Field screening of wild cotton, Gossypium hirsutum, landraces for resistance to thrips (Thysanoptera: Thripidae)

Thrips (Thysanoptera) are the most prevalent early season pests of cotton and are often detrimental to the establishing crop. Heavy reliance on prophylactic insecticides, primarily used as seed treatments, has led to insecticide resistance in populations of Frankliniella fusca Hinds. Sustainable, alternative management tactics are needed to protect seedling cotton from thrips. We evaluated day-neutral, exotic landraces (N = 164) of Upland cotton (Gossypium hirsutum L.) for thrips-resistance traits in three field trials using insecticide-treated and -un-treated plots. The cotton landraces were chosen for their diverse backgrounds, unique genotypes, and day -neutrality, allowing them to easily cross with USA breeding lines. The trials were conducted at the North Car-olina State University Sandhills Research Station in Jackson Springs, NC (NC2018), and the Clemson University Pee Dee Research and Education Center in Florence, SC (SC2018 and SC2020). A selection index was created to quantify thrips resistance based on densities of thrips and ratings of injury by thrips at the first and third true-leaf stages in untreated plots, and genotypic differences in above-ground dry biomass between treated and untreated plots at 42 days after planting. Subsamples of adult thrips were collected for species identification. Dominant thrips species were F. fusca in NC2018 and SC2020 and Frankliniella tritici in SC2018. Eight putatively thrips-resistant genotypes (TX-1109, TX-1975, TX-2320, TX-2383, TX-101, TX-2347, TX-2362, TX-251) and four pu-tatively thrips-susceptible genotypes (TX-203, TX-1212, TX-1094, TX-2403-2) were identified in the field trials. Further investigation of the putatively thrips-resistant and-susceptible genotypes should focus on mechanisms of thrips resistance to enhance future breeding efforts.