Eggs and provisions of various solitary bees.

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 existing 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 certain 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. Philanthinid 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 segments, an egg that soon develops into a fat maggot occupying. the 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 contents 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 pre-pupa within. The invader then repairs the hole in the cocoon with salivary material and covers up the nest, leaving her offspring to fatten on its cell mate in security.

In general, the gregarious species, more than the strictly solitary ones, are seriously harmed by parasites. Anthophora occidentalis, 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, alkaline 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 (Heterostylum robustum) nearly wiped out several large aggregations in Cache Valley in 1947; since then it has held them down, with parasitism as high as go 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 are among honey bees. Infections resembling the foul broods of honey bees have not been observed among the native species, but very likely they exist. Certainly, larvae 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 mold), food supply. On the wet soil used by 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 California in the spring of 1934 a large population of Andrena complexa gathering food from buttercups became infested with a fungus (probably Empusa 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 3o 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 irrigation 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 aggressive 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, until there is enough moisture for blossoming of their host plants.

Forage for bees is not generally abundant in densely timbered territory, 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. Forage 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 existing 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.