A dilute solution of potassium chloride and magnesium sulfate induces oviposition in artificial eggs by the egg parasitoids Trichogramma pretiosum and T. minutum. The ability to obtain large numbers of eggs through the use of this inexpensive solution is a major advance toward the development of diets and the large-scale production of Trichogramma spp. in vitro.

K +, Mg 2+, CI-, and 5042" were important components eliciting oviposition by Trichogramma pretiosum in aqueous solutions in wax eggs. Ca 2+ inhibited oviposition in a KCI-MgSO~ solution. The solutions most active in stimulating oviposition were 124.7--36.5 and 83.1--24.3 mM, respectively, of KCI-MgSO~.

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Three methods, which are more efficient than previously reported techniques, are described for in vitro production of Trichogramma pretiosum Riley in numbers ranging from a few to thousands. Several antibiotics with concentrations as high as 0.5% were not toxic to the parasitoids, reduced microbial growth, and permitted the successful rearing to the pupal stage of T. pretiosum on artificial diet composed of insect hemolymph, egg yolk, and milk. The likelihood that Trichogramma can be mass produced efficiently in vitro has been significantly increased, but it is essential that the cost and nutritional quality of the artificial diet be significantly improved.

Evidence for the role of host components in the growth and development of parasitoids i s briefly reviewed, and implications concerning the development of in vitro culture media are discussed. In vitro development of the entomophagous parasitoids Eucelataria bryani Sabrosky (Diptera: Tachinidae) and Trichograrnma pretiosum Riley (Hymenoptera: Trichogrammatidae) i s dependent on low-molecular-weight host chemicals. Asparagine, but not aspartic acid, and several other free amino acids are essential dietary ingredients for E. bryani. From insect hemolymph, small, unidentified polar molecules with characteristics similar to sugars are needed for pupation of T. pretiosum. Both adaptive and nonadaptive changes likely are responsible for the dependence of parasitoids on host factors. The genetic changes have no apparent adverse effect unless the parasitoids are fed artificial media devoid of certain chemicals that are obtained in vivo from host insects. The rearing of at least several species of important entomophages to control pest insects of great economic importance is dependent on the identification of constituents from host insects and inclusion of these chemicals in artificial diets.