Influence of brood rearing temperature on honey bee development and susceptibility to poisoning by pesticides

Adult honey bees (Apis mellifera) usually maintain colony brood rearing temperature between 34-35 degrees C by thermoregulation. The brood may, however, also be subjected to suboptimal temperature. Here we investigated whether a decrease of brood rearing temperature may have effects on larval mortality, adult emergence, longevity, morphology and susceptibility to poisoning by pesticides (dimethoate). Using the in vitro rearing protocol of Aupinel (2005), we were able for the first time to control the brood temperature not only during the pupal stage but also during the larval stage. Honey bee larvae were reared in vitro at 35 degrees C (optimal) and 33 degrees C (suboptimal) from 12 h after hatching for 15 days. Dimethoate was tested by ingestion either on 4-day old larvae or on 7-day old adults. Our results showed that lower rearing temperature had no significant effects on larval mortality and adult emergence, but adult bee mortality was strongly affected. Moreover, adult workers emerging at 33 degrees C were significantly more susceptible to dimethoate. Larval LD50 (48 h) was, however, 28 times higher at 33 degrees C than at 35 degrees C. The striking differences between larvae and adults may be explained by differential larval metabolism at 33 degrees C and resulting slower active ingredient absorption. We conclude that adult honey bees reared at even slightly suboptimal brood temperature may be more susceptible to pesticide poisoning and be characterised by reduced longevity. Thus, low temperature brood rearing could be another stress factor for colonies.