Foraging Strategies, Population Models and Biological Control: A Case Study

Abstract

The interaction between the winter moth [Operophtera brumata (L.)] and one of its parasitoids, Cyzenis albicans (Fall.), is examined from several viewpoints using data collected at Wytham Wood, Berkshire, [UK] between 1950 and 1968 and in Nova Scotia in Canada. Cyzenis at Wytham Wood causes a higher percentage parasitism on trees with high winter moth density than on low density trees. This density dependent response to host distribution is marked in years of low overall winter moth abundance, parasitism becoming more evenly distributed when winter moth are plentiful. This general trend is predicted by 3 foraging models when applied to a comparable hypothetical situation. The frequency distribution of Cyzenis larvae within winter moth hosts is described by the negative binomial distribution, with the estimated values of k in different years being correlated with the overall winter moth abundance per year. This is in accord with the observed pattern of responses that change with host abundance. A population model based on that of May (1978), but with k from the negative binomial now a function of host density as observed for Cyzenis, is analyzed and the stability properties displayed. This general model for parasitism is included in a more detailed model for the winter moth and Cyzenis at Wytham Wood. It shows Cyzenis to play a minor part in the winter moth population dynamics, which is largely governed by the destabilizing effects of the key-factor and the stabilizing effects of the density dependent soil mortality. A model for the winter moth and Cyzenis in Nova Scotia is developed. Cyzenis is assumed to search in the manner found for Wytham Wood, but the other components are derived from the studies of Embree (1965, 1966). The model predicts the successful biological control of the winter moth observed in Nova Scotia following the introduction of Cyzenis. The reasons for the different performances of Cyzenis at Wytham Wood and Nova Scotia are discussed.