Experimental Studies in Insect Parasitism

Abstract

Trichogramma evanescens is able to distinguish healthy from parasitised hosts, and when few hosts are available can restrain itself for 8 hours to the deposition of 5 per cent. of its available eggs. As far as is yet known, the discriminative ability is perfect; but the restraint is limited and, in a longer period of time or when several parasites are together, it breaks down and superparasitism occurs. When the superparasitism is slight and only two or three eggs are laid in a host, the competition between the parasite progeny leads to the victory and emer gence of one; but when it is severe either a dwarfed or imperfectly developed individual or none at all emerges. As the density of parasites in a fixed population of hosts is increased, more and more superparasitism occurs, and the following effects on the populations are observed: (1) The number of hosts that escape steadily decreases; but even at high densities of parasites some hosts occasionally escape. (2) The number of hosts that die without yielding either hosts or parasites steadily increases. (3) The number of parasite progeny reaches a maximum and then decreases. (4) The number of progeny of the individual parasites steadily decreases. (5} An increasing proportion of the emergents are imperfectly developed. (6) An increasing proportion of the emergents are males.