Will the Outbreak of Black Oak Gall Wasp be Suppressed by Natural Enemies?

The recently described black oak gall wasp, Zapatella davisae Buffington, a stem galling wasp, has caused extensive damage and mortality to black oak trees, Quercus veluntia, of coastal Massachusetts, USA. Natural enemies in the form of parasitoid wasps exist to regulate outbreaking populations of gall wasp in native habitats; however, parasitoids have not yet established enough in novel habitats to cause sufficient mortality to Z. davisae and thus do not prevent gall wasps from attaining outbreak densities. The goals of this study were to determine present year parasitism rates of Z. davisae, and to collate historical gall wasp densities to determine if a top-down relationship between gall wasps and parasitoids exists. Our hypothesis was that parasitoids serve as top-down pressures and function to control populations of Z. davisae. Present year parasitism rates were obtained through an emergence census of gall wasp communities from sites on Nantucket, Martha’s Vineyard, and Cape Cod, Massachusetts, USA. Historical gall wasp densities were obtained through dissection of branches from the same sites. Percent parasitism rates were determined to be 28.6%, 90%, and 68.8% on Nantucket, Martha’s Vineyard, and Cape Cod respectively. The trend of historical densities is consistently shown as an increase in population size followed by a crash, likely facilitated by parasitoids. Z. davisae populations in Nantucket have not yet experienced a population crash however, based on data from Martha’s Vineyard and Cape Cod, outbreaking populations of Z. davisae are likely to crash in coming years.