Chalkbrood Susceptibility Among Alfalfa Leafcutting Bee (Hymenoptera: Megachilidae) Larvae Reared at Different Temperatures

The effect of laboratory rearing temperature was related to chalkbrood susceptibility among larvae of the alfalfa leafcutting bee, Megachile rotundata (F.). Bees were reared at several temperature regimes. Mortality was inversely correlated with warmer rearing temperatures. LC 50 estimates (spores per larva) for the 6 temperature treatments ranged from 34 at constant 23°C to 8,511 at constant 37°C. Slopes of probit regression lines ranged from 0.77 to 1.29. Larvae died more quickly with increasing dose for all temperature treatments except for constant 37°C. Proportionally more cadavers sporulated with increasing dose for all treatments, but this proportion was lower on average for the warmer temperature treatments. Cadaver weight decreased with increasing dose, but was higher for bees reared at higher temperatures. Rearing temperature did not affect weight of adult females. Adult males reared at constant 37°C were significantly smaller than those reared at other temperatures. Spores of Ascosphaera aggregata Skou were tested for germinability in vitro using each of the temperature regimes. Fungus spores germinated equally well at all of the temperature regimes tested, but germination was slower at cooler temperatures.