FLOWER
RELATIONSHIPS
Flower Preferences
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 (>42oC)
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. M. rotundata is more
vulnerable to most types of pesticide than are honeybees (possibly
because they cut leaves as well as visiting the flowers). However,
a few types are less harmful (Johansen et al 1963).
M. rotundata has 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 (Butler & Ritchie 1965).
|