Source population determination for boll weevil re-infestation Events in Texas

The boll weevil, Anthonomus grandis grandis Boheman, is a pest of commercial cotton in the Americas that has been successfully eradicated throughout most of the United States. However, endemic populations in the Lower Rio Grande Valley (LRGV) along the Texas-Mexico border continue to threaten U.S. cotton production. When a boll weevil is captured in an eradicated zone, knowledge of its geographic origin benefit eradication programs and helps to implement safeguards to reduce future infestation. Here, we determine the likely geographic origin for a 2018 boll weevil outbreak in a previously eradicated region near Kingsville, TX, using independent methods of population ancestry, relatedness, and population genetic approaches. We measure and compare the genetic signatures to similar outbreaks in other eradicated zones of Texas from prior years, as well as to boll weevil individuals captured throughout the LRGV. Finally, we assess the reliability of our relatedness metrics by validating our approach with genetic lines of the model insect Tribolium castaneum Herbst. We conclude that the 2018 boll weevil outbreak in Kingsville likely originated from an independent introduction of individuals from the LRGV. Relatedness patterns among these samples also indicate they are closely related to each other, e.g., parent-offspring, sibling-sibling, or half siblings, suggesting reproduction occurred in nearby cotton fields following the introduction of a few individuals into the region. Together, these methods and results illustrate the utility of whole-genome sequencing, demographic reconstruction, classical population genetics, and contemporary pairwise relatedness for non-model insects to support precise agricultural management and conclusive source estimation.