The Circumstances of Species Replacement Among Parasitic Hymenoptera

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Abstract
Species replacement is most commonly observed among the parasitic Hymenoptera because of their dominance in biological control. Such replacement is necessarily a reproduction phenomenon involving interspecific competition in population-saturated areas. The competitive interactions of parasitic populations are differentiated into two types, “intrinsic” and “extrinsic”, with characteristic conditions and mechanisms. Intrinsic competition consists of interference between individuals of the interacting populations, being manifested by such factors as cannibalism, mutilation, starvation, suffocation, production of toxic or repellent excretions and secretions, and the differential susceptibility of individual hosts to parasitic attack. Extrinsic competition concerns the interaction of populations as groups and is manifested largely by host-habitat finding, host-finding, and host acceptance. Replacement through intrinsic competition takes place regardless of host abundance; replacement by extrinsic competition is an effect of decreased host abundance.