Estimating the Impact ofLygus hesperuson Cotton: The Insect, Plant, and Human Observer as Sources of Variability

Abstract

Unexplained variation in the relationship between herbivore densities and the short-term appearance of crop damage is sometimes observed in pest management. Here we used a field survey of commercial cotton fields and a linked questionnaire for cooperating pest control advisors to document the existence of such unexplained variation in the impact of the western tarnished plant bug, Lygus hesperus, on upland cotton, Gossypium hirsutum. L. hesperus feeds preferentially on flower buds (“squares”), and the plant may respond to this damage with abscission of the square. We explored four classes of factors that might contribute to unexplained variation in square abscission. First, misperceptions by the human observer (i.e., sampling problems) may play a role, because commercial field scouts significantly underestimate densities of L. hesperus nymphs. Second, we found no support for the hypothesis that variable behavior expressed by L. hesperus contributes to unexplained variation in square abscission. L. hesperus seems to generate relatively predictable levels of square feeding damage; the variation that was observed was unrelated to grower categorization of fields as exhibiting normal versus unexpectedly high or low levels of square abscission. Third, variable plant responses to damage may instead be the key source of unexplained square abscission. Younger plants and plants with higher petiole phosphate concentrations expressed increased sensitivity to L. hesperus feeding; these correlations must, however, be tested experimentally before definitive conclusions are drawn. Fourth, another arthropod might be generating damage that was erroneously being attributed to L. hesperus. The omnivore Geocoris pallens was a candidate agent of cryptic damage to squares; however, an experiment showed that G. pallens generated only trivial square damage and no detectable increase in square abscission. Thus, this study has focused our attention away from the arthropod side of the interaction and toward the host plant as the primary source of greater than expected square abscission generated by L. hesperus.