Diversity, density, efficiency, and effectiveness of pollinators of cicer milkvetch, Astragalus cicer L.

Abstract

Diversity, density, efficiency, and effectiveness of pollinators of cicer milkvetch, Astragalus cicer L., grown at two locations in southern Alberta were studied from 1978 to 1983. Twenty-seven species of bees were identified as pollinators. At Lethbridge, honey bees (Apis mellifera) comprised 74% of the observations, bumble bees 16%, and leafcutter bees 10%, while at Spring Coulee, the proportions were honey bees 14%, bumble bees 69%, and leafcutter bees 17%. The rate of foraging by pollinator species from flower to flower varied; bumble bee species, especially Bombus nevadensis Cress., foraged consistently more efficiently than honey bees or alfalfa leafcutter bees, Megachile rotundata (F.). A theoretical approach used to predict the bee populations required to pollinate varying flower densities shows that the population of B. nevadensis required is about half those of Bombus huntii Greene and M. rotundata and less than one-quarter that of the honey bee. Pollination by B. nevadensis consistently resulted in more seeds per pod than with any other bumble bee species, the honey bee, or M. rotundata. Of the nine species of bumble bee that established colonies in artificial domiciles near the field, B. nevadensis established the most colonies each year. The number of workers and sexuals produced per colony varied considerably among bumble bee species with only 55% of the colony establishments producing workers and 31% producing sexuals. The propagation rate and quality of alfalfa leafcutter bees produced on cicer milkvetch was excellent.