Analysis of Resistance in Cabbage Varieties to Damage by Lepidoptera and Thysanoptera

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Abstract
Differences in susceptibility to damage by Thrips tabaci Lindeman and the lepidopteran pest complex (Plutella xylostella (L.), Artogeia rapae (L.), and Trichoplusia ni (Hübner)) were documented and analyzed for fresh market and experimental cabbage varieties. Patterns of resistance to damage were different for Lepidoptera and T. tabaci. The commercial green varieties did not differ in susceptibility to damage by Lepidoptera but did differ in susceptibility to damage by T. tabaci. The two experimental varieties and one commercial red variety, used for comparison, were less susceptible to damage by Lepidoptera but varied in susceptibility to damage by T. tabaci. The best regression models for predicting damage rating, without including the effect of variety, included one measure of seasonal pest total and one plant parameter, head weight. Further regression analysis showed that, in most cases, the relationship of seasonal pest total to damage had the same slope for all varieties but different intercepts. This result indicates that, at a given population density, the models predict different damage ratings for the varieties, implying that nonpreference is not the only resistance mechanism operating and that mechanisms involving tolerance or antibiosis were involved. Use of regression models that test for varietal differences in the relationship of plant parameters and pest population densities to damage would be helpful in designing good assays of resistance for selection of new breeding lines.

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