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Effects of a nuclear polyhedrosis virus of the western tent caterpillar on individual performance and population dynamics Rothman, Lorne David


Viral diseases are an important feature of fluctuating populations of Lepidoptera. I examined the potential for a nuclear polyhedrosis virus (NPV) to explain the changes in abundance and fecundity observed in populations of the western tent caterpillar, Malacosoma californicum pluviale. Fluctuating populations are characterized by prolonged declines following high density. In M. c. pluviale, decreases in fecundity track these declines. While viral diseases of Lepidoptera are usually recognized for their ability to kill infected hosts, they may also reduce the fitness of individuals which survive infection. I surveyed studies in the literature which evaluated qualitative characteristics of individuals following treatment with virus. Debilitating effects of viral diseases of Lepidoptera included slower development rates, lower pupal and adult weights, reduced reproductive capacity and shorter adult longevity. These sublethal effects were observed more frequently in studies of less virulent pathogens (cytoplasmic polyhedrosis viruses or CPVs) than in studies of NPVs. Debilitating effects of both CPVs and NPVs could potentially suppress host population growth. In the laboratory, I treated M. c. pluviale larvae with NPV to assess whether viral infection could potentially reduce fecundity as observed in declining field populations of the host. Weights of male and female pupae and female fecundity were reduced in survivors of virus treatment, which suggests that NPV could reduce fecundity of field populations of the western tent caterpillar. In small-scale field and related laboratory experiments, I examined immediate and delayed effects of NPV introduction and density on M. c. pluviale. In a factorial experiment, larvae at high density showed increased feeding and development rates and decreased reproductive potential. The introduction of NPV significantly increased mortality of the host, particularly at high density, and generally reduced host reproductive potential. Adults from this experiment were mated and their offspring reared in the laboratory. No treatment effects on egg viability or larval performance occurred. Treatments did have significant delayed effects on pupal weights of female offspring. However, in a correlational field study maternal fecundity was not related to pupal weights of offspring. M. c. pluviale introduced to host trees used in the initial factorial experiment showed no effects of previous caterpillar density on performance as predicted by host plant induction theory. However an interactive effect of previous virus introduction and density on mortality was observed owing to persistence of NPV particles in the environment. Female pupal weights were also slightly reduced in the year following NPV introduction. I assessed the potential for NPV and density to reduce population growth by calculating net reproductive rates for treatments from the above field experiments. The introduction of NPV had a large immediate impact on population growth, particularly at high host density. Delayed effects of NPV introduction at high density were insufficient to prolong this decline, but did appreciably suppress predicted population growth. Treatment effects on individual quality (fecundity) had little effect on predicted population change. Because delayed density dependence may lead to population instability, I concluded that viral disease may have a more destabilizing influence on tent caterpillar populations than density alone. Persistence of virus particles in the environment could contribute to, but not explain prolonged declines in field populations of M. c. pluviale. High density may be sufficient to initiate decreases in fecundity in these populations while viral disease may explain continued fecundity decreases during prolonged declines.

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