Subtropical boll weevil ecology

Abstract

Although a great deal of research has been conducted on the boll weevil, Anthonomus grandis grandis Boheman, originally a Mesoamerican insect, in temperate cotton, Gossypium hirsutum L., growing regions of the United States, relatively little is known about boll weevil ecology in the subtropics and tropics, which extend from South Texas to Argentina. Unlike in temperate regions, boll weevils do not necessarily diapause in the subtropics. Rather, they are active and can reproduce year-round provided that adequate host plants are available. Instead of predators (excluding limited areas where the imported fire ant, Solenposis invicta Buren, has become established) and cold winter temperatures being key factors behind boll weevil population dynamics, the availability and quality of food sources are most important. During the cotton-growing season, boll weevil populations respond to the changing quality of the cotton plant, and component parts, which can profoundly affect gravidity, fecundity, and longevity, and susceptibility on the plant to injury from both feeding and oviposition. Immature boll weevils developing within fallen squares are often killed by heat and associated desiccation unless they are buried and insulated by in-season cultivation. During and after harvest operations, substantial numbers of boll weevils remain in the field, presumably in cotton plant debris, including fallen squares and bolls. Although heat and desiccation also kill these weevils until conditions cool as the typically mild subtropical winters approach, weevils inside fruiting bodies on the soil surface can survive better than those buried by tillage. In the subtropics, boll weevils can reproduce during winters on volunteer cotton plants and a limited number of malvaceous plant species. Overwinter survival, however, is likely facilitated more by nutritious food sources enabling longevity that spans the cotton-free period. Some common subtropical food sources that can sustain adult boll weevils in this way are the endocarps of oranges, Citrus sinensis (L.) Osbeck.; grapefruit, C. paradisi Macfad.; and prickly pear, Opuntia engelmannii Salm-Dyck ex. Engel.; and other such food sources are possible. Although some researchers have maintained that boll weevils “migrate,” long-distance boll weevil movement during the off-season is probably passive and opportunistic because nutritious food sources for overwintering already exist in most subtropical cotton-growing areas. Instead, boll weevils generally remain in the area, surviving winter in citrus orchards, and possibly other places where nutritious food is available. Flaws with the efficiency of the commonly used boll weevil trap design, and the field sampling method used in South Texas (and possibly other areas) are discussed, as well as weaknesses of the boll weevil spray regime that relies on pre-emptive sprays and sprays triggered by thresholds determined by sampling randomly-selected cotton squares. An alternative, proactive spray approach based on the relationship of boll weevil populations to the changing nutritional quality of cotton fruiting bodies is described, as well as other potential in-season and off-season control tactics, including careful choice of planting date, burial of post-harvest cotton debris, citrus orchard sanitation, and mass-trapping for adult boll weevils in overwintering “hot spots” where boll weevils accumulate until cotton is planted in the spring.