PLANTÐINSECT INTERACTIONS Nest Establishment, Pollination Efficiency, and Reproductive Success of Megachile rotundata (Hymenoptera: Megachilidae) in Relation to Resource Availability in Field Enclosures

The alfalfa leafcutting bee,Megachile rotundata (Fabricius), is used to pollinate alfalfa, Medicago sativaL., for seedproduction in theUnited States andCanada. It is difÞcult to reliably sustain commercialM. rotundata populations in theUnited States because of problemswith disease, parasites, predators, and unexplained mortality. One possible explanation for early immature mortality is that, relative to ßoral availability, superßuous numbers of bees are released in alfalfa Þelds where resources quickly become limited.Our objectivewas to determine howM. rotundata density affects bee nesting, pollination efÞciency, and reproductive success. Various numbers of bees were released into enclosures on an alfalfa Þeld, but only 10Ð90% of released female bees established nests. Therefore, a “bee density index” was derived for each enclosure from the number of established females and number of open ßowers over time. As the density index increased, signiÞcant reductions occurred in the number of pollinated ßowers, number of nests, and number of cells produced per bee, as well as the percentage of cells that produced viable prepupae by summerÕs end and the percentage that produced adult bees. The percentage of cells resulting in early brood mortality (i.e., pollen balls) signiÞcantly increased as the density index increased. We conclude that bee nest establishment, pollination efÞciency, and reproductive success are compromised when bee densities are high relative to ßoral resource availability. Open Þeld studies are needed to determine commercial bee densities that result in sustainable bee populations and adequate pollination for proÞtable alfalfa seed production.