Maternal condition can generate resource-related maternal effects through changes in egg provisioning which can greatly influence offspring growth and survival (for reviews see; [1–4]). In the present study, we observed age-related effects on embryonic development time; with eggs laid by old mothers have longer embryonic development times. Maternal flight treatment influenced maternal reproductive output and this study is the first to demonstrate that increased flight during the oviposition period can reduce both maternal fecundity and the quality or composition of eggs. Changes in maternal egg provisioning due to flight treatment had a direct influence on larval mass and development time. Offspring with lower larval masses had reduced survival after exposure to the viral pathogen.
In P. aegeria, a decline in reproductive output with maternal age has been previously reported and is hypothesised to result from a decline in maternal resources over time [7, 51, 52]. In line with these previous studies, we observed that daily fecundity (after an initial peak on day 3 of oviposition) and egg size both decline over time with maternal age. Contrary to previous studies on other insect species, we found that offspring from older mothers did not differ in; survival during the egg and larval stages [5–8], larval development time [9–12] or pupal mass [9, 10]. We did find, however, that maternal age significantly influenced embryonic development time; with eggs laid by older mothers having longer embryonic development times. This suggests that age-related changes in maternal condition generate resource-related maternal effects that influence P. aegeria embryonic development [1, 30].
Many previous studies, across a wide range of taxa, have observed that egg size can influence offspring fitness [1–4]. In the present study, we found that egg size significantly influenced P. aegeria offspring development and survival. Offspring from large eggs had significantly shorter embryonic development times, increased larval masses after 21 days of development, shorter larval development times, and higher egg to adult survival. Although larvae from small eggs had longer larval development times, offspring from large and small eggs did not differ in pupal mass. This suggests that offspring from small eggs compensate for a smaller size early in development by growing and feeding for longer (and see ).
Forced flight treatment significantly affected female reproductive output. Relative to control females, females forced to fly laid significantly fewer eggs per day, particularly during the first six days of the oviposition period. There was, however, no effect of flight treatment on egg size. This is an interesting finding because a previous study demonstrated that increased flight prior to the onset of oviposition reduces egg size (but not fecundity) in P. aegeria . In the present study, however, a much more intensive flight treatment was used and females were stimulated to fly several times across the oviposition period (rather than once prior to the onset of oviposition as in the previous study). A resource allocation trade-off between fecundity and dispersal has been previously demonstrated in range expanding populations of P. aegeria . It is possible, therefore, that the intensive flight treatment used during this experiment also resulted in a decreased resource investment to egg production rather than to egg size. Further experiments are required, however, to determine whether these differences observed across studies are due to differences in flight duration (or intensity) per se, or due to other context-dependent factors including (amongst others), population-specific differences in response to flight, or across-study differences in the temperature used during the flight treatment and the oviposition period.
Larvae from eggs laid by females forced to fly had significantly smaller masses (at 21 days of development) and longer larval development times. Given that there were no differences in egg size across flight treatment groups, these results strongly indicate egg provisioning differences between flight treatment groups that are over and above those related to egg size per se. As far as we are aware, this is the first study to show that flight during the oviposition period can affect reproductive output both in terms of reducing fecundity and in reducing the quality or composition of resources in eggs. This result also indicates the potential for flight-induced changes in maternal egg provisioning to continue exerting effects later in offspring development. There was, however, no effect of maternal flight treatment on pupal mass. This suggests that offspring from forced flight mothers compensate for a smaller size early in development by growing and feeding for longer.
Viral infection negatively affected survival to both pupation and the adult stage. Effective resistance to pathogens is often dependent on resource availability , because resource availability improves body condition and also immune system deployment . It is well recorded that lepidopteran larvae can demonstrate developmental resistance to baculovirus infection and this is often attributed to increasing body weights during larval development . In the present study, offspring with a small larval body mass (at 21 days of development and hence at the time of viral inoculation) had a lower survival to pupation, suggesting that these offspring had reduced resistance to AcMNPV. Further experiments are required, however, to distinguish whether this effect on body masses arises through developmental resistance or through resource allocation to the immune system. Although flight-induced changes in maternal egg provisioning did result in larvae with small masses (at 21 days of development and hence at the time of viral inoculation) we found no significant main effect of flight linking the impact of maternal flight on larval mass. An absence of a direct effect of flight per se may not mean, however, that this maternal effect has no importance from an ecological perspective because the net effect is still a reduction in offspring fitness. As such, given the higher susceptibility of smaller larvae to viral infection it would be anticipated that there would be an indirect effect of maternal flight on offspring survival.
Viral infection had no detectable sub-lethal effects on larval development time and pupal mass. There are inconsistencies in studies of the sub-lethal effects of baculoviruses on lepidopteran larval development rate with some demonstrating little or no effect when early instars are treated (e.g.[54, 55]) whilst others demonstrate significant effects [32–36]. Contrary to our expectations, relative to control larvae, infected larvae did not grow for longer to gain sufficient resources for both immune system deployment and to obtain the critical mass required for pupation. This result probably reflects a reduction in variation in offspring phenotypes due to differential survival at each stage of development; larvae from small/poorly provisioned eggs fail to hatch, small larvae die during early larval development, more small larvae then die due to viral infection, leaving only the most robust, faster growing larvae to complete their development. Overall, this leads to the conclusion that although a decrease in maternal egg provisioning does reduce larval mass, which in turn decreases offspring survival probability, this reduction in mass does not necessarily confer an additional sub-lethal cost to larvae when they are exposed to a viral infection.