Sub-lethal viral exposure and growth on drought stressed host plants changes resource allocation patterns and life history costs in the Speckled Wood butterfly, Pararge aegeria

Abstract

This study investigated the interactive effects of growth on drought stressed host plants and pathogen challenge with the baculovirus Autographa californica nucleopolyhedrovirus (AcMNPV) on survival and fitness-related traits using the Speckled Wood butterfly, Pararge aegeria (L.). Exposure to AcMNPV significantly reduced survival to pupation. For surviving larvae, sub-lethal infection significantly decreased daily mass acquisition rates and pupal mass. Growth on drought stressed plants increased daily mass acquisition rates resulting in heavier pupae, and increased resource allocation to adult reproduction. The interaction between host plant drought and viral exposure resulted in different resource allocation strategies, and thus different growth trajectories, between larvae. This in turn resulted in significantly different allometric relationships between larval mass (at inoculation) and both development time and investment in flight muscles. For larvae with relatively lighter masses there was a cost of resisting infection when growth occurred on drought stressed host plants, both within the larval stage (i.e. longer larval development times) and in the adult stage (i.e. lower investment in flight muscle mass). This multi-factor study highlights several potential mechanisms by which the complex interplay between low host plant nutritional quality due to drought, and pathogen exposure, may differentially influence the performance of P. aegeria individuals across multiple life stages.

Viral exposure effects on life-history, flight-related traits, and wing melanisation in the Glanville fritillary butterfly

“…Previous studies have shown that negative effects of infection are often weaker in older or larger animals (e.g. Grove and Hoover, 2007 , Gupta et al, 2007 , Gibbs et al, 2010 , Tseng and Myers, 2014 , Gibbs and Weir, 2017 ). Here we show that even though all larvae were inoculated the same number of days after the diapause was broken, those larvae that had heavier body masses at the time of inoculation with AcMNPV developed faster during the larval stage, had longer pupal stages and higher survival to pupation.…”

“…Notably, however, low concentration of viral exposure had no detectable effects on larval developmental time or pupal mass in the second experiment, even though it still induced increased mortality. In general, studies of the effects of baculovirus infections on larval developmental time show inconsistencies with either no, little or significant effects when infecting early instars ( Milks et al, 1998 , Matthews et al, 2002 , Monobrullah and Shankar, 2008 , Gibbs and Weir, 2017 ). In this study, larvae also reached heavier pupal mass when developing for a longer time, especially after being exposed to AcMNPV.…”

“…A stock suspension of the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) was produced as described previously in Gibbs et al, 2010 , Gibbs and Weir, 2017 . To enumerate the concentration of the stock, occlusion bodies (OBs) were counted in an improved Neubauer haemocytometer at magnification 400× (<10% error in counts) and this was replicated three times to give an estimated average concentration of OBs ml −1 .…”

“…prior to the inoculation) larval body mass was recorded (Precisa 2625MA-FR analytical balance accuracy: ±0.1 mg) and the larvae were randomly divided into eight different treatment groups (virus concentration exposure) across the families. Weighing larvae prior to inoculation allowed us to examine (during analyses) potential differences in response due to variation in larval mass at inoculation ( Gibbs et al, 2010 , Gibbs and Weir, 2017 ). Following a 24 h starvation period, larvae were exposed to AcMNPV (during the sixth instar).…”

“…We hypothesized that physiological constraints caused by resources being allocated to immune defence during growth would result in resource allocation trade-offs across life stages with fewer resources available to invest in life-history traits, such as developmental time or daily mass acquisition within the larval stage, and/or fewer resources available to invest in wing morphological traits in the adult stage. Little is known about the effects of baculovirus infection on wing development and wing morphological traits, but previous studies on another butterfly species found that exposure to viral infection during development affects resource allocation to thorax mass ( Hesketh et al, 2012 , Gibbs and Weir, 2017 ). We therefore predicted that effects of baculovirus infection on wing development (i.e.…”