was a scale insect, it was natural to search for an enemy which would approximate the rôle of Novius cardinalis, but no one knew the original home of the San Jose scale.
Australia was considered for some years as responsible. Finally, they concluded that it might be Japan, and Mr. Marlatt, first assistant of the Division of Entomology, Department of Agriculture, was sent on a mission, in 1901-2, to the extreme orient to solve the question, and he established in a positive manner the fact that the original home of the San Jose scale was the north of China, where he found it occurring upon small wild apple trees, in the mountainous country. There he found, at the same time, with the scale insect a ladybird, Chilocorus similis Rossi, which, both in the larval and adult stages, feeds on the San Jose scale. This ladybird is an insect widely spread, not only in China, but throughout all of Asia and the south of Europe. The San Jose scale is, then, not its only food, but it can live at the expense of different scale insects. Therefore, samples of this insect, coming from China and offering the best possible conditions for adaptation to the struggle against the San Jose scale, were sent to Washington, and all precautions being taken, they were bred with great care in the Bureau of Entomology, first in cages and afterwards in an experimental orchard.
They were thus produced in sufficient quantity, so that for several years they could be sent to different States. The colonies which were established in the north did not succeed. But, on the other hand, colonies installed in the Georgia orchards and other southern states rapidly spread, naturalized themselves, and still fill a useful role in attacking the San Jose scale. However this may be, this importation can never be compared to that of Novius cardinalis. Mr. Marlatt himself states that this insect does not seem to have found in America, up to the present time at least, conditions as favorable to its development as in its native country. Moreover, the time when it was introduced coincided with the period of employment especially of the lime-sulfur-salt wash, and obviously the use of such an efficacious remedy should not be interrupted to allow the ladybird to spread.
Scutellista cyanea.—The success gained by beneficial parasites, properly speaking, is at present rare and less startling than those which the predaceous insects, and particularly ladybirds, have brought about. An especial rank, however, should be given to a Hymenopterous parasite of the family Chalcididæ, Scutellista cyanea Motsch., which is among the most useful of the American importations. It was first described from Ceylon, where it was found attacking parasites of the coffee scale. Then it was found again by Berlese in Italy, where it attacked the wax scale of oranges and other plants.
Howard, with the help of Berlese, tried in 1898 to introduce it into Florida and Louisiana, to combat the wax scale, injurious in that part of the country. This first attempt at acclimatization failed. In the meantime, Lounsbury, State Entomologist at the Cape of Good Hope, drew attention to this parasite as one of the most efficacious enemies of the black scale of the olive. The olive scale is not abundant enough at the Cape to be considered as injurious, and the damage which it does is always less than in America and particularly in California. On this account, and considering Scutellista to be the cause, the State Board of Horticulture of California, always looking for new assistance of this kind, tried to get this parasite. In 1900-01, branches carrying parasitized black scale were sent from the Cape to California. Some parasites were obtained by breeding from these different sendings, but their number was not sufficient to undertake a rearing in the large cage constructed around the tree infested by scales, but, in 1902, numerous colonies were sent into all the districts of the State of California where the black scale was injurious. Since 1903, numerous orchards have been found which have been practically cleaned of the black scale by this parasite. It may be affirmed that this introduction is one of the most fortunate ones for fruit growers in California.[2]
The Struggle in America, by means of Parasites, against the Gipsy Moth and Brown-tail Moth.—If parasitic insects (hymenopterous and dipterous) are at the present time behind the predatory ones, it does not make their efficacy any the less that the work that they accomplish is not so immediate and is less easily brought about.
Everybody knows how some ladybirds will free a tree from plant lice or scale insects, while there is some trouble in observing how a parasitic insect lays its eggs in the interior of a caterpillar. Moreover, while the victims of a predaceous insect are killed immediately, the insects pierced by the hymenopterous insect continue in most cases to feed and grow, and it is only in the following generation that the good work can be seen. Finally, to appreciate the just value of parasites, it should be remarked that several species, in certain cases more than thirty, live at the expense of a single plant-feeding species and join forces to hold it in check. To reestablish the equilibrium in a country into which a plant-feeding species has been imported, not only one of these parasitic species, but as many as possible, should be sought for and should be naturalized.
In a few years we will be much more certain concerning the advantages to be drawn from the utilization of these beneficial species.
No experiment in any case can be better conceived to illustrate this question than the gigantic undertaking now carried on by the government of the United States which has for its object the importation of the European parasites of Bombycids, up to the present unmasterable scourges, which ravage without interruption the trees of Massachusetts.
These two insects, Liparis dispar and L. chrysorrhœa, are European insects which have been accidentally introduced into Massachusetts, the first in 1868, and the second in 1890. It is difficult to imagine the intensity of the ravages of these two insects. The damage occasioned by the first of them, which is popularly known in America under the name of the gipsy moth, is to-day celebrated in certain localities, notably in the suburbs of Medford, which was the first point of infestation. The caterpillars became so abundant that all the trees in the parks, woods and public streets were entirely defoliated, and presented, in mid-summer, a winter aspect. These trees, deprived of their vitality, were killed by thousands. In certain suburban quarters, one could see the walls of the houses carpeted with caterpillars, and the roads themselves so invaded that it was impossible to walk without crushing them by hundreds. A special committee was started to organize the fight, and from 1889 to 1895, $525,000 was spent in work against the destruction of this species. For the year 1897 alone, $150,000 was voted by the legislature.
As to chrysorrhœa, known to Americans under the name of the brown-tail moth, although it has shown itself extremely injurious, it is to-day eclipsed by its congener, and it is only in these later years that it has taken an importance of the first order, tending even in certain districts to take the first rank over the gipsy moth.
The caterpillars of these two species are extremely common in Europe, their original home. They are injurious and from time to time appear in great number. It is to be remarked that in a year following their large multiplication, the caterpillars of these insects become quite rare, and that they remain so for a long time. They are, then, very far from being responsible for damage similar to that which they cause every year on the other side of the Atlantic. With us their presence is tolerable, and they do not cause notice since they do not threaten the vitality of the trees. In Massachusetts, on the contrary, they constitute a permanent plague which has commenced to invade neighboring states.
The difference in these conditions appears to be that in Europe the insects are held in check by parasites, which are much more numerous than in the United States.
Some American parasites have adapted themselves to destroying the gipsy moth. There are 5 hymenopterous and 6 dipterous parasites, without counting several predaceous species which attack it. But this is small in comparison with the 27 hymenopterous and 25 dipterous parasites of the gipsy moth in Europe. While the parasites of the brown-tail moth are less known, it is perfectly sure that in Europe this insect is kept in check much more efficaciously by its natural enemies than is the case in America. On account of these considerations it was only natural to seek to introduce into Massachusetts the original parasites of these two insects. For a long time it was not judged wise to undertake the enterprise. A law obliging the systematic destruction of the gipsy moth and the use of insecticidal mixtures seemed to render it inadvisable. Moreover, there was confidence in the fact that the native parasites would increase. Now the conditions have changed. In 1900, the appropriations were stopped, at a time when the insect was well in hand. In five years, however, it has spread over a territory four times as great as that which it occupied in 1900 and has commenced to spread into the neighboring States of New Hampshire and Rhode Island.
On the other hand, in the 36 years that the insect had infested the country about Boston, American parasites, if efficacious, would have manifested it in an appreciable way. The same considerations applied to the brown-tail moth.
Americans resolved, then, to attempt a last and great effort to master the plague against which a long struggle had given insufficient results. In the appropriation bill of the Federal Congress, in 1906, $2,500 were appropriated to begin the importation of parasites of these two insects into the United States. At the same time the state of Massachusetts appropriated $10,000 a year for three years for the same end. A special superintendent, Mr. Kirkland, with a staff of agents and assistants, was charged to preside over the execution of the work in America, and Mr. Howard, during the three years 1905, 1906 and 1907, was sent on a mission to Europe to seek for the parasites of the two species, visiting France, England, Italy, Germany, Austria, Hungary and Russia. He interested in his enterprise all of the official entomological bureaus, as well as the principal specialists, who promised him their help and active cooperation.
It is by hundreds of thousands that the nests of the brown-tail moth have been sent to Boston for two winters. It is in innumerable quantities that, during the months of June and July, caterpillars and chrysalids of the two species have been sent to both destinations. All these insects, upon their arrival in Boston, where they have been received by Mr. Kirkland, are sent to a laboratory specially constructed for this work. It is in the suburbs of a small village named Saugus, in a house which is constructed in the midst of woods infested by the caterpillars of the two species. Aside from the rooms devoted to research and rearing work, this house contains the local or resident assistant, who has charge of the work, and also the specialists who are sent by the bureau from Washington, at the time when the insects are appearing. The insects are reared in boxes constructed for that purpose, and somewhat like those employed by the State Board of Horticulture of California. To avoid the issuing of hyperparasites or of suspected species not existing in America, and accidentally mixed with the sendings, the cages are kept in closed rooms with double doors. They are arranged side by side in several longitudinal rows, and so abundantly that it is difficult to walk between them. When issued, the parasites are generally not set at once at liberty, but are allowed to breed in large outside cages.
To what practical results will these experiments conduct us? It is difficult to answer this question in a decisive way. The experiments have been in any case carried on under conditions most perfectly constituted to assure the success of the enterprise, and it was impossible to confide their execution to a savant of greater authority than the eminent director of the Bureau of Entomology, at Washington. Having a great number of parasites imported, an abundance of food which they find at their disposition, and a climate which they will encounter analogous to that of Europe, it does not appear doubtful that many species will acclimatize themselves, and as soon as acclimatized, they can not fail to strongly influence the balance of nature to the prejudice of the destructive species.
The time necessary for this movement of the see-saw may be long, and it seems that one could hardly expect appreciable results before four or five years.
But what does this matter in any event? since we are trying to obtain a result of indefinite duration which will bring about exemption from the ruinous methods of destruction by insecticides and which will mark the end of a public calamity menacing the trees of the whole United States.
General Considerations and Conclusions
The exposition of facts we have presented in this memoir allows us to take stock of the importance gained during these recent years by the method of utilizing beneficial forms. It can not be denied that, practised in a judicious manner, it can render very great services, and the initiative of governments which have been encouraging large experiments destined to show its value, must be applauded. It would be bad taste indeed to criticize those who have brought about a check, for it is only by trying experiments that one can understand the conditions which may prevent success, and far from implying failure under such conditions, the experiments almost always teach a useful lesson.
Those accustomed to the experimental method and to laboratory research know well how the discovery of a new fact in science is made at the price of much groping, of misconceptions and of failures, and how these have to be conquered before the truth is learned. Is it reasonable, then, to suppose that it can be otherwise for these great experiments in economic entomology, of which we have just spoken? And if by forced circumstances those who are carrying them on can not be protected by the silence which the learned men of the laboratory enjoy, if the work which they are undertaking is exposed to distortion or exploitation by persons anxious to boast and to give out sensational information. . . these are circumstances which, to our eyes, can only add to their merit and to increase the rights which they have to our esteem.
If eulogy without reserve should be given to those who have taken part in this great movement in favor of the utilization of beneficial insects, we would have the right, on the other hand, to discuss the too-exclusive and too-optimistic conclusions to which some of the most fervent adherents of the method have been led.
It is in California above all, and in Australia, as we have said, that the theory has been formulated in the most absolute way. According to the claims of the State Board of Horticulture, of California, no insect is in its original home a pest of sufficient gravity to menace a crop in a serious way, because nature has always placed by its side a parasite capable of holding it in check. Each time that a new enemy reveals itself in a region and begins to undergo exaggerated multiplication, its orginal country should be searched for parasites living at its expense, and these should be procured and naturalized. This is, if I am not mistaken, the theory of Alex. Craw and his school.
In objection to this doctrine it should be urged that there exist insects which can be considered as veritable plagues to our crops, and which, however, are undoubtedly indigenous, such, in Europe, as the cockchafer, the apple anthonomus, the pyralis of the vine, the cochylis; and for America, the Colorado potato beetle, which would have ruined the culture of the potato in the United States if the use of Paris green had not been discovered.
But other stronger objections may be urged, if not against the principle of the theory, at least against its too great application and against the exclusive way in which it has been propounded. Admitting that it is incontestible that certain insects can become terrible plagues where they are introduced into a new country because they are not accompanied there by their natural enemies, it is manifestly going too far to hold that it is always to the absence of the natural enemies of an insect of exotic origin, taking the proportions of a plague, that it owes its virulence.
We know well, for example, that it is for entirely different causes, depending upon the nature of the affected plants, that the Phylloxera occasioned an unprecedented disaster in Europe; and it would have been taking a false step at the time of invasion of this insect to undertake long researches to procure its natural enemies.
An indigenous insect, which has been for a long time practically harmless, can become more dangerous and even arrive at the condition of a plague simply because man, by new crop conditions, in favoring the extension of some plants at the expense of others, and substituting for an extremely varied natural vegetation an immense supply of a single plant, has himself broken the equilibrium of nature and favored to a very large degree the multiplication of the insects that attack his privileged crop. It is in the same order of things that an insect living upon a wild plant becomes adapted to a cultivated plant, and multiplies excessively at the expense of the cultivated plant whose conditions are particularly favorable to its nutrition. One of the most striking examples of this phenomenon is the case of the Colorado potato beetle, of which we have already spoken. This insect, originally from the Rocky Mountains, lived solely upon wild Solanum, but about 1855 it invaded the potato fields which began to be cultivated in its country, and then gradually spread into all of the potato fields of the United States and Canada, causing terrible damage.
Finally, it is not necessary to believe any longer that all exotic enemies, whose appearance is signalized by extreme virulence, will bring disaster unless their natural enemies are introduced. It is rational to believe, on the contrary, that at the end of a longer or shorter period there will be brought about an accommodation between the plants and their enemies, analogous to that which is produced between animals and bacterial, trypanosome or piroplasmic diseases. It is well demonstrated that certain insects like the scale insects inoculate in the sap certain toxic products, and it happens that varieties of plants that have never been attacked before by these insects are at first peculiarly subject to their action, then at the end of a longer or shorter time they begin to acquire a relative immunity—an immunity acquired in a way quite different from the formation of anti-toxins may be conceived, that is to say, an immunity attributable to many defensive adaptations of the plant, consisting in modifications of the plant tissue and tending either to render the attack of the insect more difficult, or to diminish the quantity of food which it can get, or again to render the lesions which it produces less dangerous.
If, for example, the San Jose scale is less injurious in its native country, can it be said that this is the case only because of the parasites which hold it in check? Are there not serious reasons for admitting that the trees of this country may be formed of varieties adapted to this insect, capable of resisting it?
Finally, we should also take into consideration the fact that if, during the first years following its introduction, an injurious species of exotic origin can multiply freely without any parasite to interrupt its multiplication, there often comes a time sooner or later when the parasites of a country living at the expense of the indigenous forms most nearly related to the exotic species progressively adapt themselves to the latter and end in limiting its propagation. This cause, added to the progressive adaptation of plants, appears to be bringing about the actual diminution of the virulence of the San Jose scale in America.
It results from what precedes that in the problem with which we are occupying ourselves the factors are multiple, and that it will be a great mistake to consider one or the other and not all.
If, because of the too absolute manner in which it has been formulated, the theory of the utilization of beneficial insects must surrender somewhat, it is necessary also to point out the errors and the exaggeraions to which it has given rise in practise, for they give rise to excessive hopes, provoke serious disillusions, and are a discredit to the whole method.
In the first place, a grave fault resulting from an excessive confidence in the action of parasites consists in advising the suppression of insecticides in a region where it is desired to acclimatize the beneficial species. In the great majority of cases, at least in regions where insecticides are employed with success to hold a crop enemy in check, the desire to acclimatize a beneficial species should not cause the general use of sprays to be stopped. The two methods are not incompatible, for in a given region it is very rare that you can regularly spray all of the trees. This one, or others, will not be treated, and the ladybirds and other useful insects will therefore have a free field to carry on their beneficial work, and centers from which they can be dispersed will be created.
One of the greatest dangers in introductions consists of the possibility of introducing into a region an animal which considered as useful in its original home, is capable of becoming absolutely injurious in the new country into which it is introduced, on account of the conditions of the environment which it encounters. The examples of the sparrow imported from Europe into America and Australia, of the mongoose introduced from the East Indies into the West Indies, of the rabbit imported from Europe into Australia, are too well known to be described. It has been stated that no danger of this sort exists in such cases as these, since parasitic insects of other insects can live only at the expense of these last, and it is the same with predaceous insects. There is no doubt of this, but there exists another danger of a direct character in the importation of the insects which are desired for acclimatization, and that is the danger of importing at the same time either injurious insects sent along as food, or hyperparasites which can prevent the propagation of the useful insects and which becoming acclimatized themselves, endanger even certain useful indigenous species.
It is very easy to take the necessary precautions so that the insects which serve as food during the journey should present no danger, and it will suffice to make sure that they belong to a species existing already in the region where they are to be acclimatized.
The history of the naturalization of Icerya purchasi in Florida shows us that the method of utilization of beneficial insects, practised by incompetent people, may have sad consequences.
As to the danger from hyperparasites, while it is apparently not so serious as the preceding, it is, on the other hand, much more difficult to avoid. Preliminary rearings are necessary before the beneficial species are definitely set at liberty, and all precautions are necessary after the issuing to separate the primary parasites from the hyperparasites. It is for this reason that the application of the method of the utilization of beneficial insects, in order to render all the services which are expected of it, should be carried on indispensably and exclusively by learned men, especially informed concerning insects and their reciprocal and biological relations.
We have shown in this memoir about all that can be drawn from the utilization of predatory and parasitic insects in the struggle against enemies of crops. One conclusion may be drawn also from this study, and that is that one can not count upon beneficial forms as a substitute for the methods of destruction commonly used in applied entomology. Their rôle does not consist of exterminating a species, but of maintaining a natural equilibrium, or of reestablishing it when it has been disturbed by human intervention. In such cases, their action can make itself felt in a more or less prompt way. It may happen that, immediately after having been imported into a country, they stop with an extraordinary rapidity the plague which they have been brought to combat. This was the case with Novius cardinalis, in California, and in different countries. It was also the case for Crypotolæmus montrouzieri, which made very rapid spread in Hawaii. It must be stated, nevertheless, that this is rather exceptional and that, in general, a number of years are required—the number varying according to the species and to the circumstances under which it is brought—before it can be completely naturalized in a given country, and before, thanks to its spread, it brings about a sufficient retardation of the multiplication of the plant-feeding species for which it is imported, to reduce it from the condition of a scourge to that of a supportable species.
The services that parasites and predaceous species render are sufficiently great so that there is no necessity for exaggerating them.
Far from lulling ourselves with illusions, we should keep on the watch and foresee the dangers with which other injurious species menace us, such as Icerya purchasi, which may any day invade Provence or Algeria on plants imported from Portugal and Italy.
There is no doubt that, however great may be the efficaciousness of a ladybird, like Novius cardinalis, it will be still better not to have the enemy at all than to be obliged to fight it by the intervention of its natural foe. We do not know that Novius cardinalis will with us develop with the success which marked its spread in California, in Portugal, and in Italy. We are ignorant whether the climatic influences or some parasite, recently adapted to this new strange host, will not limit its propagation and diminish its beneficial action. Finally, other plagues than Icerya menace us, and it is unfortunately certain that not all of these may be mastered by the equivalent of Novius cardinalis.
Confidence in the assistance which we can get occasionally from parasites and predaceous insects should not make us lose all prudence nor prevent us from seeking a guard against the perils which surround us, in organizing at our large ports an inspection and disinfection service like those which have been started at foreign ports, notably Hamburg, and in a general way taking every measure possible to protect our crops.
- ↑ The attempt to acclimatize Rhizobius ventralis in India and Ceylon, undertaken by Froggatt and Green, did not succeed, probably on account of unfavorable climate.
- ↑ The acclimatization of this insect appears also to have been brought about in Australia, where it was introduced in 1904, and in Hawaii, where it was imported in 1905. Mr. Lafont has lately announced the presence of this insect in France, where he considers it a very efficacious parasite of the black scale.