Field Estimation of the Functional Response of Itoplectis behrensii1 , a Parasite of the California Oakworm, Phryganidia californica2

Abstract

The functional response of ltoplectis behrensii (Cresson), an epidemic pupal parasite of Phryganidia californica Packard, was estimated from field data involving various host densities on the trunks of coast live oak, Quercus agrifolia Nee. Pupal hosts were detected as uniformly distributed on the trunk surface at high host densities using a nearest neighbor index of dispersion. The number of parasitized pupae were recorded at 3-4 day intervals throughout host pupation for 4 consecutive P. californica generations. A second degree polynomial curve was fitted to observed % parasitization at different host densities at 14 day intervals. Most generated curves were significant (at 5% level) to observed data. Minimum thresholds of parasite response were between 1 and 2 pupae/m2 of trunk surface. Optimum host densities ranged between 6.6-13.2 pupae/m2. Maximum parasitization ranged between 20.3 and 55.2%. Host densities beyond the optium range showed a decreasing rate of parasitization indicating inverse density dependence. Based on the estimated functional response, I. behrensii is more efficient at lower host densities between the minimum threshold and the optimim range.