Older beetles are stronger than young: Influence of maturation, aging and mating on response to an entomopathogenic fungus

Insect susceptibility to a pathogen challenge can be influenced by a variety of physiological factors, including the age of the insect and its reproductive history. The age of the insect can impact its susceptibility and there can be tradeoffs between reproduction and immunity. Fungal pathogens are used as biological control organisms for a variety of insect pests,  and Metarhizium brunneum is being developed to control the Asian longhorned beetle (Anoplophora glabripennis), an invasive wood-borer. Because female Anoplophora glabripennis take 1-2 weeks to mature after eclosion and both sexes can be long-lived, we wanted to determine how age, maturation and mating would impact A. glabripennis susceptibility to Metarhizium brunneum. Young (6-7 days-old) virgin, mature (27-33 days-old) mated and virgin, and older (57-71 days-old) virgin and mated adult beetles were inoculated with a lethal dose of M. brunneum and monitored daily for death. We determined whether beetles of different ages and mating status were resisting or tolerating the pathogen by quantifying M. brunneum presence in the hemolymph on days 5 and 9 after inoculation. Bioassay results to date indicate that young male beetles are more susceptible to M. brunneum infection than mature or older male virgin beetles. Additionally, our results suggest there is a cost to reproduction, at least for mature beetles which were more susceptible to M. brunneum infection than older beetles or unmated mature beetles.