Pollination by Insects - NALDC

bage Group, California Agricultural Experi- ment Station Bulletin 532, 1932.

G. L. Philp: Cherry Culture in Cali- fornia, California Agricultural Extension Circular 46, revised. 1947.

F. W. Rane: Fertilization of Muskmelon, Society for the Promotion of Agricultural Science, Report of the I9ih Annual Meet- ing, pages 150-151. 1898.

A. J. Pieters and E. A. Hollowell: Clover Improvement, Yearbook of Agriculture i937> P^g^s 1190-1214.

C. R. Ribbands: The Foraging Method of Individual Honey Bees, Journal of Ani- mal Ecology, volume 18, pages 42—66. 1949.

R. G. Richmond: Red Clover Pollination of Honey Bees in Colorado, Colorado Agri- cultural Experiment Station Bulletin 391. ^932-

A. N. Roberts and C. A. Boiler: Pollina- tion Requirements of the English Holly, Ilex Aquifolium, Proceedings of the American Society for Horticultural Science, volume 52, pages 501-509. 1948.

R. H. Roberts: Better Cherry Yields in Wisconsin, Wisconsin Agricultural Experi- ment Station Bulletin 344. 1922.

J. T. Rosa: Fruiting Habit and Pollina- tion of Cantaloupe, Proceedings of the American Society for Horticultural Science, volume 21, pages 51-573 1924: Pollination and Fruiting Habit of the Watermelon, Pro- ceedings of the American Society for Horti- cultural Science, volume 22, pages 331-333- 1925-

C. W. Schaefer and C. L. Parrar: The Use of Pollen Traps and Pollen Supple- ments in Developing Honey Bee Colonies, Bureau of Entomology and Plant Quaran- tine Circular E-531. 1941.

J. S. Shoemaker: Cherry Pollination Studies, Ohio Agricultural Experiment Sta- tion Bulletin 422. 1928.

C. E. Shuster: Pollination and Growing of the Cherry, Oregon Agricultural Experi- ment Station Bulletin 212. 1925.

Sardar Singh: Behavior Studies of Honey Bees in Gathering Nectar and Pollen, New York Agricultural Experiment Station Mem. 288. 1950.

F. W. L. Sladen: How Pollen Is Collected by the Social Bees, and the Part Played in the Process by the Auricle, British Bee Jour- nal, volume 39, pages 491-494. 1911.

John C. Snyder: The Pollination of Tree Fruits and Nuts, Washington State College Extension Bulletin 342, reprint. 1947.

F. E. Todd and O. Bretherick: Composi- tion of Pollens, Journal of Economic Ento- mology, volume 35, pages 312-317. 1942.

W. P. Tufts: Almond Pollination, Cali- fornia Agricultural Experiment Station Bul- letin 306, 1919; and California Agricultural Experiment Station Bulletins 346, Almond Pollination {1922)^ 373, Pear Pollination {1923), and 385, Pollination of the Sweet Cherry {1925), with G. L. Philp.

Pollination by Native Insects

Géorgie E. Bohart

The earliest flowering plants in the fossil record were related to the mag- nolias, which to this day depend for pollination on the visits of beetles. Beetles, w^hich comprise the order Coleóptera^ w-ere the most abundant and adaptable insects during the dawn period of flowering plants and thus, quite naturally, were the first pollinators. The flies and the sawflies and wâsps were present but poor in variety and primitively developed. In the en- suing ages, however, their adaptation to the products of flowers became a dominant feature of their structure and habits. The moths and butterflies, w^hich first appeared in the early days of flowers, soon adapted themselves completely to floral ofí'erings. Now nearly all of them are highly developed for taking nectar from flowers.

While the insects wêre thus becoming specialized to take advantage of flowers, plants were likewise becoming specialized to make more efficient use of insects. Certain flowers developed characteristics limiting them to pollination by certain types of insects, which in turn become highly adapted to these specialized flowers. Today we have many plants so constructed that only a few specially adapted insects can visit them successfully. Figs, orchids, Span- ish-bayonet, and monkshood are examples.

The so-called hawk-moth orchids (in the genera Hahenaria^ Angrae- cum y and others) exemplify the many intricate modifications possessed by orchids to insure pollination by specific kinds of insects. In these flowers the nectar, lying at the bottom of a long narrow tube, is accessible only to the long-tongued hawk moths. While

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probing for nectar, the moth brings each eye a.gainst a sticky disk to which a mass of pollen is attached, and flies away^ carrying the masses on its eyes. The masses (called pollinia) then bend forward on their stalks in such a way that, when the moth inserts its proboscis into the next flower, they fit perfectly against the stigma and adhere to it. From the presence in Africa of an orchid of this type, with a nectar tube 12 inches long, there is inferred the existence in that region of a hawk moth with a tongue equally long.

In most acts of pollination the insect has no interest in the plant beyond its store of nectar or pollen, pollination on its part being an accident. It is the plant which, by its offering of nourishment and by the arrangement of floral parts, insures that such "accidents" will occur.

The yucca moth, which is the sole pollinator of yucca (Spanish-bayonet), is a unique exception and provides a good example of symbiotic relation- ships between plants and animals. It is no mere nectar sipper. At first, oper- ating somew^hat in the manner of the fig wasp, the female stabs the ovary of the yucca flower with her ovipositor and inserts an ç^g. That is common- place insect behavior, but her next acts, though instinctive, seem to display careful planning and an uncanny knowledge of botany. She mounts a stamen, scrapes together a wad of pollen, carries it back to the pistil containing her egg, and thrusts it into the funnel-shaped stigma. She takes neither nectar nor pollen for herself but performs the only act that will guarantee the proper food for her ofT- spring, the developing ovules of the plant. The yucca plant in its turn may lose a few seeds to the young wôrms— surely a small price to pay for such perfect pollination service.

SOME YEARS AGO, scientists argued hotly whether insects or flowers became specialized first or whether it wâs simultaneous. Voluminous papers at- tempted to explain why and how the

Yearbook of Agriculture 1952

process of mutual adaptation developed, but the subject finally became so controversial and unproductive that it was all but dropped. Recently, however, technical advances in agriculture have demanded that progress on problems of pollination keep pace. Knowl- edge gathered by the early wôrkers in defense of their philosophical argu- ments is now being put to work in the applied field, but many of the old chal- lenging questions of insect-flower evo- lution remain unanswered.

Granting the influence of pollinating insects on biological history, what would happen if they should suddenly disappear? It would certainly not mean the end of the flowering plants, because many important plant types have secondarily become adapted to pollination by other agents than insects. The great family of the grasses depends upon cross-pollination by wind or automatic self-pollination within closed flowers. Most of the nut and acorn trees have become adapted to pollen transfer by wind. Even many species within the family of legumes, which is highly specialized for pollination by bees, have come secondarily to depend mainly upon automatic self- pollination within the young blossoms. Peas and beans are familiar examples.

It is likely, therefore, that man could carry on without insects for pollination. The grasses and self-pollinating legumes could form the basis of his agricultural economy. Many of the insect-pollinated plants could be main- tained by vegetative propagation, although most of them would be barren of fruit. Tomatoes and potatoes he would still have, but he would have difficulty finding substitutes for clover and alfalfa, and he would have to get along with reduced yields of a variety of crops ranging from cotton to onions. Perhaps the most drastic effects would be in uncultivated areas where a large share of the soil-holding and soil-en- riching plants would die out. Further- more, it would be a bleak springtime if no gay-colored flowers were to grow in the forest glens and open hillsides.

Pollination by Native Insects . So much for what did not happen

and is not likely to happen. Let us ex- amine what has happened or may happen in the future. Probably the insect pollinators will not disappear, and we can go right on eating apples and finding pieces of okra in our vegetable soup.

When the first settlers arrived in America they found no honey bees but there were flowers, fruits, and vegetables in the forests and fields. Further- more, they were able to produce native American and introduced Euro- pean crops of many kinds for more than 50 years before honey bees were well established. Native insects were still abundant enough to pollinate the native and introduced insect-pollinated plants. Honey bees were colonized in North America before 1638, but for several decades they were probably more important as honey producers than as pollinators. So long as cultivated areas were composed of small fields surrounded by wild land, native insects were able to handle the pollination job without help from foreign labor. Inevitably, however, as the plow turned under large tracts of sod the native beneficial insects began to disappear. At the same time the available pollinators were spread more thinly over the ever-enlarging orchards and seed fields.

Our native pollinators have sufiPered the same fate as other forms of wildlife. Certain species have been able to persist and even increase in cultivated areas by taking advantage of road cuts, outbuildings, eroded areas, and the like for nesting places. But most species have had to retreat into fence rows, stream gullies, wood lots, and waste fields to maintain themselves. In recent years, as clean cultivation and in- tensive land utilization have become the rule, such havens are fast disap- pearing within flight range of the crops that need insects for pollination.

The logical question at this point is : How important or necessary are native pollinators to our agricultural set-up now that honey bees can be brought in

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large numbers to any field or orchard? There is no single answer to such a question. An estimated 80 percent of the insect pollination of our commercial crops is performed by honey bees— but that figure, which is only an esti- mate, does not tell us which crops are involved in the 20 percent pollinated by native insects and whether the whole 100 percent is adequate.

Honey bees, unlike most of our native pollinators, collect nectar or pollen or both from a wide assortment of plants. Consequently, there are few crops, whose blossoms are attractive to any insects, that do not hold some at- traction for honey bees. Besides, honey bees can be increased and moved about easily. Nevertheless, in the forests and ranges many herbs, shrubs, and trees will always have to depend on native insects for their reproduction. Like- wise, many forms of wildlife and range stock depend in whole or in part for food upon the plants or the seeds and fruit that the native pollinators make possible. Bee for bee, various native species are more efficient pollinators of certain crops, such as alfalfa, red clover, and sometimes even fruit, than honey bees. About that, more later; first let us look at the insects themselves.

Thousands of species of insects assist in the pollination of our entire fauna of insect-pollinated plants. They are distributed principally among the bees and wasps, the butterflies and moths, the flies and gnats, and the beetles. Even minute thrips may be "important in the self-pollination of certain plants like carrots and some of the composites, which have tiny, closely aggregated flowers. The order Hymenoptera, even without the honey bee, is by far the most important order of insects in the pollination of commercial crops. Flies probably rank next in importance although the moths, which are very abundant, may do more pollinating under cover of darkness than they arc given credit for. However, the value of moths and butterflies as pollinators is more often than not offset by the dam-

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no age they do as larvae. Flies, likewise, are frequently harmful as larvae and many species are carriers of disease as adults.

Among the Hymenoptera, bees, which comprise the superfamily Apoi- deaj are the most useful pollinators. Some other members of the order, such as the thread-waisted wasps, visit many flowers to partake of nectar, but theirs is a supplementary role on commercial crops and it is difficult to conceive of methods for making better use of them.

THE WILD BEES (after the honey bee) have rightfully received most of the attention accorded to our insect pollinators.

At least 5,000 species of bees probably exist in North America, many of them still undescribed by the taxono- mists. Most of the species are important only to wild plants, but at least several hundred take part in the pollination of cultivated crops. For exani- ple, more than lOO species have been reported as visitors to flowers of alfalfa alone.

All but a few of our many species can be grouped in families thus: Gol- letidae, obtuse-tongued bees; Halicti- dae, sweat bees and their allies; An- drenidae, mining bees; Megachilidae, thick-jawed bees; Anthophoridae, flower-loving bees; Xylocopidae, carpenter bees; Apidae, honey bees and bumble bees.

The first three families are com- monly called short-tongued bees and the last four long-tongued bees, although that is not an invariable distinc- tion.

Wild bees have great diversity in habits and habitats. Biological infor- mation has been published on fewer than 5 percent of the species. We do not even know where many of our more important pollinators nest. Rea- sonably complete biological studies have been made in this country for fewer than a score of species. By piec- ing knowledge of species in this country with the more complete knowledge of their European relatives, however.


we have a ground work on which to build.

Bees are characterized by the habit of providing a store of honey and pollen for their ofl'spring, although many species, cuckoolike, preempt the stores of their more industrious relatives.

Most wild bees are solitary. Each female constructs, provisions, and lays eggs in her own nest without help from her neighbors. Each cell in the nest is sealed up as soon as it is provisioned and provided with an egg, and there is no further contact between parent and offspring. A number of species of the family Halictidae have advanced to the stage where the overwintered mother bee remains with her daughters and assists them by guarding the communal nest entrance and laying fertilized eggs.

Many of the so-called solitary bees are gregarious to a greater or lesser de- gree. Highly gregarious species may dig their burrows in the soil only an inch or two apart and cover acres with their bee towns. Populations in such sites are sometimes comparable to those of moderate-size apiaries of honey bee colonies. A nesting site of alkali bees in Utah was estimated to contain 200,- 000 nesting females. This site and another large one nearby provided good pollination for the alfalfa-seed fields within a radius of at least 2 miles.

Most bee species are strictly solitary, showing no tendency toward neighbor- liness and often nesting in well-hidden places. In order to persist in effective numbers as pollinators, such species must have extensive areas suitable for their nesting. The recent experience of alfalfa-seed growers in Saskatchewan is a case in point. Their alfalfa is pollinated principally by leaf-cutting bees, which nest in beetle burrows in the forest timber. A few acres of seed surrounded by forest usually had plenty of leaf-cutting bees and good seed crops, but when the same area was given over to extensive cultivation, only a few seed fields next to the wild country were adequately pollinated.

Social life is a striking but rather un-


common attribute of bees. The true social habit, involving division of labor and cooperation between parents and offspring, reaches its culmination in the complex society of the honey bee, but the glimmerings of social behavior arc exhibited by several divergent stocks in various parts of the world. Among our native forms, bumble bees have the most complex society but their hive is a humble and untidy affair compared to that of the honey bee. The bumble bee, like the honey bee, belongs to the family Apidae, most of whose members are social. In the Tropics are many species of small stingless Apidae which, in some regards, are as highly developed socially as the honey bee. They are the most abundant bees in many tropical areas and have been used by the Indians of South America for honey production, but attempts to col- onize them in this country have failed.

Nesting places are nearly as varied as the bees themselves. The bumble bees choose well-protected cavities, which may be above ground or subter- ranean, depending upon the species. Carpenter bees [Xyiocopa) and representatives of many genera of the Mega- chilidae nest in beetle burrows in wood or chisel their own tunnels. The small carpenter bees {C er atina) and again many representatives of the Megachili- dae nest in the natural channels of hollow or pithy-stemmed plants. Broad- tongued bees, sweat bees, mining bees, and fíower-loving bees almost invari- ably construct burrows in the ground.

Some of the less common environments chosen by certain species include abandoned snail shells, small limb crotches, burrows of other bees, nests of mud-dauber wasps, and cavities in porous types of rock. Various megachilids are especially prone to develop tastes for unusual nesting places.

Most bees nest in the soil. Depending upon the species, the soil may be moist or dry, loose or packed, or even solid rock. The surface may be bare or vege- tated, flat or vertical. Few species nest in rich organic soil or in densely shaded places. Most seem to like soil that packs

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firmly, at least at the level of the brood cells.

Nests of solitary bees in the soil are usually in the form of burrows with short or long branches containing brood cells. Some species make their tunnels only an inch or two long, but

The large mountain carpenter bee and a series of brood cells tunneled in cedar.

others drive the main shaft down for 2 or 3 feet. Some have vertical and others have horizontal cells. Some have several cells in a linear series, and others have only one. Some have cells in tight clusters like bunches of grapes, and others have them at the ends of short, horizontal branches along the main vertical shaft. Each genus of bees usually has a distinctive plan of archi- tecture, but plenty of leeway is left for one species to differ from another. Above the generic level, basic archi- tectural patterns are discernible in some cases ( for example, in all halictid nests the entrance tunnel is wider than the branch tunnels) but in some families, like Megachilidae, the diversity of nest types defies satisfactory classi- fication.

Bees, like all insects that undergo complete metamorphosis, pass through four principal life-history stages, the egg, larva, pupa, and adult. The ç^gg is always laid within a brood cell. Bumble bees and honey bees generally lay it in an empty cell, and the young larva is fed progressively by nurse bees in the hive. In this country all other bees lay their eggs on, within, or under

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Nest of Andrena subaustralis, exterior view (one cell cut open).

Nest of Diadasia enevata, exterior view (one cell cut open).

a mass of honey-moistened pollen, which becomes the sole nourishment of the growing larva. After laying the egg, the mother bee seals the cell. The eggs and developing larvae of most bees dry out readily. Consequently some sort of seal coat is applied to the inner walls of the cells by the mother bee. Mega- chilid larvae, which are less delicate, usually are not protected in such a fashion, although some of them are protected by cells lined with sections cut from leaves (leaf-cutting bees), with plant fibers (cotton bees), or with pitch (resin bees).

As I mentioned, the young of all bees are fed a combination of honey and pollen. Larvae of the honey bee are also fed a gland-secreted material called royal jelly. Queens particularly are fed large amounts of the material. Royal jelly or its equivalent may be added to the food of the solitary bees, but this has not been actually observed.

The eggs of solitary bees, being rela- tively few in number, are much larger than those of honey bees and perhaps contain substances that are provided for honey bee larvae in royal jelly.

The beebread, as the store of food is often called, is prepared in a variety of ways. Hylaeus, which is generally considered one of the most primitive bees, does not collect pollen on her body but takes it into her honey stomach with the nectar. This distinctly liquid material is regurgitated into transparent waxen envelopes. The ^gg floats on the liquid in the envelope. The halictids fashion a flattened or egg-shaped ball of pollen, to which a small amount of nectar is added just before the tgg is laid. This pellet of food material is measured exactly to serve the needs of the larva, and none is ever left over. The andrenids make a similar but more spherical ball. Sometimes differ- ent species within a genus may be dis-

Pollination by Native Insects "3

Nest of Nomia melanderi, exterior view (one cell cut open).

Nest of Halictus farinosus, exterior view (two cells cut open).

tinguished by the shape of the pollen loaf. Size and shape of the egg and its method of placement also vary widely.

The eggs generally hatch within 2 or 3 days, and the larvae attain full growth within from i to 3 weeks, molt- ing and usually eating their cast skins twice during the process. There follows a period of several days during which the great quantities of pollen in the digestive tract are absorbed and waste materials arc discharged as fecal strips or pellets. Most bees, like the honey bee, maintain sanitary quarters during the feeding period. Among honey bees, at least, the midgut does not commu- nicate with the hindgut until the feeding period is over. In the large family of megachilids, however, the larvae may begin defecating when only one- third grown. Abandoned nests can often be identified by the type and manner of placement of the feces. Many of the megachilids use their pel-

lets as building blocks in the construc- tion of their cocoons.

Some bee larvae spin cocoons in which they pupate. Some do not. Honey bees and bumble bees, which are supposed to be at the top of the evolutionary scale, spin cocoons as do most of the megachilids, which are also thought of as advanced forms. Cocoon formation is scattered throughout the anthophorids, which are considered intermediate in the evolutionary scale. It is rare among the more lowly halictids and andrcnids, and absent among the most primitive of bees, the colletids. As bees are supposed to have developed from hunting wasps, most of which spin well-made cocoons, one would ex- pect the evolutionary trend, if any, to be away from rather than toward the cocoon-spinning habit.

Mature bee larvae, after defecating and perhaps spinning a cocoon, become nearly motionless prepupae. The pre-

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Nest of Anthophora occidentalis, exterior view (two cells cut open).

Nest of Colletés, interior view.

Yearbook of Agriculture 1952 pupa is more resistant to cold, drying out, and disease than the growing larva and is for many bees the overwintering stage. Beginning with this stage, the major differences in the life cycles of the various genera become manifest.

ANDRENA is the largest genus of bees. Most of its species have a simple life cycle, which may serve as a standard for comparison. Adults of both sexes emerge in the spring from their larval cells in the soil. After mating, the females construct, provision, and lay eggs in nests of their own. The larvae grow rapidly and by the end of spring have pupated and become adults within their larval cells. They remain thus im- prisoned throughout the summer, fall, and winter; they escape the following spring when their host plants are in bloom.

The alkali bee has a similar life history except that the first activity of adults takes place in the summer, and overwintering is in the prepupal stage. In most localities, a second brood of adults appears in the late summer. It is composed predominantly of females. The scarcity of males at this time may account for the high percentage of males usually occurring in the overwintering generation since males develop from unfertilized eggs.

This life-history pattern, allowing for variation in time of adult activity and number of generations in the active season, is the predominant one for wild bees. It is safe to say that most genera of bees pass the winter in the prepupal stage.

Halictus is a large and familiar genus of ground-nesting bees. Many of the species show a tendency toward social behavior. They are not related to bumble bees, but their life cycle is similar in several wâys. Many Halictus have a life history somewhat as follows: In the spring, overwintered females leave their hibernation burrows to construct and provision brood nests. Within a month or 6 weeks their progeny, all females, make their nests in the form of side burrows of the

Pollination by Native Insects parental nest, or else dig new ones of their own. Their progeny, being unfertilized, develop into males. The development of males from unfertilized eggs (parthenogenesis) is a general but not infallible rule among Hymcnop- tera. The old, overwintered female continues to lay eggs, this time on pollen balls of her daughters. These d'e- velop into females, which emerge in the summer and mate with the males, of which there is a large crop. The males soon die and the females dig themselves into hibernation burrows for the winter. Some species produce a third generation in the late summer composed of both males and females.

BUMBLE BEES carry this pattern to a higher social level. The mated, overwintered females are large individuals known as queens. In the spring the queen leaves her hibernation quarters and spends considerable time feeding and searching for a nesting place. After finding one, she prepares a small bed of woolly material in which she constructs a ball of pollen and a waxen cup or two filled with honey. She then lays a group of eggs in a cavity in the pollen and feeds the young larvae honey, increasing the pollen supply as needed. This progressive feeding is a step forward in social development.

Bumble bees have the birdlike habit of brooding on the eggs and young larvae. The queen's first brood generally develops into four to eight small worker bees. The workers are females with small bodies and poorly developed ovaries, apparently resulting from a limited food supply in the larval stage. Shortly after emerging, the worker bees take over the field and hive duties. The queen then retires to a life of egg laying.

Successive broods of workers tend to become larger as there are increasing numbers of bees to feed them in the larval stage. By the middle of the summer a large share of the larvae are fed a maximum diet and develop into queens. At the same time the males (drones) begin to appear. Some of the males may come from unfertilized eggs

cells lined uifh

(iljnlja ¡eavex

(md If of pieces

Nest of Megachile dentitarsis, interior view.

Nests of Colletés, interior view.

laid by the queen, but apparently most of them are the progeny of laying workers. The queens mate with males outside the nest, and after a few days or weeks of freedom they dig into sod or other material for hibernation. In the early fall, when no more female eggs are being laid by the exhausted old queen, the proportion of males in- creases and the colony gradually dies out. During the senescent period, scav- enging larvae of moths and beetles rapidly destroy the nest.

Many genera of bees have become specialized as social parasites—they live not on the tissues but on the food of their hosts. They are parasitic on other bees in all cases and, since they belong to various branches of the bee family tree, it is apparent that the para-

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Anthopliora occidental i s Osmia lignaria

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and provisions of various solitary bees.

Osmia montana

sitie habit arose independently many times. A number of genera of parasitic bees are closely related to their hosts.

Despite their diversity, parasitic bees all operate in much the same manner. The female spends most of her time searching for nests of her host. When she finds one, she waits for a propitious moment to slip in and place an tgg on a completed pollen ball before the cell is sealed. Apparently the host bee then seals the cell without recognizing that one of the eggs is not her own. The parasite is well protected with heavy armor and a long sting in case the foraging host returns and finds her in the nest. The young larva of the parasitic bee has long jaws adapted for pierc- ing the eg% or young larva of the host. When this is accomplished, the in- truder develops on the stored food just as if it were the rightful progeny of the host.

LET US now consider the usefulness of wild bees in the pollination of spe-

cific crops. Although wdld bees supple- ment the activities of honey bees in the pollination of many crops such as swêetclovcr and most of the fruits and cruciferous vegetables, honey bees are apparently at least as efficient and need only be supplied in reasonable numbers to handle the job alone.

Red clover was recognized by Charles Darwin as a plant that re- quires bumble bees for satisfactory seed production. The flowers have a deep corolla tube and tend to produce little nectar. Consequently honey bees often find it difficult or unprofitable to take nectar from red clover. They can obtain pollen from it readily but more desirable sources of pollen in the vicinity may satisfy their needs. In New Zealand, where red clover is well adapted, seed production was almost nil until the end of the nineteenth cen- tury, when several species of bumble bees were successfully introduced and established. Bumble bees have longer tongues than honey bees, and most


species regard red clover with special favor as a source of nectar and pollen. Apparently they have declined drasti- cally in numbers in the United States since 1900. In only a few districts are they adequate for the pollination of red clover; even there they are unre- liable because of yearly fluctuations in numbers. Fortunately in most regions honey bees^ in sufficient numbers and properly managed, can be induced to pollinate red clover. Undeniably, however, a general increase in bumble bees in the red clover seed areas would be a boon.

Alfalfa presents a diíTerent problem. It is a favorite source of nectar for honey bees, especially in the West, but is not a preferred source of pollen. Honey bees pollinate most kinds of flowers equally well when gathering nectar or pollen, but in the case of alfalfa the nectar gatherer is able to "steal" nectar from the flower without tripping a special mechanism involved in the pollination process. Nectar gatherers accidentally trip a small percentage of the flowers they visit, but for effective pollination they must be present in great numbers—greater, in fact, than the beekeeper is generally willing to supply when a honey crop is his primary goal. In some places the seed growers have largely overcome this -difficulty by paying beekeepers to overstock the alfalfa fields with honey bees, but this does not seem to work equally well everywhere. It also in- volves difficult problems in financial arrangements between seed growers and beekeepers and in maintenance of colony strength. In some areas, where 5 to 25 percent of the honey bees visit- ing alfalfa collect pollen, the problem is much less acute, and overstocking is practiced only for exceptionally high yields.

Many kinds of native bees visit alfalfa and most of them pollinate it efficiently, because they work it pri- marily for pollen, and trip the major- ity of the flowers they visit. Some species even seem to prefer alfalfa to neighboring pollen sources. Despite the

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variety of bees that visit alfalfa, however, there are not enough of them in most seed fields to provide adequate pollination—especially when the fields are large or an entire district is given over to seed production. The alkali bee is one of the few species that can build up sufficient numbers on small pieces of wasteland to pollinate extensive acreages of alfalfa. In a new seed district near Yakima, Wash., alkali bees are responsible for most of the pollination on thousands of acres of high- yielding alfalfa-seed fields. This bee is an important pollinator in localized areas in most of the States west of the Great Plains. In Canada leaf-cutting bees (Megachilidae) are generally credited with most of the pollination of alfalfa, although they can do a good job only on small acreages surrounded by much wild land. An important future development in pollination by wild bees may be in the production of foundation and registered seed stocks which require isolation in order to maintain their purity.

■ Fruit trees are pollinated principally by honey bees in this country. In most districts, however, various vernal species of wild bees have a supplementary role. Even syrphid flies and blow flies are important in some localities, notably in pear orchards. Honey bees are generally satisfactory pollinators of fruit except in parts of New England and eastern Canada where weather unfavorable for honey bee activity is customary during the apple-blossoming season. When they are present there, bumble bees and a few other species active at cooler temperatures are more satisfactory.

Tomatoes, peas, and string beans are examples of automatically self- pollinated crops. The principal exist- ing varieties are highly self-fertile and apparently receive no benefit from the cross-pollination accomplished by insects. Various wild bees are more attracted than honey bees to those crops. For example, bumble bees collect pollen readily from tomatoes. Leaf-cutting bees are strongly attracted by cer-

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tain varieties of peas. It is possible that wild bees could be important in the development of a hybrid-seed industry for several such vegetable crops.

Several small species of sweat bees appear to be the only bees that visit the flowers of beets in Utah. Beets are generally considered to be wind-pollinated, but insects are known to assist in the transfer of beet pollen. In Utah, where hybrid seed of sugar beets is being produced on experimental plots by planting alternate rows of male- sterile and pollen-parent varieties, the set of seed on the male-sterile lines is greatly enhanced by the presence of sweat bees. Such isolated experiences indicate that more seed crops are benefited by wild bees than is generally recognized.

ANY ATTEMPTS TO CONSERVE wild bees must be based on a knowledge of their habits and on a knowledge of the natural and man-made factors that operate against them.

Even in environments undisturbed by man, wild bees fall prey to an assortment of natural enemies. Philan- thinid wasps store them as food for their larvae. Robber flies pounce on them in the air and drain them of blood. Ambush bugs and crab spiders lie in wait on the flowers for a meal of bee blood. Back at the nests, conopid flies perch on spears of grass and seize passing bees for long enough to force an egg between their abdominal seg- ments, an egg that soon develops into a fat maggot occupying thé entire body cavity of the host bee. Cuckoo bees lurk about the nesting sites and seize an opportunity when the mother bee is foraging to slip in and lay an egg in the cell being provisioned. Bee flies hover over the nest entrances and spray them with minute eggs. The eggs develop into hordes of spiny little maggots, which work their way into the bee cells before they are sealed and remain there until the bee larvae are full- grown. Only one maggot develops on a bee larva, but its persistent sucking gradually transfers the semiliquid con-

Yearbook of Agriculture 1952 tents of the bee larva into its own growing body and leaves only a dried-up husk. Toward the end of the nesting season, wingless velvet ants crawl over the ground in the late afternoon, searching for any evidence of a nest. Once they find it, they force their way in, chew a hole through the host cocoon, and deposit an egg on the prc- pupa within. The invader then repairs the hole in the cocoon with salivary material and covers up the nest, leav- ing her ofl'spring to fatten on its cell mate in security.

In general, the gregarious species, more than the strictly solitary ones, are seriously harmed by parasites. Antho- phora occident alls, a large western bee that nests gregariously in clay banks, is parasitized in Utah by a chalcid wasp, three meloid beetles, a clerid beetle, a velvet ant, two parasitic bees, and a bee fly. Total parasitism in some sites runs as high as 50 percent. The alkali bee, which nests by thousands in flat, alka- line ground, is parasitized in Utah by one parasitic bee, one meloid beetle, one conopid fly, and one bee fly. The first three are of minor importance, but the bee fly {Heterostyium rohustum) nearly wiped out several large aggre- gations in Cache Valley in 1947; since then it has held them down, with parasitism as high as 90 percent. Strangely enough, this same fly occurs in the large nesting areas of central Utah, but only a few maggots have been found in thousands of cells examined.

Diseases are found among wild bees just as they arc among honey bees. In- fections resembling the foul broods of honey bees have not been observed among the native species, but very likely they exist. Certainly, lar\âe in their cells in the ground are frequently seen to sicken and die. Probably the development of organisms on the stored food is more serious to the wild bees. Various types of mold attack the pollens and some invade the bodies of the bee larvae, although that may usually be secondary after the larva is weakened on account of the moldy food supply. On the wet soil used by

Pollination by Native Insects the alkali bee the pollen balls may suddenly liquefy, in which case the larva quickly dies. In some sites this has been observed in as many as one-quarter of the cells. Diseases of adults are not often seen but would usually be difficult to observe or evaluate. In Califor- nia in the spring of 1934 a large population of Andrena complexa gathering food from buttercups became in- fested with ä fungus (probably Em- pusa sp.) and most of them died, still clinging to their host plants.

The impression should not be gained that predators, parasites, and diseases are so serious that wild bees have no chance to increase. In central Utah the many kinds of insects and pathogens attacking brood of the alkali bee prevented only 30 percent from emerging over a period of 3 years. During this period the known nesting sites increased in size and several new sites were founded.

Predators and parasites of wild bees will probably prove difficult to control. The life history of many of them is so tied to that of their hosts that selective control measures may be impossible.

Spoilage of the stores and molding of the larvae have been seen to increase following rain during the active nesting period of several species that nest in the soil. It is obvious that irri- gation and floodwater over the nests would be harmful then. Even during the dormant season, standing water would cause trouble, depending on the soil type and the species of the bees.

The principal limiting factor in numbers of wild bees appears to be available forage. Particularly is that true in wild or thinly settled land. The close association between species of bees and particular genera of flowers was probably developed as a response to competition for forage; the less ag- gressive types had to specialize to survive.

Competition has similarly forced many bees to restrict their season of activity to avoid periods of drought. In desert areas most bees can remain dormant for several years, if necessary,

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until there is enough moisture for blossoming of their host plants.

Forage for bees is not generally abundant in densely timbered terri- tory, in deserts, or in open prairies. It is more often suitable for large populations of bees in transitional zones at the edges of deserts or forests, in hilly country, or in abandoned agricultural areas that are reverting to forest. For- age and bees are also usually abundant for limited periods in semiarid country where rain falling during a restricted season gives rise to short but intense periods of bloom. Some cultivated areas are highly productive of forage; a common condition is for flowers to be produced in greater quantity but lesser variety than before cultivation.

For bees to build up sufficient numbers of overwintering forms for a good emergence the following year, there must be a continuity of bloom during the season of foraging activity. The interrupted bloom common to most agricultural areas is thought to be largely responsible for the small exist- ing populations of wild bees. For example, it has been stated that wild bees will increase in alfalfa-seed-producing areas when the cutting schedule allows for a constant supply of bloom. Applied to wild bees in general, the statement is based on an oversimplified notion of their life histories. It would apply best to leaf-cutting bees, most of which, in Utah at least, have activity periods involving two to three generations, which last through the blossoming period of alfalfa. Good forage and weather conditions in the spring before alfalfa blooms are probably more important for bees like honey bees, bumble bees, and sweat bees, which have a long season. Bees like Nomia, which do not appear until late summer, or Osmia, which disappear shortly after the first blooming of alfalfa, would be benefited more by a single cutting designed to achieve the maximum bloom at the proper time.

The value of spring forage for bees with a long season is illustrated by events in an isolated alfalfa-seed dis-

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trict near Fredonia, Ariz. Bumble bees were abundant in the summer of 1949 and provided excellent pollination for the alfalfa. An unprecedented drought in the area in 1950 prevented any spring bloom and^ although queens were seen in the spring, there were no workers in the summer for pollination. It is likely in this instance that a few irrigated acres, of an early-blooming crop like vetch would have allowed the bumble bees to increase as usual.

More than half of our species of bees have a short season of activity. In most cases the timing of emergence of such bees with the first blooming of their natural host plants is remarkable. In the Sacramento Valley of California, where a nesting site of two species of Andrena was under observation, emergence of the bees and the first appear- ance of willow blossoms took place on the same day. Bad weather during the short period of activity of such bees is apt to be their most serious hazard.

The presence of permanent and suitable nesting sites may be as important as abundant forage for the maintenance of effective numbers of wild pollinators. The decline in populations of wild bees in agricultural areas has probably been brought about at least as much by destruction of nesting sites as by destruction of forage. In this connection it is interesting to speculate upon the probable history of populations of the alkali bee in central Utah. In view of the fact that common, introduced plants like alfalfa, sweet- clover, and Russian-thistle are almost the sole forage plants for these bees in the area, it appears that they must have actually increased following the ap- pearance of white settlers. Many state- ments from the older farmers in the region attest to their abundance in the early days of alfalfa-seed growing. However, as cultivation increased, the nesting sites, although generally in poor soil, were plowed up and planted to alfalfa. Now only scattered areas are close enough to remaining nesting sites to be benefited. The best seed district in Utah from the standpoint of polli-


nation by alkali bees is adjacent to many acres of permanent saltgrass pas- ture that furnishes plenty of suitable land for nesting.

Intensification of land utilization has played havoc with the nesting sites of wild bees. The old rail fences provided sites for many timber-inhabiting bees like leaf-cutting bees and Osmia and provided a network of areas of undisturbed ground for nesting and of wild plants for forage. The clean cultivation now practiced to destroy weeds and soil-inhabiting insects is wiping out many of these last sanctuaries. It may soon become necessary to determine in each area how valuable the wild bees are for the pollination of . crops and whether nesting sites can be reserved for them in a manner compatible with good agriculture.

Destruction of harmful organisms together with conservation of beneficial ones should be our aim. Too often the ravages of the destructive forms are so conspicuous that we lose sight of the value of the beneficial forms. This is clearly evident in the use of insecticides. The necessity for insecticidal control for many insect pests is unques- tioned. But it is becoming increasingly apparent that the simple question, "Will this application provide economic control of the pest concerned?" must be expanded to, "Will this application fit into a general program cal- culated to control all important pests without presenting a hazard to health or seriously afiPecting beneficial parasites, predators, and pollinators?"

Conservation programs for wild bees have never been tried or even formu- lated on an area-wide basis. Although it is encouraging to know that a few seed growers are taking steps to protect known nesting sites, it is disheart- ening to know that most farmers do not appreciate the value of wild bees and are unlikely to take readily to conservation measures involving setting aside pieces of land and complicating the cropping procedures.

The following general measures should tend to conserve and even in-


crease the numbers of many kinds of wild bees. Details for carrying them out would depend upon many local factors; local conditions would probably call for certain additional measures.

1. Apply insecticides to blossoming plants only when there is no other way to control the harmful insects. Such applications should be made between 7 p. m. and 7 a.m. and should contain only toxaphene, methoxychlor^ or other toxicants demonstrated to be rel- atively safe for bees when used at the proper strength.

2. Provide a continuous supply of bloom throughout the season. Forage crops such as vetch, clover, and alfalfa make a good series lasting from late spring through summer. ÎFruit trees, maples, hawthorns, elderberries, and other hedgerow plants generally provide needed spring forage. Of course, each area would be best served by the plants suited to its own climate and agricultural needs.

3. Establish and maintain hedge- rows around agricultural fields and along roadways, ditch banks, and canals. Pithy-stemmed plants such as elderberry, sumac, and tree-of-Heaven should be encouraged in such hedge- rows. Light browsing would make them more suitable for nesting than if they were left undisturbed.

4. Hollow-stemmed plants such as milkthistle, wild parsnip, canebrake, and teasel should be broken over after the stalks are well developed. These will provide nesting places for leaf- cutting bees and harbor many hiber- nating species.

5. Establish and protect areas of bunch-type perennial grasses, especially along the tops of banks. They will provide nesting places for bumble bees and tend to stabilize and shelter the banks. Banks so protected, especially if nearly vertical, are ideal nesting places for many kinds of bees.

6. Preserve known nesting sites of gregarious bees from being cultivated, flooded, trampled, or encroached upon by dense vegetation. Expand the available nesting ground if necessary, and

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establish new areas with the same conditions as populated sites. In the past few years many nesting sites of the alkali bee have been discovered by alfalfa-seed growers. Once apprised of their value, the growers have usually been willing and even anxious to keep them in an unaltered state. Several and perhaps most of the gregarious species of bees migrate in large groups to newly prepared areas. If other conditions for population increase are fav- orable, it should not take long for new areas to be populated.

Another approach to the problem is through better utilization of available populations of native pollinators. The following principles should apply to many crops.

1. Grow the crop in areas where native pollinators are known to be abundant. In most cases such areas will be adjacent to or surrounded by untilled land.

2. Limit the acreage of the crop in bloom at one time to that which thé native pollinators can handle.

3. Reduce competitive sources of pollen and nectar.

4. Time the blooming of the crop with the period of greatest natural abundance of the pollinator. (In general, only forage crops v/ould be concerned here.)

GEORGE E. BOH ART, a member of the division of bee culture of the Bu- reau of Entomology and Plant Quar- antine^ is in charge of the pollination studies in connection with the production of legume seed, conducted at the Legume Seed Research Laboratory in Logan, Utah,

The attention of the reader is di- rected to the section of color drawings in which appears a drawing of an alkali bee {Nomia sp.) tripping an alfalfa blossom and the nesting sites and life stages of the bees. Opposite the drawing is a description of the life history and pollination activities of alkali bees.

Documents

Pollination by Insects - NALDC