41st Annual Meeting of the Society for Invertebrate Pathology
and 9th International Conference on Bacillus thuringiensis

Viral disease is a major component of the cyclic population dynamics of some Lepidoptera including western tent caterpillars. Epizootics of nucleopolyhedrovirus and host population subdivision provide an arena in which selection on virulence of virus and resistance of hosts could act. Theory predicts that epizootics should select for host resistance and that viral isolates should respond to this change on a population-by-population basis. Experiments provide evidence that these interactions are occurring but that patterns are weak as compared to other factors that determine the cyclic population dynamics. In addition there is no evidence for induced immunity or selection within a generation of tent caterpillars. The factors that promote the rapid development of NPV epizootics remain a mystery and are the topic of future research.

Chalkbrood is a disease of bee larvae caused by fungi in the genus Ascosphaera (Ascomycetes: Ascosphaerales). These fungi have been found only in association with bees, either as pathogens or saprophytes on pollen that is stored in bee nests. Like most entomopathogenic fungi, spores are the infective stage, but these spores differ in that they are unable to infect through the host cuticle; they must be consumed by the larvae and infect through the gut. Thus, the epizootiology of chalkbrood is affected by the ability of the fungus to transmit spores to the host’ pollen food stores. This is most likely achieved by phoresy on the adult bees. In addition, epizootics are uncommon in honey bees, but can be very common in managed solitary bees such as the alfalfa leafcutting bee and the blue orchard bee. This difference is probably a result of the capacity of honey bees to control hive temperatures above that which is optimal for disease development, and to remove diseased larvae from the nest. But it may also be due to the nest construction differences. Newly emerging adult cavity nesting bees may have greater exposure to the spores than do adult honey bees.