Parasite‐Induced Alterations in Host Reproduction and Survival

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Abstract
The effects of a parasitic water mite, Hydryphantes tenuabilis, on the reproduction and survival of its aquatic insect host, Hydrometra myrae, were determined through laboratory rearing experiments. The parasite attaches to any of the five immature instars of the host but uses these stages only as an indirect route to the parasitism of the adult, the only stage from which the parasite extracts appreciable amounts of nutrient. Accordingly, the parasite had no effect on the immature stages of the host but changed the reproductive and survival characteristics of adult ♀ ♀. Highly significant differences between parasitized and unparasitized female hosts were shown in survivorship and age at which oviposition began. Once oviposition began, formerly parasitized ♀ ♀ laid as many egges as formerly unparasitized ♀ ♀ in the earlier segments of their respective m(x) distributions. The results suggest that parasitic water mites in loads observed in natural populations should reduce the rate of population increase of the host. Approximate values of the instantaneous rates of increase of each experimental and control host group were calculated, and the rates of the control (unparasitized) groups were found to be higher at all food levels offered to the host. Adjusting the rates of increase by assuming that survivorship was perfect for all hosts and that oviposition of experimental hosts began on the same day as the controls produced a high level of convergence in the rates of increase of all host groups within any food level and emphasized the importance of parasite—induced alterations in survivorship and age of first oviposition in determining rates of population growing of the host.

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