Influence of oviposition strategy of Nemorilla pyste and Nilea erecta (Diptera: Tachinidae) on parasitoid fertility and host mortality

Abstract

This study examined fertility of the tachinid (Diptera) parasitoids Nemorilla pyste (Walker) and Nilea erecta (Coquillett), and mortality of the host, Choristoneura rosaceana (Harris) (Lepidoptera: Tortricidae). These are common parasitoids of mature C. rosaceana larvae in apple and sweet cherry orchards in central Washington where C. rosaceana is an important pest. The parasitoids have similar phenology and use the same stages of the host, but their modes of attack and development in the host differ. N. pyste oviposits on the host and develops gregariously, while N. erecta ovolarviposits on foliage near the host and develops solitarily. Life tables were used to compare adult longevity and fertility of these flies at three different host densities in laboratory cage experiments. Results suggested that although population growth rates of these species were similar under the experimental conditions, N. erecta had a greater effect on mortality of the host population. This additional mortality was attributed to superparasitism because it occurred in host larvae at a higher rate than expected from natural causes, and it occurred more frequently when fewer hosts were available to N. erecta females. A supplemental experiment demonstrated that host larvae that were manually infected with N. erecta maggots to simulate superparasitism expressed high rates of larval mortality. Thus, even though N. erecta had a greater effect on the host population, reproductive opportunity was lost to superparasitism. These findings may demonstrate certain situational disadvantages of ovolarviposition as a reproductive strategy, and illustrate how parasitism effects may be underestimated in field sampling.