Conservation biological control in urban landscapes: Manipulating parasitoids of bagworm (Lepidoptera: Psychidae) with flowering forbs

We conducted field experiments to test the hypothesis that regulation of herbivorous pests in urban landscapes can be enhanced with forbs that provide floral resources for adult natural enemies. The herbivore was bagworm, Thyridopteryx ephemeraeformis (Haworth) (Lepidoptera: Psychidae), which is attacked by a guild of hymenopterous parasitoids. We established host shrubs in common garden plantings, encircling some with flowering forbs, released bagworm larvae on shrubs, and assessed survivorship. The hypothesis was supported: parasitism rates of bagworm were 71% higher in shrubs that were surrounded by flowering forbs than in shrubs that lacked flowers. This study also suggested that white-footed mice and European sparrows were important predators of bagworms. The most abundant parasitoid species was the exotic ichneumonid Pimpla (=Coccygominus) disparis (Vierick), an introduced biological control agent of gypsy moth. In a second experiment, parasitism rates were at least three times higher in shrubs encircled by a high density of forbs compared to those having fewer or no forbs. In a third experiment, parasitism rates exceeded 70% in shrubs that were adjacent to a central bed of flowering forbs, but less than 40% in shrubs that were farther away. We conclude from these studies that flowering forbs have a localized effect on host-searching behavior of female parasitoids, encouraging them to parasitize bagworms in the immediate vicinity. This study provides further evidence that ecological methods of pest management can be integrated into the design of urban landscapes to improve regulation of herbivorous pests.