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Influences of weather and temperature on disease : implications for the population dynamics of western tent caterpillars Frid, Leonardo

Abstract

Studies on insect population outbreaks have failed to identify a single, general cause for population cycles. However, few studies have considered how various factors may together cause periodic outbreaks. Here I consider how disease and weather might interact to cause population cycles of western tent caterpillars, Malacosoma californicum pluviale (Dyar). A multiple regression analysis indicates that nuclearpolyhedrovirus (NPV) prevalence in western tent caterpillar populations increases with both host density and hours of sunshine during the larval feeding season. I explore the potential impact of climatic variability on population dynamics by modifying an existing model of caterpillar-disease dynamics to include uncorrelated stochastic variation in transmission rates between seasons in a manner that mimics the relationship between variability in spring weather and viral infection. Including stochasticity in transmission creates cyclic dynamics over a broader range of parameter space than in the deterministic model. Furthermore, simulated patterns of host density are more realistic when stochasticity in transmission rates is included in this model with outbreaks that vary among years in timing, amplitude and duration. Western tent caterpillars hatch in the early spring when temperatures are cool and variable. They compensate for suboptimal air temperatures by basking in the sun. To determine the impact of basking behaviour on the disease ecology of western tent caterpillars I examined the environmental determinants of larval body temperature and the effects of temperature on larval susceptibility to NPV. In the field, larval body temperature was determined by ambient temperature, solar irradiance and larval stage. The pathogenicity (LD50) of NPV was not influenced by temperature but the incubation time of the virus declined asymptotically as temperature increased. Basking behaviour of larvae and the associated increase in body temperature can strongly influence growth and development but has little impact on infection by NPV. Thus, an effect of temperature on the pathogenicity of NPV is not the mechanism by which disease prevalence is higher during sunnier years. Wanner temperatures attained by basking may increase the number of infection cycles in sunny springs but do not protect larvae from viral infection.

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