Field Crops News

Resistant Sorghum Cultivars Affect Convergent Lady Beetle

It is often assumed that plant resistance to insects and biological control are two compatible nonchemical controls that can be used in integrated pest management systems. This assumption was tested with grain sorghum, greenbugs and convergent lady beetles. Greenbugs were reared on 3 grain sorghum hybrids (2 resistant; Dekalb DK-41Y, DK-59E, and 1 susceptible; Horizon 101G) and then fed to convergent lady beetle for all of their larval development. Lady beetles fed on greenbugs from resistant sorghums took significantly longer to complete larval development (although this difference was only about 0.1 day), and had significantly lower survival (5-21% lower, dependent on hybrid). Female convergent lady beetles weighed significantly less when fed greenbugs from one resistant sorghum hybrid, but not when fed on another. These studies point to the need to carefully evaluate interactions between control practices. Benefits of resistant sorghums include decreasing greenbug survival and reproduction and increasing the development period of greenbugs, which must be balanced with the negative effects on convergent lady beetles documented in this study.

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Trichogramma nubilalis Best for European Corn Borer Control

Losey & Calvin (1995) studied four commercially available species of Trichogramma (T. pretiosum, T. platneri, T. minutum and an undescribed Trichogramma species from Georgia) in comparison with T. nubilalis to determine their efficacy in parasitizing European corn borer eggs. T. nubilalis is known to be an effective parasitoid of European corn borer, but is not available commercially. In 24 hour laboratory tests, only 5 and 10% of T. pretiosum and T. sp. (Georgia), respectively, laid eggs in corn borer eggs. T. minutum and T. platneri did not parasitize any corn borer eggs. In contrast, 58% of T. nubilalis females parasitized at least a portion of egg mass to which they were exposed, and parasitized the highest proportion (66%) of eggs in each mass of any species. Although T. nubilalis is more expensive to rear, it is a better candidate for control of corn borers than the other species tested. In fact, the performance of the commercial species tested was so poor that the advisability of purchasing them at any price to control corn borers is questionable.

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