CHALKBROOD SUSCEPTIBILITY AMONG LARVAE OF THE ALFALFA LEAFCUTTING BEE(HYMENOPTERA, MEGACHILIDAE) REARED ON DIFFERENT DIETS

Bioassays were conducted to determine effects of diet on chalkbrood, a devastating mycosis of larvae of the alfalfa leafcutting bee, Megachi le rotundata (F.). Bees reared in the laboratory on four sterilized di ets were assayed for chalkbrood susceptibility, bee development, and s urvival. Bees reared on a pollen + pollen substitute diet had an LC50 of 138 spores per larva, the shortest time to death, the smallest cada ver size, and the most frequent sporulation. Uninfected larvae develop ed into normal adults. Bees reared on a pollen + sugars diet had the l owest LC50 (79 spores per larva), the longest time to death, and the l owest sporulation rate. On this diet, the development and survival of uninfected larvae were delayed, and only a few small adults emerged. B ees reared on a pollen + sugars diet with a higher pollen concentratio n had the highest LC50 (1,549 spores per larva). These bees also had d elayed development and reduced survival to the adult stage, but adults were of normal size. Bees reared on sterilized natural provisions had an intermediate LC50 (219 spores per larva), time to death, and sporu lation rate, and the largest cadaver size. Development and survival ra tes for bees reared on this diet were similar to those obtained with b ees reared on the pollen + pollen substitute diet, but adults were muc h larger than those reared on any other diet. Thus, diet factors that promote bee survival and growth are different from those that determin e chalkbrood susceptibility.