Density-dependent resistance of the gypsy moth Lymantria dispar to its nucleopolyhedrovirus, and the consequences for population dynamics

Abstract

The processes controlling disease resistance can strongly influence the population dynamics of insect outbreaks. Evidence that disease resistance is density-dependent is accumulating, but the exact form of this relationship is highly variable from species to species. It has been hypothesized that insects experiencing high population densities might allocate more energy to disease resistance than those at lower densities, because they are more likely to encounter density-dependent pathogens. In contrast, the increased stress of high-density conditions might leave insects more vulnerable to disease. Both scenarios have been reported for various outbreak Lepidoptera in the literature. We tested the relationship between larval density and disease resistance with the gypsy moth (Lymantria dispar) and one of its most important density-dependent mortality factors, the nucleopolyhedrovirus (NPV) LdMNPV, in a series of bioassays. Larvae were reared in groups at different densities, fed the virus individually, and then reared individually to evaluate response to infection. In this system, resistance to the virus decreased with increasing larval density. Similarly, time to death was faster at high densities than at lower densities. Implications of density-resistance relationships for insect-pathogen population dynamics were explored in a mathematical model. In general, an inverse relationship between rearing density and disease resistance has a stabilizing effect on population dynamics.

Rapid induction of immune density-dependent prophylaxis in adult social insects

“…However, animals did not show general depression of their immune system either in the social or the solitary context (in each case, one branch of the immune system was upregulated and the other downregulated), as would be expected if 'stress' were the cause (Reilly & Hajek 2008). Similarly, although workers in social groups compete for reproductive dominance (Honk et al 1981), this is unlikely to lead to similar immune changes in dominant and subordinate animals (Sapolsky 2004), and because our samples were taken at random with respect to dominance from the social groups it is unlikely to be an explanation for our results.…”

Crowding and disease: effects of host density on response to infection in a butterfly–parasite interaction

“…An alternative hypothesis to density-dependent prophylaxis is the 'crowding and stress' hypothesis, which predicts that individuals reared at lower population densities will be in better physical condition, and hence will be better able to resist the negative effects of parasitism ( Goulson & Cory, 1995;Adamo, 2006;Reilly & Hajek, 2008 ). This idea is based on the assumption that increased stress and intra-specific competition will make hosts more susceptible to infection, or less able to tolerate the negative consequences of parasitism.…”

“…In insects, this hypothesis was initially examined by Steinhaus (1958) in studies of caterpillars and their natural pathogens. More recent experimental studies on lepidopteran hosts have shown that animals reared at higher densities experience reduced disease resistance and/or decreased time to death ( Goulson & Cory, 1995;Reilly & Hajek, 2008 ). However, Brown et al (2003) found no effect of host-resource stress on infection or immunity in bumblebees.…”

Maternal effects in disease resistance: poor maternal environment increases offspring resistance to an insect virus

“…There has been some theoretical examination of the effect of within generation DDP on host-parasite population dynamics. White & Wilson [52] use a discrete-time model, representing non-overlapping insect generations and found that DDP stabilizes the dynamics, while Reilly & Hajek [53] using a continuous-time model within the season and a discrete-time map between seasons reported that DDP has a destabilizing effect on the population. Given the intrinsic delays involved in maternal effects, the population dynamical implications are likely to be even more complex.…”