Source tracing of Anthonomus grandis captured in areas of the USA where the species had previously been eradicated

Anthonomus grandis Boheman (Coleoptera: Curculionidae) is comprised of two subspecies: the boll weevil (A. g. grandis) and the thurberia weevil (A. g. thurberiae). The thurberia weevil is generally regarded as a non-pest variant of the species, whereas the boll weevil is an important economic pest of commercial cotton. In the USA, a nationwide eradication program has been largely successful against the boll weevil, but reintroductions of eradicated zones remain a threat. In 2015, A. grandis was trapped in the functionally eradicated Winter Garden growing region in Texas, where it had not been found since 2012. Weevils established in the area and triggered a multi-year, multi-million-dollar mitigation effort. In 2017, the species was trapped in Hidalgo County, New Mexico, another area where it had been previously eradicated. In this study, we obtained specimens from both trapping events and used double digest restriction site-associated DNA sequencing to obtain genetic data in homology with previously published A. grandis population genetic data. We conducted source tracing to match the captured weevils to previously described populations to determine a likely source population. We also compared the accuracy of the approach to other diagnostic methods currently used by the United States Department of Agriculture. The results suggested that a small founder population of weevils dispersed from the Lower Rio Grande Valley population in southern Texas and northern Tamaulipas and established in the Winter Garden. In Hidalgo Co., NM, the weevils were found to be genetically indistinct from non-pest populations of thurberia weevil associated with Arizona wild cotton (Gossypium thurberi). Thurberia weevils in Arizona remain unlikely to threaten commercial growers. Improved rapid diagnostic tools may be desirable to enable better assessment of trap captures and allow for a reduction in the application of mitigation efforts when the non-pest variant is identified.