Susceptibility to Chalkbrood of Alfalfa Leafcutter Bees, Megachile rotundata, Reared on Natural and Artificial Provisions

Chalkbrood susceptibilities of leafcutter bees reared on either natural or artificial provisions were compared. The dose-mortality responses of larvae reared on artificial diet were similar to those reported earlier; the LD50 was estimated to be 100 spores/larva with a probit slope of 1.13. Larvae reared on natural provisions showed different dose-mortality responses; mortality increased with an increase in dose up to 101 spores/larva. Above this dose, mortality decreased linearly with increasing dose. Differences in disease symptomatology were also detected; larvae reared on artificial diet had decreased numbers of sporulating cadavers at doses greater than 103 spores/larva. In contrast, such an effect was not apparent when larvae were reared on natural provisions; percentage sporulation was little affected by dose. It is concluded that diet has a profound influence on susceptibility of leafcutter bees to chalkbrood.