Within and between population variation in disease resistance in cyclic populations of western tent caterpillars: a test of the disease defence hypothesis

Abstract

Summary 1.Epizootics of nucleopolyhedrovirus (NPV) are an obvious component of the population fluctuations of several species of temperate forest Lepidoptera, including the western tent caterpillar, Malacosoma californicum pluviale (Dyer). An observed relationship between epizootics and the subsequent reduction in fecundity of populations led to the formulation of the disease defence hypothesis. This hypothesis predicts that viral epizootics in peak populations select for more resistant moths and that their reduced fecundity in declining populations reflects a cost of disease resistance. 2. To test the disease defence hypothesis, we carried out bioassays to measure the variation in larval resistance to NPV infection for families of western tent caterpillars from four spatially distinct populations over 3 years of peak and declining host densities. 3. Each female moth lays a single egg mass and larvae are gregarious and remain together through development. We found that the resistance to disease of larvae within families was not related to the number of eggs in the mass from which they hatched (the fecundity of their mother). 4. Disease resistance of larvae varied among populations and over time in a manner consistent with selection for resistance. One population that had not experienced a strong viral epizootic during the last population decline was more susceptible to infection in the first year of the study. Larvae from a second population that experienced an early epizootic became significantly more resistant. The resistance of two other populations increased slightly before the viral epizootic occurred in the field however, and thus could not be explained by selection. 5. As population densities declined from peak density, the background mortality of larvae increased and the fecundity of moths decreased. This indicates a general deterioration in the quality of field populations of tent caterpillars associated with the declining populations. 6. Although some evidence suggests that viral epizootics can select for increased resistance of field populations of tent caterpillars, the general deterioration in quality, elevated background mortality, and the reduced fecundity after the epizootic are stronger influences on the population decline. These are possibly related to sublethal viral infection.

The effect of food limitation on immunity factors and disease resistance in the western tent caterpillar

“…An important characteristic of declining tent caterpillar populations is a reduction in the sizes of egg masses (Myers 2000;Cory and Myers 2009). The experimental reduction of food availability in our experiment caused the egg masses of food-restricted parents to be smaller than those of fully-fed parents, even though they received full-food in the 5th instar.…”

“…Early studies of Wellington (1960) highlighted the relationship of changing population density to variation in the condition and behavior of western tent caterpillars and their susceptibility to pathogens. In recent work, we observed a rapid increase in viral infection from essentially zero in one generation to high levels in the next in peak populations (Cory and Myers 2009). The cause of this sudden epizootic is unclear.…”

“…Population densities were beginning to increase in this area following the decline in 2004(Cory and Myers 2009). The eggs were kept outdoors for 1 month and then refrigerated at 4°C for another month before they were surface-sterilized with 2.5% sodium hypochlorite, rinsed with tap water, and transferred to individual paper cups at room temperature (approximately 22°C).…”

“…The M. c. pluviale NPV stock used in the bioassays originated from a Weld collection in 1999, and one of the aliquots prepared previously as described in Cory and Myers (2009) was used. For each treatment, three larvae from every family, a total of 30 larvae per dose, were exposed to a 0.8-cm-diameter alder leaf disc with 3 l virus or distilled water.…”

“…A good example of population cycles are those of western tent caterpillars, Malacosoma californicum pluviale, in the PaciWc Northwest of North America where, since 1936, outbreaks have been recorded to occur every 6-11 years (Myers 2000;Cory and Myers 2009). As with several other members of the families Lymantriidae and Lasiocampidae that demonstrate population cycles (review in Cory and Myers 2003), epizootics of nucleopolyhedrovirus (NPV) are associated with population declines of western tent caterpillars.…”