Interspecific competition among insect parasitoids: Field experiments with whiteflies as hosts in cotton
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Manipulative field experiments assessing the importance of interspecific competition on the dynamics of parasitoid populations and the impact of multiple parasitoids on host populations are virtually absent from the ecological literature. We report findings from such experiments assessing competitive interactions among three species of parasitoids (Hymenoptera: Aphelinidae) of the silverleaf whitefly Bemisia argentifolii Bellows & Perring (Homoptera: Aleyrodidae), a pestiferous species of worldwide importance. The parasitoids used in the study (Encarsia pergandiella Howard, Eretmocerus mundus Mercet, and Encarsia formosa Gahan) encompass the range of life histories found within Aphelinidae, and they exhibit a high degree of niche overlap. The objectives of our studies were (1) to investigate the occurrence of interspecific competition among the whitefly parasitoids, (2) to quantify shifts in resource utilization patterns as influenced by experimental manipulations of the parasitoid assemblage, and (3) to assess the impact of interspecific competition on the ability of the parasitoids to suppress their host populations. Field cages enclosing cotton plants were inoculated with whitefly by releasing adults at either a high or a low rate followed by releases of all possible combinations of the parasitoid species (including a no-parasitoid release control). The treatments were replicated three and four times each in 1996 and 1997. The presence of interspecific competitors reduced population growth rates of E. formosa and E. pergandiella but not the population growth rate of E. mundus. The spatial distribution of parasitism by E. pergandiella along the vertical axis of cotton plants differed in the multiple species treatments compared to the single species treatments. Changes in parasitoid activity in the presence of competitors may reduce potentially adverse effects of competition and promote species coexistence. Only releases of E. formosa together with E. pergandiella resulted in lower levels of host mortality than would be expected based on observed mortality by individual species. Our results demonstrate competitive interactions among three parasitoids of B. argentifolii under field manipulations but indicate that, in most cases, competitive interactions among parasitoids do not affect host population suppression.
