Leafcutter and Mason Bees: a Biological Catalogue of the Genus Megachile of the Neotropics Anthony Raw

FLOWER RELATIONSHIPS

Innumerable scattered publications refer to the flower taxa that Megachile species visit. Many of the bees are polylectic, visiting the flowers of various species of plants for food. For example, M. centuncularis in southern England averaged more than seven plant families per cell (Raw 1988). However, nesting females of polylectic species visit the flowers near their nests so they often collect most of their pollen from only a few plant taxa. Often they visit particular families of plants, like Compositae and Leguminoseae. Some species are more oligolectic, the bees restricting their choices to a particular species or genus. For example, M. gravita and M. pascoensis are oligolectic on Clarkia in western U.S.A. (MacSwain et al 1973).

In the species which have been studied the males fly around the flowers frequented by the females looking for mates. The searches of some species seem to be more general, the males quartering the areas of flowering plants. In others the males hold territories around the plants in bloom which the females visit. In the West Indies, males of M. lanata are commonly hold territories at flowering Crotalaria (Raw 1984b).

Pollination

Innumerable species of Megachile pollinate crops and wild plants like forest trees and other useful plants. They are especially common at species of Leguminosae, Compositae and Labiatae. Like all species of Megachilidae, nesting females carry pollen on the ventral side of the abdomen; an arrangement which facilitates the pollination of these flowers.

In much of the world, species of Megachile are the major pollinators of alfalfa. The role played by Megachile in the pollination of this plant was first recognized by researchers in North America (Brand & Westgate 1909, Piper et al 1914, Aicher 1917, Sladen 1918a, Tysdale et al 1943). Undetermined species of Megachile tripped 84% of the flowers visited, whereas honeybees tripped only 1% (Tysdale 1940). In order to achieve a yield of 1,300 kilos of seed per hectare in the absence of other possible pollinators a density of 500 foraging females of pollinating Megachile species per hectare of flowering crop is recommended (Hobbs 1956).

In most of U.S.A. M. rotundata is the most important pollinator, its presence resulting in yields of 2,200 kilos of seed per hectare; a twenty-fold difference over its absence. M. rotundata has been semi-domesticated and farmers are able to buy bees and receive advice on their care (Hobbs 1973). The use of M. rotundata in regions with a severe winter is complicated by the need to store the overwintering stages in frost-free conditions. However, in southern Alberta two native species, M. dentitarsus and M. perihirta, effectively fill the role (Hobbs & Lilly 1954). In hotter climates M. concinna is an efficient pollinator of alfalfa, the bees tripping on average 10 flowers per minute. An advantage of this species is that the developmental stages can survive high daily temperatures (>42^oC) which kills the larva of M. rotundata in Arizona and Mexico (Butler & Wargo 1963).

The effectiveness of these bees as pollinators was demonstrated by observations in an individual female of M. perihirta, which tripped 372 alfalfa flowers per foraging trip. Averaging 75 foraging trips per cell and 15 cells per nesting female, each foraging bee may thus trip 418,500 flowers to produce two kilos of seed (Hobbs 1956). An important revelation of these data is the high relative importance of the individual bee and, hence, the need to exercise great care when applying pesticides on the crop so as not to kill the pollinators. The susceptibility of M. rotundata to pesticides used on alfalfa differs from that of honeybees. M. rotundata is more susceptible to most types of pesticide (possibly because the bees cut leaves as well as visiting the flowers). However, a few types are less harmful (Johansen et al 1963).

M. rotundata has also been used to pollinate alfalfa in glasshouses (Aubury & Rogers 1971). M. concinna is also a candidate as a pollinator in small enclosures, having nested successfully in a flight room of 12 m³ and (Butler & Ritchie 1965).

IDENTIFICATION

Identification of the species of Megachile is difficult; a result of both the large numbers involved, the very fine differences which distinguish them and the frequent question with Hymenoptera of associating the two sexes. Unfortunately, the difficulties encountered in their identification have deterred useful investigation on the bees. As numerous species are major pollinators of many plants, botanists often collect the females. Males are less often collected, possibly because they fly more quickly and make only fleeting visits to flowers. Despite their acknowledged importance, citations in many publications are made only to generic level, reflecting the difficulties that exist in their identification. In order to identify the members of this large group of superficially similar insects it is first necessary to determine the subgenera to which they belong. The two sexes of many species of Megachile are very different in appearance. Fortunately, many are adventive nesters so the nests are relatively easily obtained and the collector may rear both sexes from a nest. For several species, this method and the rare capture of a pair in copula have been the only means of associating the sexes.

For identification, even to subgeneric level, the bees must be adequately prepared. In both sexes it is essential that the mandibles be opened to expose their inner surfaces. The fore and mid legs of the males must be sufficiently spread to allow examination of the lower surfaces. Determination of the subgenera of females is often difficult if the apex of the abdomen is not opened to allow examination of the sixth sternite. The simplest method to discover how to prepare the bee is to identify freshly caught specimens. Often pollen grains obscure the scopa of the female and must be removed before its structure can be examined. The most useful keys to the identification of neotropical Megachile are those of Michener et al (1994) Michener (2000) and Silveira et al 2002.