Modification of reproductive schedule in response to pathogen exposure in a wild insect: support for the terminal investment hypothesis

Trade-offs in the time and energy allocated to different functions, such as reproductive activities, can be driven by alterations in condition which reduce resources, often in response to extrinsic factors such as pathogens or parasites. When individuals are challenged by a pathogen, they may either reduce reproduction as a cost of increasing defence mechanisms or, alternatively, modify reproductive activities so as to increase fecundity thereby minimizing the fitness costs of earlier death, a behaviour consistent with the terminal investment hypothesis (TIH). The TIH predicts that individuals with decreased likelihood of future reproduction will maximize current reproductive effort, which may include shifts in reproductive timing. We examined how wild, adult female click beetles (Agriotes obscurus) responded after exposure to the fungal pathogen Metarhizium brunneum. Field-collected beetles exposed to a high concentration of M. brunneum died earlier and in greater numbers than those exposed to a low concentration. Using a multivariate approach, we examined the impact of pathogen challenge on lifespan and a suite of reproductive traits. Stepdown regression analysis showed that only female lifespan differed among the fungal treatments. Fungal-induced reductions in lifespan drove changes in the reproductive schedule, characterized by a decrease in preoviposition period. Moving the start of egg laying forward allowed the females to offset the costs of a shortened lifespan. These changes suggest that there is a threshold for terminal investment, which is dependent on strength of the survival threat. From an applied perspective, our findings imply that exposing adult click beetles to M. brunneum to reduce their population density might not succeed and is an approach that needs further investigation.