Parasitoids, Patches, and Phenology: Their Possible Role in the Local Extinction of a Cynipid Gall Wasp Population

Abstract

The cynipid gall wasp Xanthoteras politum (Bassett) and its associated chalcid and inquiline parasitoid community were studied from 1976 to 1978 at a postfire site near Lake Oswego in the New Jersey Pine Barrens. X. politum and two other cynipid species selectively attacked new sucker shoots growing from larger fire—damaged trees. Chalcid and inquiline parasitoid attack was the major mortality factor in the population in all 3 yr. In each successive year a higher frequency of the host population was killed by parasitoid attack, and by 1978 on 76 host galls occurred at the site. Inquiline cynpids were minor parasitoid elements the 1st yr of the gall outbreak, but dominated the parasitoid guild in subsequent seasons. Their dominance is attributed to two major factors. First, they attack galls earlier than other parasitoids, and those galls attacked remained at a smaller size and were not chosen for oviposition by later arriving chalcids. Secondly, inquilines did not emerge in spring and infest other gall species as did the chalcid parasitoids. These inquilines remained in the leaf litter and reinfested the next asexual generation of X. politum which appeared on trees in mid—July. The parastioid community displayed neither strong stability nor consistent composition. The apparent overexploitation of the host population by parasitoids may be to their selective advantage as phenological changes in host plants eventually eliminate X. politum from postfire sites anyway.