The Effect of Multiparasitism on Populations of Two Parasites and Their Host

Abstract

In an experimental study of multiparasitism attention has been paid primarily to its effects on individual hosts and to the determination of the factors which control the result of competition. It has been established that the time of oviposition is of prime importance, but that the rate of preimaginal development, which in turn is dependent upon the species of host attacked and the environmental temperature, is an important modifying factor (Fisher 1959, 1961b). The ichneumonid parasites used in this study were Horogenes chrysostictos Gmelin and Nemeritis (Idechthis) canescens Grav. Both are natural parasites of larvae of the moth Ephestia sericariumi Scott (= E. kulhniella Zeller) (Lepidoptera: Phycitidae). On simultaneous oviposition in this host by both parasite species Horogenes wins in competition because the duration of its egg stage is shorter than that of its opponent, Neweritis and it is thus able to attack the latter with the well developed mandibles of the first instar larva. But since simultaneous oviposition is unlikely to occur in the field and the difference in the duration of egg development in Ephestia is relatively small (10 ? 2 hr at 250C), for practical purposes neither species has an intrinsic advantage and the result of multiparasitism is determined by the time of oviposition. Any extrinsic superiority (sensu Smith 1929) of either species therefore depends upon the fecundity and maximum population density attained by each. Although the fecundity of the 2 species is approximately the same (Fisher 1959), the potential capacity for increase is very much greater for Aremeritis since it is a wholly parthenogenetic species. In consequence this parasite has an additional intrinsic advantage over Horrogeucs which, in natural populations, should far outweigh any superiority which the latter could show in multiparasitism. Since Flanders (1948) has shown that populations of Ephestia and Nemeritis can exist indefinitely on a continuously available source of food for the host, it seemed possible to set up experimental ecosystems consisting of Ephestia, Nemeritis and Horogenes to study the effect of multiparasitism on the population densities of all 3 species. The method used was adapted from that described by Flanders (1958). Each ecosystem consisted of a rectangular w-ood cage of internal dimensions 16 X 16 X 10 inches, with nylon gauze side panels, a glass top and a wooden floor. The floor was divided into 25 squares, each of about 10 sq. inches in area. The basis food of Ephestia, equal parts of sterilized rolled oats and whole wheat flour, was contained in 10 g lots in small waxed paper cups. One cup of food was added each week to a square selected at random until all the squares were filled. On the 26th week the first cup, then exhausted of food, was replaced by a newly filled one and the continuation of this procedure ensured constantly enduring host populations with larvae at every stage of development.