Mortality of light brown apple moth egg masses in coastal California: Impact of resident Trichogramma parasitism and predation

Abstract

Egg mortality of light brown apple moth (LBAM), Epiphyas postvittana (Walker), was studied during the early years of its discovery and establishment in California from 2008 to 11. Sentinel egg cards were used to quantify mortality by parasitoids and predators at an intensively monitored location in Santa Cruz to reveal seasonality, and at other sites in Santa Cruz and San Francisco to measure spatial variation and parasitoid species composition. In addition to egg mortality, egg mass use efficiency by Trichogramma parasitoids was also examined under field conditions. The egg parasitoids primarily consisted of three Trichogramma species, T. deion, T. fasciatum and T. platneri. Previously unknown to California, T. fasciatum was shown to displace the indigenous T. platneri over a two-year period as the predominant egg parasitoid attacking LBAM egg masses. Based on the number of parasitized eggs per sentinel egg card, it is argued that individual females of at least some Trichogramma species have the capacity to oviposit more than 50 eggs in the egg mass of a host, warranting further studies to document the egg mass use efficiency and oviposition traits of other trichogrammatids under field conditions. Although egg parasitism was limited to causing elevated mortality during the summer and fall only, egg predation occurred more consistently throughout the year and is likely to be of considerable significance in suppressing LBAM populations in California. Combined with the use of a Type II survivorship curve, the importance of using a temperature-dependent physiological time scale to quantify egg mortality is illustrated when sentinel hosts are exposed to natural enemies in the field for less than the entire duration of the ‘window of susceptibility’ to parasitism and predation.