Interactions between native predators, an introduced parasitoid, and a native parasitoid in the population regulation of an invasive geometrid
A tachinid parasitoid, Cyzenis albicans, has been introduced to the northeast United States to control the outbreak of winter moth (Operophtera brumata). A large change in winter moth densities with only a relatively small percent parasitism (35%) was documented at one of the release sites. Further, in Nova Scotia in the 1950s and British Columbia in the 1970s, predation of winter moth pupae increased following parasitoid establishment. It appears that mortality from parasitism and predation is greater than if the two factors were acting independently. We tested three hypotheses proposed by Roland (1990) as to why this may be: (1) Mortality due to parasitoids is enough to bring the winter moth population down to a level that can be effectively controlled by generalist ground predators, (2) Parasitized pupae experience lower mortality rates than unparasitized pupae, (3) The presence of C. albicans induces a numerical response in generalist ground predators because C. albicans is available in the spring. To test our three hypotheses, we (i) analyzed cumulative pupae mortality against the corresponding winter moth pupae density, (ii) compared mortality rates between parasitized and unparasitized pupae and (iii) analyzed each site’s community composition from pitfall traps collections.
Our results did not support our hypotheses, but the study uncovered a fourth possible source of the apparent synergistic relationship. There was (i) no evident predator saturation threshold (linear fit: R2 = 0.10, p = 0.43, quadratic fit: R2 = 0.25, p = 049), (ii) only an early season difference between parasitized and unparasitzed pupae (June and July but not August or September), and (iii) limited support of a predator numerical or functional response to the presence of C. albicans (predator numbers to pupae density: p-values > 0.05; Carabid number to pupae mortality: p = 0.007). However we found a native Ichneumonid wasp (Pimpla aequalis) parasitizing deployed pupae (1-33%) in areas where winter moth is well established. We suggest that the aggregation of this wasp after winter moth establishment may be responsible for the apparent synergistic effect. Further studies will be conducted this coming season (2015) to evaluate parasitism rate across a winter moth density and establishment gradient. Locally, these findings have implications to the biological control approach in regulating winter moth population outbreak. From a broader prospective, these findings will build on the theories of population cyclicity, showing that generalist predators and parasitoids can influence outbreak regulation.