The relation of wild parsnip, Pastinaca sativa L., to parasitoid populations associated with soybean pests in central Iowa
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The effect of wild parsnip on parasitoids of soybean pests in central Iowa was examined in three components: assessment of the possible impact of parsnip on nearby crop-pest populations, confirmation of parasitoid visits at parsnip flowers, and assessment of the possible contribution of parsnip flowers to parasitoid longevity. Larval Lepidoptera in 7 soybean fields with parsnip borders and 4 fields without parsnip were sampled during two growing seasons. Green cloverworms (GCW), Plathypena scabra (F.), were 99% of the caterpillars present in soybeans. GCW populations were 10-50% higher in soybeans without nearby parsnip than in parsnip-bordered soybeans, but statistical anlyses showed no signficant differences between the numbers of insects in the 2 field types. Presence of parsnip had no significant effect on lepidopteran parasitization rates by the parasitoids tested;Yellow sticky traps caught many families of parasitoids in 7 parsnip plots near or distant from soybeans. Most identifiable Diptera were Tachinidae; 7 species were known GCW parasitoids. More of these species were trapped in the GCW outbreak year. More species, and generally larger numbers, of beneficial Diptera were trapped in soybean-associated wild parsnip than in parsnip alone, but the proximity of soybeans did not significantly affect trap catches of Diptera;More parasitic Hymenoptera were caught in soybean-associated wild parsnip. Five species of braconid GCW parasitoids were trapped in very low numbers, with no significant differences in numbers caught between plot types. Rogas nolophanae Ashmead, the braconid most commonly reared from GCW, was nearly absent on traps. Among the 14 subfamilies of Ichneumonidae trapped, only Vulgichneumon brevicinctor (Say) was known to attack GCW. More than 20 families of Hymenoptera-Parasitica (excluding Ichneumonoidea) were caught, but no species was a known GCW parasitoid. None of the Diptera and Hymenoptera parasitic on GCW differed significantly in abundance between types of parsnip plots;Live, reared adult GCW parasitoids were held in the laboratory in the presence or absence of wild parsnip flowers. Parsnip presence failed to significantly enhance survivorship in any species tested;These combined results suggest that wild parsnip fulfills a minor role in maintaining GCW parasitoids; thus, weed management practices directed at parsnip near soybeans should not have a significant impact on natural control of GCW populations exerted by parasitoids.