Popular Science Monthly/Volume 81/August 1912/Bees Which Visit Only One Species of Flower
|BEES WHICH VISIT ONLY ONE SPECIES OF FLOWER|
ONE warm afternoon on the twentieth of July I was collecting insects from a boat on the Medomac River. A thunder-shower was coming up in the northwest. The air was very still and in that peculiar condition which precedes an electric storm. At such times insects are very sluggish and seek shelter against the approaching tempest. The silence was broken only by the rumbling peals of the distant thunder, following the bright flashes of lightning, which illumined the dark thunder-heads of the advancing clouds. It became necessary for me to hasten homeward. To my surprise I noticed on almost every one of the violet-blue spikes of the pickerel-weed (Pontederia cordata), a species of water hyacinth, which in countless numbers fringed the winding stream on both sides, one to several small bees. They had crept within the bilabiate flowers as far as possible, and were evidently intending to await there the passing of the storm. They were so inactive that no net was required, and I could easily knock them off into the cyanide jar. I collected about forty specimens and could have easily collected hundreds. This phenomenon has never been repeated to my knowledge.
On examination the bee proved to be Halictoides novae-angliæ, or the pickerel-weed bee. Every season when the pickerel-weed is in bloom I find both sexes of this bee on its flowers, and though I have carefully observed the visitors to many other plants in this locality I have never met with it elsewhere. Apparently in this region it never visits any other flower—it is a monotropic bee. When a female bee in gathering pollen for brood-rearing visits but one kind of flower it is termed a monotropic bee, or if only a few allied species an oligotropic bee; but if it visits many flowers it is called a polytropic bee. These terms were first proposed by Dr. Loew, and signify adapted to one, few or many flowers.
It is impossible not to feel some curiosity as to why this little bee restricts its visits to the inflorescence of the pickerel-weed. Notice that it flies only at the season of the year when this aquatic plant is in bloom, and that it finds within the perianth both food and shelter. Very likely its nests are built not far away. The flowers of the pickerel-weed strongly attract insects by their great numbers, bright hues, pleasant fragrance and abundant nectar and pollen; and consequently are sought out by many bees, flies and butterflies. Bumblebees especially delight in these blossoms, which they visit with astonishing rapidity—Bombus consimilis making about seventy visits per minute. On the middle lobe of the upper lip there are two bright yellow spots, which tell of the presence and guide to the exact location of the nectar concealed within the tube of the perianth. When the pickerel-weed bee makes its appearance about the middle of July, there is no other flower in southern Maine which can offer it so many inducements as the pickerel-weed. But let us look farther and see if there are any other bees, which behave in a similar manner.
In the quiet bays of the river, floating upon the surface of the water, bloom the yellow water lilies (Nymphæa advena).
Again the wild cow-lily floats
Her golden-freighted, tented boats,
O'ershadowed by the whispering reed,
And purple plumes of pickerel-weed.
The flower is securely anchored to the bottom of the stream by a long stem. At first the opening in the bud is no larger than a bee's body, and the chamber within offers a dry and snug shelter amid the waves. It may truly be called a haven of refuge. Directly below the entrance is a broad, many-rayed, crown-shaped stigma, as in the poppy. The petals are thick, wedge-shaped bodies which are orange-yellow on the outer side near the top, where they freely secrete nectar. Under a microscope both large and minute drops can readily be seen. The stamens are indefinite in number; and reveling in the pollen, their bodies completely covered, there is a large and lively company of small flies called Hilara atra. Less common are two beetles, Donacia piscatrix and Donacia rufa; but what chiefly interests us is a small bee, Halictus nelumbonis, or the water-lily bee. This bee in this locality is never found on any other flower, but elsewhere it is met with on other species of the water-lily family, or Nymphæaceæ. It is an oligotropic bee, and the only species of the great genus Halictus that is known to behave in this way.
But in Andrena this is a common phenomenon, for instance, in Washington County, Wisconsin, according to Dr. Graenicher, twenty-four of the forty-seven indigenous species of Andrena are oligotropic. This is the largest genus of North American bees. They are sometimes called ground-bees, since they build branched tunnels eight to ten inches deep in the soil of sandy pastures and hillsides. A part of the species are vernal or fly in springtime, while a part are autumnal and fly only in autumn. They provision their cells with balls of "bee-bread," about the size of a garden-pea, composed of pollen moistened with nectar. An egg is laid on the top of the mass of bee-bread, and the cell is then closed.
The bright yellow staminate aments of the pussy-willow (Salix discolor) are great favorites of vernal species of Andrena, whence Smith calls them "harbingers of spring." The pussy-willows bloom in northern New England during the latter part of April, and their bright yellow aments are very pleasing objects in the cold gray landscape. They are very attractive to a varied company of insects, as honey-bees, bumblebees, flies, butterflies and beetles. It is a busy scene and one which the naturalist can never tire of watching; but it is not one of unmixed happiness, for little tragedies take place before our eyes. Among those which come to sip the nectar are little dance-flies (Empididæ), and not infrequently they are seized and carried away bodily by black robber ants, which roam everywhere. Honey-bees and many species of Andrena come in great numbers to procure pollen for brood-rearing. A part of the Andrenid bees gather only a portion of the pollen they require from the willows and the balance from the maples, plums, cornels and Viburnums; but there are four species (A. illinoiensis, A. mariæ, A. erythrogaster and A. moesta), which get their whole supply from this genus of plants. Of the autumnal flying species of Andrena there are five (A. canadensis, A. nubecula, A. solidaginis, A. hirticincta and A. asteris), which I have collected only on the flowers of the Compositæ, or aster family; and four of these bees confine their visits very largely to the golden-rods. In both Salix and Solidago the inflorescence offers an ample supply of nectar and pollen and there is little temptation for Andrenid bees to go elsewhere, when their time of flight coincides with the period of blooming of these two genera.
But in other localities Andrena erigeniæ is reported to be a monotropic visitor of the spring beauty (Claytonia virginica), Andrena violæ of the violet (Viola cucullata), Andrena geranii maculata of the wild geranium (Geranium maculatum), Andrena fragariana of the strawberry (Fragaria virginica) and Andrena parnassiæ of Parnassia caroliniana. It is not so easy to explain the behavior of these latter bees. It seems very remarkable that they should restrict their visits so closely to the flowers mentioned.
Macropis ciliata, or the loosestrife bee, usually gets its pollen from the flowers of the common loosestrife (Lysimachia vulgaris); while many species of Panurginus are taken only on the inflorescence of the Compositæ.
But this habit of visiting only one kind of flower is perhaps better illustrated by Perdita than by any other genus of bees. Only one species of Perdita is found in Maine; but in the western states some 90 species occur, of which about forty live in the arid regions of New Mexico. In Maine Perdita octomaculata is found almost exclusively on the panicles of Solidago juncea, the earliest blooming of the golden-rods; and I have never met with it on any other species except in one instance. In New Mexico two species of Perdita are found on the willows, Perdita zebrata visits only Cleome serrulata, Perdita crotonis visits Croton texensis, Perdita albipennis visits Heliantlius annuus (sunflower), and Perdita senecionis visits Senecio Douglasii. "It may be laid down as a rule," says Professor Cockerell, "that each species of Perdita visits normally but one species of flower, but occasionally specimens may be found on flowers to which normally they do not belong." But in many instances several species of Perdita frequent the same flower. The bees of this genus are small forms very frequently marked with bright yellow.
Many species of Colletes, Epeolus and Melissodes visit almost exclusively the flowers of the Compositæ, as the thistles, golden-rods and asters. Xenoglossa pruinosa confines itself to Cucurbita pepo or the common field pumpkin; while Megachile campanulæ, one of the leaf-cutting bees, is a monotropic visitor of the bellflower Campanula americana. Many other instances are recorded, and many more will no doubt be discovered when our bee fauna is better known.
This is certainly a very singular habit on the part of bees, and one which could hardly have been foreseen. On the contrary, it is generally supposed that bees fly about sipping sweets indiscriminately, as they are so commonly represented by the poets.
He woos the Poppy and weds the Peach,
And then like a tramp abandons each
For the gorgeous Canada Lily.
It is really getting unsafe for poets to write about nature in their old, haphazard way, trusting chiefly to their imagination as a guide. Fancy can supply nothing half so wonderful as the true facts about flowers and insects. Let us consider what theories naturalists have advanced to explain this curious habit!
In Kerner's day only a few oligotropic bees were known, and he believed that they gave the preference to certain flowers because they found their odors so highly attractive. But it is incredible that so many bees should be dominated in their flight to such an extent by various floral odors, and besides they not infrequently visit several flowers which differ in scent. No doubt, though, bees have their preferences in odors and nectars, and probably they prefer pollen that has a roughened or spined surface to that which is smooth.
A more probable explanation claims that female oligotropic bees have adopted this method of visiting flowers to avoid competition in gathering pollen for brood-rearing. This theory is only partially satisfactory and certainly is not always applicable, even assuming that such a partition is beneficial or required. The four species of Andrena, which in this locality visit exclusively the willows, do not thus avoid competition nor do they thus benefit other bees. The willow aments have pollen enough for all comers. In this particular case this habit seems to have arisen because it was advantageous to these bees to restrict their visits to flowers so abundantly supplied with pollen and nectar, combined with their early and short time of flight, which lasts only about a month, and perhaps also to their nesting near these shrubs. Where bees fly only during the latter part of the season it seems very natural for them to restrict their visits to the Compositæ. These flowers, as in the case of the golden-rods and thistles, are very common, contain ample food supplies and are easy to visit. They are actuated not by the need or desire of avoiding competition, but by the same motives which lead honey-bees to visit the white clover exclusively while it is in bloom.
The oligotropic habit is not beneficial to flowers, it concerns the bees alone. The oligotropic bees are almost without exception solitary forms, to which there are no flowers specially adapted. The social bees, as a rule, visit a great variety of flowers, though in Europe it is stated that there is a bumblebee (Bombus gerstaeckeri) which visits a single species of monkshood (Aconitvm lycoctonum). Here, of course, the adaptations are mutual. This mode of flight, however, has not in general been determined by floral adaptations. Certain species of bees have become satellites of certain flowers because of the advantage thus gained for themselves, and partly also perhaps as the result of habit. Just as there are fly-flowers, butterfly-flowers and bumblebee-flowers, so, on the other hand, there are willow-bees, golden-rod bees, a pickerel-weed bee, a loosestrife bee, a violet bee and a strawberry bee.
Two most important influences are the season of the year and the length of time the bee is on the wing. It is clear that bees which fly only in spring or autumn for about a month have not a great choice of flowers; and, of course, we never look for autumnal bees on spring flowers. Usually the length of time an oligotropic bee flies and the flower it visits is in bloom are about the same. The honey-bee is practically a monotropic bee at certain seasons of the year. While the basswood and white clover are in bloom the honey-bee visits these flowers almost exclusively. Again in the fall in Maine it confines its attention solely to the golden-rods. In California at times it collects nectar exclusively from the sages; in Michigan from the willow-herb, and in other regions from other plants. If from any one of these plants it also obtained its supply of pollen and was on the wing only while it was in bloom, it would be regarded as a monotropic bee in the strict sense of the word. That it exhibits a strong tendency, when collecting pollen, to be constant to one plant species is well known; and the little packets of pollen it brings into the hive seldom consist of two kinds of pollen. But, when a bee flies from spring till fall and requires a large amount of stores, it is evident that it can never become oligotropic.
Another important factor is the small size of many oligotropic bees. This is true of Prosopis nelumbonis, Halictus nelumbonis, and many species of Panurginus, Perdita and Andrena. These bees have a weak flight and are not fitted to travel long distances. It is known that in some instances they build their nests near the flowers they visit. Probably this generally true. The medium-sized oligotropic bees, belonging to the genera Colletes, Epeolus and Melissodes, fly in the fall and visit chiefly the Compositæ, a family which offers a wide choice of flowers. It is not always easy here to draw the line between an oligotropic and a polytropic bee.
There are still in existence many intermediate stages between monotropic, oligotropic and polytropic bees. While many bees visit a great variety of flowers, others visit only one family, as the Compositæ or Nymphæaceæ, others only a single genus, as Salix, and others only a single species, as the violet, strawberry or spring-beauty. Many exceptions no doubt occur and will be recorded when the habits of these bees have been more carefully observed. For instance, I have often seen the loosestrife bee on the umbels of the prickly sarsaparilla. It is evident that if a monotropic bee extends into a region, where the flower it visits elsewhere does not occur, it must of necessity visit other flowers. Dr. Graenicher writes me that the pickerel-weed bee (Halictoides novæ-anglicæ) is found in Wisconsin; but the pickerel-weed does not flourish in the same locality, and so this bee is compelled to visit the blossoms of other plants. Evidently this habit did not originally exist among bees, but has gradually been acquired.
We may sum up the matter as follows. All bees including the honey-bee show a strong tendency in collecting both nectar and pollen to be constant to one species of flower. This is manifestly for the advantage of both insects and flowers. In the case of a number of bees flying for only a small part of the season this habit has become so specialized that they visit only one or a few allied species of flowers, which offer an abundance of pollen and nectar. Primarily it seems to be the direct advantage gained rather than the avoidance of competition that has led to the rise of the oligotropic habit. As the honey-bee for a time restricts its visits to the white clover, so in like manner a monotropic bee visits but a single kind of flower. But in the former case the bee flies throughout the whole season, but in the latter when the flower fades the bee's period of flight is over.
The idiosyncrasies of bees in visiting flowers present many remarkable peculiarities, and undoubtedly offer an attractive field for observation. There are certain bees, which though they are not oligotropic obtain the larger part of their supplies from comparatively few flowers, as the plums, thornbushes, cornels and Viburnums. In this locality one of the leaf-cutting bees (Megachile melanophæa) shows a decided preference for the purple vetch (Vicia cracca), and if I desired a specimen I should look for it on the blossoms of this plant. As the parasitic bees do not provide stores for their brood and seek nectar for themselves alone, they show little preference for special flowers. For a similar reason the males of any species of bee may not visit the same flowers as the females, though the attraction of the female may largely influence their course, in which respect they exhibit quite human sentiments. It would, of course, be in vain to look for the males of Bombus and Halictus on the flowers of spring, since they do not appear until mid-summer. In the case of diœcious plants, or plants in which the sexes are on different individuals, the bees visiting the staminate flowers are more numerous and are sometimes widely different from those visiting the pistillate. The common sumach is a good example. Indeed the bees visiting a flower in its early stages may differ from those visiting it in its later stages. Again the visitors to a flower may differ, both in number and kind, in different seasons.
The depth at which the nectar is concealed is another most important factor in controlling the visits of bees. In some flowers it is fully exposed on a flat surface where it is accessible to all insects; in others it is at the bottom of a slender tube, where it can be reached only by the larger moths. The familiar fable of the crane and the fox is constantly illustrated among flowers. As a matter of fact, bumblebees and butterflies avoid rotate, flat flowers containing little nectar, since their long tongues do not permit them to suck easily on such a surface. On the other hand, it would be useless to look for the smaller bees with short tongues on the larkspurs and clovers, for the nectar is quite beyond their reach.
As we take our leave of the oligotropic bees it may be inquired if there are any other insects, which visit only one species of flower. There are many others, especially among butterflies and moths. The flag beetle (Mononychus vulpeculus) passes its entire life on the blue flag (Iris versicolor). This small weevil feeds both on the pollen and nectar and sometimes gnaws the flower-leaves badly. The eggs are laid in the young seed capsules, where the larvæ feed on the ripening seeds. Both the adult beetles and larvæ are supported at the expense of the blue flag. The legitimate pollinators are bees and while the flag beetle may rarely effect pollination it does far more harm than good. This symbiotic relation is a benefit to the insect, but anto the plant.
Two slender metallic-hued beetles (Donacia rufa and Donacia piscatrix) find very comfortable quarters within the flowers of the yellow water lily, where they idle away much time drinking nectar and eating pollen. They lay their eggs on the leaves and the larvæ burrow in the stems. As in the case of the flag beetle this arrangement is evidently more to the advantage of the beetles than of the plants.
The night-blooming yucca, or Spanish bayonet, which flourishes throughout the southern states, is pollinated exclusively by a small nocturnal moth. The larvæ of the moth live in the seed-capsules. Thus both plant and moth are reciprocally dependent on each other, and the destruction of the one would be followed by the disappearance of the other. But in most instances the insect receives the greater benefit.