Experimental Studies in Insect Parasitism

Abstract

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 emergence 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 : The number of hosts that escape steadily decreases; but even at high densities of parasites some hosts occasionally escape.The number of hosts that die without yielding either hosts or parasites steadily increases.The number of parasite progeny reaches a maximum and then decreases.The number of progeny of the individual parasites steadily decreases.An increasing proportion of the emergents are imperfectly developed.An increasing proportion of the emergents are males.