A mathematical analysis of superparasitism by Collyria calcitrator Grav

Abstract

1. An outline of the life history of Cephus pygmaeus Linnaeus is given.2. The parasites of Cephus are listed and are discussed briefly in relation to the particular parasite Collyria calcitrator Gravenhorst.3. It is shown that if the host larvae collected from wheat-stubble in autumn are used for the investigation of the extent of parasitism by Collyria, allowance must be made for the fact that the parasite Pleurotropis benefica Gahan is intrinsically inferior to Collyria, and that its cocoons must represent hosts left unparasitized by Collyria.4. In summer the adults of Collyria emerge, on the whole, earlier than those of the host Cephus. This suggests that at the beginning of the season of oviposition there will be a scarcity of hosts in comparison with the numbers of parasites wishing to oviposit. The relative activity of host and parasite during the egg-laying period is worked out on hypotheses of the average length of life of the individuals, and is shown diagrammatically.5. The Cephus larvae travel down the stem and hibernate at the base. When the wheat is cut, a proportion of the larvae and their parasites are removed with the straw. The way in which this may prevent the stubble material from giving a true indication of the degree of parasitism in the population as a whole is discussed.6. On the basis that 3910 hosts receive 3600 parasites, various hypothetical distributions are worked out arithmetically in order to find which type of discrimination between healthy and parasitized hosts gives an arrangement of parasites most in agreement with what has been found for Collyria in Cephus. The most satisfactory result is that obtained when it is assumed that the probability of an already parasitized host being re-attacked varies with the proportion of parasitized hosts. It is also shown that a shortage of hosts at a time of parasite activity may cause a high degree of superparasitism, thus giving a false impression of the parasite's power of discrimination.7. Suggestions for further field work are given.