Assessment of Potential Methods of Measuring Parasitism by Brachymeria intermedia (Nees) (Hymenoptera: Chalcididae) of Pupae of the Gypsy Moth

Abstract

This study was conducted to assess potential biases associated with three methods of calculating percentage parasitism of gypsy moth, Lymanttia dispar (L.), pupae by Brachymeria intermedia(Nees). We determined the timing of four processes: (1) recruitment of gypsy moth (host) larvae to the pupal stage, (2) recruitment (oviposition) of B. intermedia to its immature stage, (3) losses of hosts from the pupal stage because of mortality or adult emergence, and (4) losses of immature B. intermedia because of mortality or adult emergence. B. intermedia emerged after most unparasitized pupae had died or after moths had emerged. As a result, the proportion of pupae in samples that were parasitized increased with time. Sample percentage parasitism peaked late in the season and overestimated levels of stage-specific parasitism. Parasitism extended the residence time of parasitized pupae, with the result that they were available to predators for a longer period of time than un parasitized pupae and suffered a higher level of predation. Examination of pupal exuviae at the end of the season, therefore, underestimated the effect of B. intermedia. No biases were observed that affected estimation of percentage parasitism through the direct measurement of host and parasitoid recruitment. Of the three methods studied (percentage parasitism in samples, examination of exuviae, and measurement of recruitment), direct measurement of host and parasitoid recruitment appears to be the best method for measuring the effect of B. intermedia on L. dispar.