THE STUDY OF WILD BEES P . A . RACEY
Introduction MOST laymen often take the term wild bees to mean escaped colonies of the domesticated honeybee Apis mellifera. In fact, instances of successful escapes are rare, and such colonies seldom survive the winter. Not so rare, however, are the true wild bees, which in this country comprise 239 species of 26 genera. The most familiar are the Bumblebees, but also included are the Mining Bees, which burrow into the ground, the Mason Bees which construct nests of clay, and the Leaf-cutters, whose favourite nesting material is the garden rose. Considering the interest that has been focused on the truly social Honeybees and Bumblebees, it is surprising that so little is known about the rest of our native Apidae, especially since they raise some of the most stimulating and tantalising questions in entomology. In addition, they are important pollinators, a fact which has been exploited with great effect in North America. As well as discussing the biology of selected examples, I shall in this article be making an attempt to illustrate some of the evolutionary pathways that have led to more complex insect societies. PROSOPISâ€”the Bramble Bee The members of the primitive genus Prosopis build their nests in bramble stems or use the bore Channels made by other insects in old wood, banks of clay, vacated Lipara galls in reeds, and similar cavities. After mating, the small black female Prosopis begins the work of improving her tunnel by smearing the walls with a hardening slime, which resembles a very thin layer of cellophane. This is spread by the tongue, but whether it originates from glands in the head or from the Malpighian tubules of the alimentary canal is not known. It has been observed, however, that the bee continually bends its head ventrally towards the tip of its abdomen before a thread of slime is laid down. Functionally it is presumed that the slime makes the tunnels waterproof and proof also against leakage of the mixture of nectar and pollen which the female deposits there. Prosopis is only sparsely clothed with hairs, and does not appear to collect pollen on its body. It reveals its primitive lineage in the structure of its crop; whereas many bees have filtration devices to separate the pollen and nectar, Prosopis lacks this elaboration. Instead, the pollen and nectar must be swallowed together and regurgitated in the form of a thin gruel. As soon as a cell is stocked, an egg is laid in it and it is sealed with slime. A series of such cells are constructed along the length of the Channel.
T h e nearest relative of Prosopis, but slightly more advanced, is Colletes, found nesting in banks of sand in which it excavates its own burrows. It lines them with a similar cellophane-like secretion. Colletes is roughly the same size as a worker honeybee, and can collect pollen on its legs, not, however, packed into a discrete lump but loosely deposited on the scopa. W h e n a "cellophane" cell has been prepared in the lined burrow, it is filled with pollen and nectar, which is mixed into a semi-liquid paste. A n egg is attached to the wall of the cell just above the paste and the cell is then sealed with several layers of "cellophane". T h e egg hatches into a tiny larva about a millimetre long, which floats on top of the paste and eats its way through its nutritious provisions. A s the alimentary canal at this stage is blind, it cannot discharge any faeces until is has devoured its whole food supply and the canal becomes continuous. It then squeezes the faeces, composed of the husks of pollen grains from which the Contents have been digested, out onto the wall of the cell in a uniform layer, rather like toothpaste from a tube. Once this layer has hardened, the process is repeated, b u t this time a clear gelatinous secretion from the anus is applied. T h e developing bee is now encased in a three layered cell in which it spends the winter (FIG. 1).
lining of burrow
of foecal pollen impregnated
gelatinous secretion -sandstone
Single cell of Colletes with larva removed
While studying the life history of Colletes, I found the inconvenience of travelling long distances to colonies to observe the behaviour of these bees could be overcome by transferring overwintering cells to a small galvanised metal tank filled with sand
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and sandstone. The bees hatched during the summer, mated, and provisioned cells for a new generation, thus building up a small artificial colony in the tank. Two of the common British species of Colletes are C. daviesana and C. succincta; the former often constructing a series of cohesive cells in its burrow, whereas the latter constructs only a single cell at a time (FIG. 2).
Nest of Colletes daviesana
Nest of Colletes
Most genera of wild bees have other bee genera which parasitise them, and whose habits are clearly analagous to the cuckoo. The parasitic bee waits at the nest entrance until the rightful owner departs to forage, then quickly enters and lays an egg in a stocked cell. This hatches into a larva which first kills the host and then eats its provisions. In this way, Epeolus makes use of Colletes. More common than cuckoo bees in the nest sites of Colletes and all wild bees, are flies such as Miltogramma punctatum. I have taken as many as three larvae of this fly tightly packed in a single C. daviesana cell. When they have devoured the Contents of that cell, they were certainly not fully grown and later, in the laboratory, they were observed to eat their way down the series. Colonies of C. daviesana must therefore sufFer more heavily than C. succincta, and since the larval parasites cannot burrow through the hard sand from cell to cell, the construction of single cells may be a selective evolutionary advantage. ANDRENA and HALICTUSâ€”the Mining Bees The members of the genus Andrena are the harbingers of spring and on the bright cold days of March are often seen gathering loads of pollen from yellow sallow catkins. Although the genus is the largest in the British wild bee fauna, very little is known about its biology. The females of the American species A. erythronii usually construct two nests one after the other, by
burrowing into the soil, thus provisioning four cells in the first nest and two in the second, making a total offspring of six. Occasionally two females occupy a single nest, either simultaneously or one after the other. In the English colonies of A. ferox and A. bucephala, however, all the females share the same entrance hole at the same time. This may be an important clue to the way in which social behaviour was established in the Hymenoptera, and if the females can be shown to co-operate within the nest, would be the first report of social activity in this genus. After the period of flight in early summer the adult Andrenids die, males before females. T h e larvae develop in their cells in the soil during the summer, pupating in the autumn. T h e adults hatch in late autumn, but remain in their cells throughout the winter, burrowing to the surface in the spring, when colonies, sometimes consisting of many thousands of nests may develop. Since there is no known social co-operation between females, the factors determining the build up of such colonies have aroused interest, and in A. flavipes it has been found that females mate and nest only within the nest-site in which they themselves were reared. T h e nature of the soil and the lack of encroaching Vegetation, characteristic of the nest site, must favour this. Since they are strongly attracted by the odour of the site and not by that of the females, males will only seek nubile females within the nesting area, often ignoring those outside. T h e fact that this dependence on odour may be connected with the origins of social life is apparent when we consider how greatly the highly social honeybees depend on scents for their Organisation and communication. T h e origins of social behaviour have been of abiding interest to entomologists, and as the genus Halictus contains some social and semi-social species. as well as solitary species, it has commanded a great deal of attention. Although a number of social species are included among the English halictids, the most illuminating insights into their biology have come from French and Canadian work. T h e typical solitary nesting species, such as H. rubicundus, has a simple life cycle: females emerge from the ground in the spring, construct burrows and provision cells, from which males and females emerge and mate in late summer. T h e males die and the inseminated females burrow into the ground again, from which they may be dug up and put into Observation boxes in the laboratory. By simulating the conditions of spring, they may be induced to begin nesting prematurely. From this solitary nesting life, the origin of social life would theoretically require development along the following lines, each stage illustrated with present day examples:â€”
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Solitary species with one generation du ring the summer e.g., Halictus leucozonius
T Species with two generations, each generation establishing its own nest e.g., Augochlora pura
Species with two generations; smaller 2 2 and fewer ^JQ in the first brood e.g. H. ligatus H. scabiosa
Species with two generations; recognizable castes of $9. differing in size. No $ $ produced in first brood e.g., H. cinctipes H. pauxillus
Species with several broods during the summer; only in later broods e.g., H. malachurus
â€˘ Perennial species; $ 9 castes physiologically but not physically different. No males produced in the first year. e.g., II. marginatus A great deal is known about the biology of some of the examples given. The society of the Nearctic H. ligatus has, for example, been found to consist of a long lived principal egg-layer who leaves her burrow infrequently and never forages, together with several short-lived foragers. These foragers also guard the nest entrance against parasites and predators. The egg-layer inhibits the ovarian development of the other nest members, and the sex of
W I L D BEES
her progeny is determined by some "maternal switch mechanism", whereby external Stimuli are thought to act through hormones to determine whether or not a sperm is admitted to an egg. The fact that bees "drift" from one nest to another, and workers can be successfully introduced into stränge nests, suggest, moreover, an opt n type of society. H. marginatus is found in France and its complete life cycle takes, uniquely, six years to complete. It begins with a fertilized female, the queen, digging out and provisioning several cells which produce only workers. For the rest of her life, the queen lays in cells dug and provisioned for her by the workers, which increase in numbers from year to year. It is not until the fifth and sixth years that males are produced and copulation can occur to produce further generations of foundresses. It appears, however, that all females emerge from the pupa morphologically and physiologically alike, and that they are only differentiated functionally into egg laying queens and workers. Mating and fertilization seem to be the factors which determine whether or not this differentiation takes place. OSMIA and MEGACHILE—the Mud-daubers and Leafcutters So far I have been considering mainly bees which nest in the ground and which can only be studied by Observation of colonies in the field or by attempting to simulate the external environment by means of Observation boxes in the laboratory. Leaf-cutting bees of the genus Megachile and Mud-daubers of the genus Osmia can be studied conveniently if they can be induced to make use of trap-nests. The natural choice of nesting situations in these bees is very wide. Osmia will nest in any dry aperture, such as the vacated burrows of wood boring insects, between crownboard and roof in a beehive, under tiles, and even in disused locks. More specialised species make use of snail shells which they often bury or conceal after nesting. Some leaf-cutters, on the other hand, make their own burrows in soft soil, or in the rotten wood of old trees. By constructing artificial nests from drilled out blocks of wood, lengths of cane, or even more conveniently, drinking straws, and placing them in situations where the mud-daubers and leafcutters are found, these bees will eagerly accept the ready made nest, which can then be brought back to the laboratory for study. FHT
^ — r r
ffe^e fe^ fe^H^ FIG. 3.
Nest of Osmia in drinking straw showing clay partitions and eggs laid on provisions
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DĂźring the summer, osmias carry into the nest mud pellets and partition off cells which they stock with a mixture of pollen and nectar in the form of bee bread. Unlike the andrenids and halictids, the osmias and megachiles spin themselves cocoons, so constructed that all the faeces are left outside, a Situation very different from the Colletes mentioned earlier. In a trap-nest survey of these osmias, however, some Strange behaviour was often apparent. An initial examination of the traps each containing 150 straws, showed that 75 contained some sign of bee activity in the form of mud partitions. Further examination, however, revealed that very few of the straws had actually been provisioned, and contained developing bees. The rest had either their bottoms partitioned, the open ends partitioned, or both, with occasional partitions in between. One possible explanation is that the large number of similar straws confused the bee and that it made the mistake of partitioning holes which were not located again for provisioning. This is unlikely, and does not explain the partitioning of the open ends, since the bee would be able to smell a straw which it had stocked It may be very significant that the amount of mud required for the partitioning of straws is far less than that incorporated into a natural nest (FIG. 4).
FIG. 4. Natural nest of Osmia showing cocoons surrounded by clay
This suggests the explanation that when use is made of a straw the energy which motivates the instinctive drive for mud collection is not exhausted, and after provisioning has been completed, continues to motivate the now irrelevant collection and partitioning. Leaf-cutters make use of the straws more rarely and seem to prefer nesting in wooden traps such as canes, which can be spl in half for examination. The nests represent some of the most
skilful and dexterous architecture in the insect world. First, a number of small circular pieces of leaf are put into the bottom of the length of cane, and these are followed by larger pieces which are shaped and interwoven to form compact cells. It has been found recently in Megachile rotundata that sex ratios of two males to one female can be consistently obtained when the diameter of the nesting tunnel is 6 mm. Smaller tunnels with a diameter of 4 mm. result in a sex ratio of five to eleven males to one female, and as may be expected, bees reared from the large diameter straws are two to four times as large as those from the small ones. The innermost cells from each series usually contain females and the outermost males. Females, which may construct several cell series during their lifetime, repeat this pattern consistently and are, therefore, believed capable of Controlling the sex of each egg deposited. Follination One of the motives for the study of wild bees has been an assessment of their value as pollinators. In this country, for example, Andrena varians has a definite preference for the pollen of plum, cherry, pear, and apple and is therefore likely to be of value as a pollinator of fruit trees. In American, however, the value of wild bees in increasing the yields of alfalfa seed is proven, and every effort is made to provide adequate nesting sites. With the leafcutters this is relatively easy, since straws and drilled wood can be used with good effect. The trap-nests have an additional advantage in that they can be moved around from field to field, like beehives. The most important ground nesting pollinator is the halictid, Nomia melanderi and to encourage it, bee beds are constructed. Large trenches are bulldozed out, lined with polythene, and filled with soil of the right texture. These beds are then seeded with cores of earth containing the overwintering bees from a natural nesting site, and a new colony soon establishes itself over the whole of the prepared nesting area. Conclusion Although many questions about wild bees have been answered in the last few years, as many problems have been uncovered. Two of the most significant of these are the mechanism of sex determination, and the factors governing emergence, whereby in a series of Megachile cells for example, the last egg to be laid gives rise to the first bee to emerge from the nest. As a result of their value as pollinators, wild bees are now realised to be of considerable economic importance. The alfalfa seed producing areas of North America represented a huge vested interest, and the presence of such interestis always an encouraging sign that more facilities will become available in future years for the study of these insects.
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Appendix: Wild Bees in Suffolk. Where to look for t h e m Part of the trap-nest survey described in this article was carried out on Lord Cranbrook's estate at Great Glemham during the summer of 1966, and Osmia rufa was found to occur there, not surprisingly, as it is a very common species all over the country, and can often be seen searching old walls and posts during the summer looking for a hole in which to nest. Examination of such a wall in late summer often reveals mud plugs at the entrance to these completed nests. Colonies of Osmia may build up to large numbers in houses, nesting either in the damp course to which they gain access through holes in the mortar, or under the roof tiles. Megachile visits many gardens to cut leaves for its cells, often choosing roses, to the chagrin of prize growers! To watch this bee at work is well worth the patience and stillness necessary. Prosopis is less readily seen, for it is very small, but sometimes a walk along the sunny side of a wood will find this bee inspecting broken off bramble stems looking for a suitable nest. The ground nesting species are difficult to find, but the discovery of a colony will draw the observer for many hours to watch the behaviour of the bees and their parasites. Colletes prefers vertical sand faces on the south-east wall of disused sand pits or undisturbed dune banks. A colony in such a dune is currently under study on an island in the Essex Marshes and is interesting in that its parasites don't seem to have found it yet. Some species of Andrena are found in lawns, the regulĂ¤r cutting of which flattens the little tumuli of excavated earth but has no ill effect on the bee. Others nest in bare earth banks; they cannot be sensitive to disturbance for such colonies have been found along the sides of busy suburban roads. Halictus on the other hand prefers wilder situations, again choosing well insolated banks. Willows, Sallows, and orchards in blossom attract large numbers of wild bees, and a single tree may have several species gathering pollen from itâ€”each species being "shadowed" by its respective parasite. Such a Situation is a collectors' field day, for the bees are newly emerged and are in fine condition. Their hair soon becomes worn off and bleached, and their wings fray as the work of stocking their cells progresses. The males are sometimes seen swarming in large numbers in orchardsâ€”several swooping on one female in an attempt to mate.