Trichogramma pretiosum is a naturally occurring wasp parasite of southwestern corn borer eggs in Kansas corn fields. High numbers of southwestern corn borer eggs are parasitized during late summer, but overall Trichogramma parasitism has a limited effect on the borers. This suggests that augmentation biological control might be useful to increase parasitism earlier in the season. Proper timing of releases is dependent on a good understanding of the biology of Trichogramma parasitism of southwestern corn borer eggs. Laboratory studies were conducted to determine the effect of southwestern corn borer egg age on parasitism by T. pretiosum. Four types of eggs were studied; fertile and infertile fresh (<24 h old) eggs, red-bar stage eggs (48-72 h old) and black-head stage eggs (96-120 h old). There was no difference in preference between fertile and infertile fresh eggs, and wasps were equally able to develop in either egg type. Eggs in the black-head stage were least frequently parasitized by Trichogramma wasps. This nonpreference was reduced when wasps were tested in the dark, suggesting that visual cues contributed to the wasp's ability to discriminate between egg stages. Wasp eggs laid in black-head stage southwestern corn borer eggs were able to develop normally, and the resulting wasps were equal in size, fecundity and sex ratio to those developing in younger eggs.
Source: D. D. Calvin, J. E. Losey, M. C. Knapp and F. L. Poston. 1997. Oviposition and development of Trichogramma pretiosum in three age classes of southwestern corn borer eggs. Environ. Entomol. 26: 385-390.
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