Four commercially available species of Trichogramma, T. pretiosum Riley, T. platneri Nagarkatti, T. minutum Riley, and Trichogramma sp. (an undescribed species collected in Georgia), were monitored over time to assess the number of emerging adults. The number of females yielded per unit stock was substantially lower for all species tested than the number listed by the producer. The number of viable females per unit stock varied significantly across dates and species. A new method of monitoring the number of parasitoids per unit stock is described. The 4 commercially available Trichogmmma species were evaluated in the laboratory to determine their efficacy in parasitizing Ostrinia nubilalis (Hübner) eggs. The efficacy of these species was compared with that of Trichograrnma nubilale Ertle & Davis, which is known to be an effective parasitoid of O. nuhilalis but which is not commercially available. Only 5 and 10% of T. pretiosum and T. sp. (Georgia) females, respectively, oviposited in O. nubilalis eggs, and T. minutum and T. platneri females did not parasitize any O. nubilalis eggs. In contrast, 58% of the T. nubilale females tested parasitized at least a portion of the exposed egg mass. T. nubilalc females also parasitized a higher proportion (66%) of eggs in each mass.
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Losey et al. (1995) studied this question.