Viral prevalence is higher, while relative infection loads are lower, in later developmental stages of a univoltine insect herbivore

Abstract Pathogens represent critical agents of mortality for insects, yet their ecological impacts remain poorly understood in many natural systems. Developmental resistance (i.e., a reduction in disease susceptibility as hosts mature), along with age‐related changes in ecology and behaviour, are expected to give rise to variation in pathogen pressure across the host life cycle. However, the dynamics of pathogen infection across long larval periods such as those in diapausing insects are relatively unexplored. We used field‐collected Euphydryas phaeton caterpillars (Lepidoptera: Nymphalidae) to evaluate how interactions with a naturally occurring entomopathogen, Junonia coenia densovirus, varied across host development and diapause. We compared the frequency and severity of viral infection during three stages of the life cycle: post‐diapause, pre‐diapause and diapause. Field‐collected post‐diapause caterpillars were reared until death before viral screening, while field‐collected pre‐diapause caterpillars were subsampled for viral screening, allowing a portion to enter diapause to determine viral prevalence and load following overwintering. Viral prevalence was higher in post‐diapause caterpillars, pupae and adults compared to relatively immature pre‐diapause caterpillars. However, early‐instar (pre‐diapause) caterpillars that were infected with the virus harboured higher relative viral loads compared to late‐instar (post‐diapause) caterpillars, pupae and butterflies. In addition, viral prevalence was generally similar across sampling regions, while viral loads exhibited inconsistent regional differences across developmental stages. These results demonstrate that viral prevalence and load can vary considerably across the life cycle of an insect host and suggest that regional variation in infection severity may be more evident during certain stages of development than others. Moving forward, field and lab studies investigating the consequences of pathogen infection should consider host developmental stage, as infection parameters can change across development.