Evaluation of resistance in the southwestern corn borer (Diatraea grandiosella) to purified Bt proteins in the Southwestern U.S..

The southwestern corn borer, Diatraea grandiosella (Dyar), is a target pest of Bt corn in the southwestern U.S., causing yield loss due to tunneling and lodging of corn stalks. Transgenic corn expressing Bacillus thuringiensis (Bt) proteins have been effective for the management of the SWCB. However, previous reports have indicated decreases in susceptibility to the Cry1F protein in SWCB populations collected from Arizona and New Mexico in 2015. Moreover, in 2023, extensive infestation and damage of SWCB was observed on a corn hybrid expressing Cry1Ab and Cry1F proteins in Antelope Wells, New Mexico, suggesting that SWCB are exhibiting resistance to these proteins. Because it’s possible that Bt resistant alleles in SWCB from Arizona and New Mexico could move eastwardly into major southern corn production areas, it’s important to investigate the status of Bt resistance. In this study, we ascertained the level of resistance to Bt proteins in SWCB from the field population collected from Antelope Wells, New Mexico, and also a population from Jackson, Tennessee on non-Bt corn. First, we determined the diagnostic concentrations in diet-overlay bioassays of Cry1A.105, Cry1Ac, Cry1F, Cry2Ab2, and Vip3Aa39 proteins of a laboratory susceptible SWCB population. Mortality was recorded after 14 days, and Probit analysis was used to determine the (LC₅₀), (LC₉₀), and (LC₉₉) of each Bt protein. With this data, we performed full range bioassays on the field populations to detect decreases in susceptibility to the Bt proteins. Results from this study should provide valuable information regarding the status of Bt resistance in SWCB.