Alfalfa leafcutting bee (Hymenoptera: Megachilidae) pollination of oilseed rape (Brassica napus L.) under isolation tents for hybrid seed production

Abstract

In this 3-yr field study, the activities of alfalfa leafcutting bees (Megachile rotundata Fabricius) (LCB) at varying stocking levels were monitored in shade cloth isolation tents containing male-sterile and male-fertile rows of oilseed rape Brassica napus L. to determine optimum conditions for hybrid seed production. Bumble bees, Bombus impatiens Cresson, were similarly tested in 1 yr; their pollination activities were minimal, and weight of seed produced on female lines in tents containing bumble bees was similar to that in tents without pollinators. Leafcutting bee activity, as measured by the number of female bees at the entrance to tunnels in the hive, at first increased, then decreased with increasing stocking rate. Duration of time spent visiting flowers did not vary with stocking rate, but was longer on male-fertile than on male-sterile flowers. Production of hybrid seed within tents varied with leafcutting bee stocking rate, with the highest seed yield achieved at stocking rates equivalent to three charges at weekly intervals of 400 000 leafcutting bees per hectare. At optimum LCB stocking rates, the method described provided sufficient seed quantity for small-plot multi-location field evaluation of oilseed rape hybrids. Key words: Megachile rotundata, alfalfa leafcutting bee, Bombus impatiens, Brassica napus, oilseed rape, hybrid seed production, isolation tents