Early-season colonization patterns and biological aspects of natural mortality of the boll weevil (Coleoptera:Curculionidae) in central Texas cotton (Malvales:Malvaceae)

Abstract

Border sprays have been suggested as an efficient alternative to whole-field insecticide applications for control of early-season infestations of the boll weevil, Anthonomus grandis grandis Boheman, in cotton, Gossypium hirsutum L. However, a review of the literature failed to find data to support this tactic. Early-season colonization patterns of the boll weevil in cotton during 1999 and 2000 in Brazos Valley, Texas, were examined. Colonization patterns were determined on the basis of collections of adults and fallen infested squares. Adult boll weevils and fallen infested squares were not consistently significantly more abundant at field margins. Based on these findings, total reliance on border sprays for early-season boll weevil management may not be advisable. Starvation of immature boll weevils resulting from square desiccation is considered a major determinant of natural mortality. However, the critical weight required for larvae to complete development without further feeding has not been determined. Critical weights of second and third instars were investigated using food-removal techniques. Second instars examined ranged in weight from 0.23 to 5.55 mg. About 78% of the second instars weighing >2 mg survived to the third instar, whereas only 2% of the second instars weighing <2 mg molted. Third instars examined ranged in weight from 1.81 to 34.43 mg. Less than 1% of the third instars weighing <5 mg pupated, and none survived to adulthood. About 64% of the third instars weighing >5 mg developed into adults, ranging in weight from 1.61 to 21.49 mg. Larvae collected from fallen squares were predominantly second (56%) and third instars (39%), and about 92% of collected third instars weighed >5 mg. Thus, 36% of larvae in newly abscised squares were potentially capable of developing into adults without further feeding. These results, combined with the rapid rate of larval growth and development, further suggest that desiccation sufficient to deter feeding by larvae must occur within 1-3 d of square abscission in order to produce a high proportion of starvation-induced mortality in larvae.