lnundative Release of Trichogramma for the Control of Cruciferous Lepidoptera: Preintroductory Selection of an Effective Parasitoid

Egg parasitoids of the genus Trichogramma are the most widely used natural enemy for biological pest control in the world. Their use is restricted to lepidopterous pest species. Due to relatively cheap mass rearing systems employing factitious host species, Trichogramma are usually applied for inundative biological control: repeated releases of high numbers of parasitoids aiming at a direct control effect in the pest population. This paper discusses one of the central issues of biological control: the selection of a candidate natural enemy. Studies of the parasitization and searching behavior of Trichogramma have shown that interand intra-specific variability occurs for most characteristics determining their effectiveness as a control agent. A preintroduction selection procedure comparing different Trichogramma strains or species may then be useful or even necessary. This procedure requires a set of evaluation criteria and bioassays to quantify these. lnundative Biological Control Inundative biological control aims at directly increasing mortality in the pest population, whereby the released natural enemy is used as a biological insecticide (Stinner 1977). The practice of inundative release therefore centers on its cost/benefit ratio as compared to alternatives, especially pesticides. Species of the hymenopterous genus Trichogramma have been used more than any other natural enemy for inundative biological control (Ridgway and Morrison 1985). Trichogramma has a wide range of hosts, especially among the Lepidoptera (Nagarkatti and Nagaraja 1977). Suitable factitious hosts include the eggs of lepidopterous storage pests of grains, making mass production economically feasible. Experiments with Trichogramma started in the beginning of this century in the USA and the USSR. Development of a method for mass production of Trichogramma on eggs of the Angoumois grain moth Sitotroga cerealella gave rise to worldwide use of the parasitoid as a control agent. In a paper with the meaningful title ‘Is Trichogramma becoming a fad?’, Smith and Flanders (193 1) expressed their concern about the usefulness of these applications, on the grounds of a lack of convincing evidence for practical results. Because of this, and the advent of organic chemical pesticides, interest in Trichogramma faded in western countries (Ridgway et al. 1977). Research on the use of Trichogramma continued in the USSR and China, probably due to a lack of pesticides, rather than biological or economical effectiveness of Trichogramma release (Van Lenteren 1987). In these two countries application is presently carried out on several millions of hectares. Increasing problems with the use of pesticides has stimulated renewed interest in biological control, whether as a method by itself or as part of an integrated pest control system. For