Submitted to: American Entomologist
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/10/2008
Publication Date: 12/10/2009
Citation: Showler, A.T. 2009. Roles of host plants in boll weevil range expansion beyond tropical Mesoamerica. American Entomologist. 55(4):234-242. Interpretive Summary: After a century of research on the boll weevil, misperceptions based on inadequate knowledge and false assumptions have persisted and affected our understanding of its ecology and control in large areas of the pest’s range from the United States Cotton Belt to Argentina. The scientific literature has been carefully studied and combined with contemporary findings about feeding habits, reproduction, and overwintering. This work presents a new theory on the boll weevil’s dispersal from its original tropical habitat in Mesoamerica and its establishment in various environments in both North and South America.
Technical Abstract: New findings on boll weevil, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), biology and ecology have had repercussions on the current level of understanding about short- and long-range boll weevil dispersal, and range expansion from its original tropical Mesoamerican habitat. The weevil co-evolved in the tropics with perennial reproductive host plants (plants that enable boll weevil reproduction by supplying the nutrients and sites for development to adulthood) including cotton, Gossypium hirsutum L. (which flowered mostly in response to short daylength) and other Gossypium spp., Hampea, and Cienfuegosia spp. Scattered patches of reproductive host plants were reached by flight, possibly instigated by increasing competition for limited resources. Range expansion was facilitated by shifts to new host plants. Boll weevils probably consumed nectar (not pollen as is commonly assumed) for energy to facilitate foraging and searching for reproductive hosts. On reaching the subtropical Lower Rio Grande Valley, characterized by generally mild winters with occasional cool northerlies and rare, brief freezes, the boll weevil only became established when the citrus industry boomed. Grapefruit, Citrus paradisi MacFad., and orange, C. sinensis (L.) Osbeck, endocarps, like nectars, contain sugar and other nutrients which can sustain actively foraging boll weevils through the cotton-free winter period. Some weevils might also enter a state of quiescence within desiccated bolls if winter begins while they are still immature, or as active adults during particularly cold intervals. Feeding on both citrus fruit and a wide array of nectars means that nonreproductive host plants (plants used only for foraging, not reproduction) have critical roles in the ability of the pest to disperse and become established at new latitudes. Once in the subtropical Lower Rio Grande Valley, the boll weevil spread north to the temperate United States Cotton Belt and moved across it during warm summers at an average rate of 104 km/yr. Where winters were too cold to support normal metabolic processes of both boll weevils and host plants, the tropical insect was forced to rely on a poorly-adapted diapause response. This strategy necessitates the buildup of large populations on cultivated cotton during the summer to ensure that some will survive winter to infest the next season’s crop. Fringe areas of the temperate zone like southern Arizona and California are also discussed, as well as northern Argentina where the pest’s range is expanding into citrus-growing subtropics. A less-toxic subtropical integrated pest management framework, based on the boll weevil’s relationships with host plants, is presented.