Mutual Interference between Searching Insect Parasites

The theory of Nicholson (1933) and Nicholson & Bailey (1935) assumes that the searching efficiency of a parasite as measured by its 'area of discovery' is a constant and thus independent of both host and parasite density. Major drawbacks to this simple hostparasite model are (1) the instability of the interactions and (2) that, even if stabilized with an additional density-dependent mortality acting on the host, it is not possible for two or more specific parasites to coexist on the same host species (Varley & Gradwell 1970). Hassell & Varley (1969) have recently proposed a new population model for insect parasites in which the searching efficiency of the adult parasites declines exponentially as the density of searching adults increases. They base this model on the published experimental data for five species of parasites all of which show such a relationship. Fig. 1 reproduces these results and includes an additional relationship for Pseudeucoila bochei Weld. (Cynipidae) parasitizing Drosophila melanogaster Meig. larvae. The range of searching parasite densities in each of these experiments is certainly much higher than one would expect under natural conditions. However, in two of them (a and e) the lowest parasite densities are below ten/m2, which is a density exceeded by some parasites in the field. The model of Hassell & Varley is derived from the linear equations describing the data in Fig. 1: log a = log Q-m logp (la)