Magnitude and Extent of Helicoverpa zea Resistance Levels to Cry1Ac and Cry2Ab2 across the Southeastern USA

Abstract

After resistance is first detected, continued resistance monitoring can inform decisions on how to effectively manage resistant populations. We monitored for resistance to Cry1Ac (2018 and 2019) and Cry2Ab2 (2019) from southeastern USA populations of Helicoverpa zea. We collected larvae from various plant hosts, sib-mated the adults, and tested neonates using diet-overlay bioassays and compared them to susceptible populations for resistance estimates. We also compared LC50 values with larval survival, weight and larval inhibition at the highest dose tested using regression, and found that LC50 values were negatively correlated with survival for both proteins. Finally, we compared resistance rations between Cry1Ac and Cry2Ab2 during 2019. Some populations were resistant to Cry1Ac, and most were resistant to CryAb2; Cry1Ac resistance ratios were lower than Cry2Ab2 during 2019. Survival was positively correlated with larval weight inhibition for Cry2Ab. This contrasts with other studies in both the mid-southern and southeastern USA, where resistance to Cry1Ac, Cry1A.105, and Cry2Ab2 increased over time and was found in a majority of populations. This indicates that cotton expressing Cry proteins in the southeastern USA was at variable risk for damage in this region.

Extended Sentinel Monitoring of Helicoverpa zea Resistance to Cry and Vip3Aa Toxins in Bt Sweet Corn: Assessing Changes in Phenotypic and Allele Frequencies of Resistance

“…Moreover, 32% of the trials since 2020 reported higher H. zea larval densities in Cry1A.105/Cry2Ab2 ears compared to non-Bt ears. These findings concur with recent studies reporting high resistance ratios and the increased field failure of the Cry1A.105 and Cry2Ab2 toxins in controlling H. zea infestations in Bt corn and Bt cotton [28,49,75,[83][84][85]. As previously mentioned, it was not possible to estimate the RAF for each toxin, according to Venette et al [60].…”

Within-field spatial patterns of Helicoverpa zea (Lepidoptera: Noctuidae) and spatial associations with stink bugs and their injury in field corn

“…Differences in Bt hybrids used in our fields did not have a significant impact on the number of larvae or the level of injury. This finding is unsurprising, given that resistance to Cry toxins expressed in Bt hybrids planted in our study has been widely reported in H. zea in the southeastern United States (Bilbo et al 2019b, Yu et al 2021, Reisig et al 2023). The spatial patterns described herein were likely not influenced by differences in Bt toxins expressed and more dependent on other factors, including pest ecology, natural enemies, host-seeking behavior, plant phenology, or the presence of other herbivores.…”

“…Given that Bt resistance is widespread in H. zea (e.g., Bilbo et al 2019b, Reisig et al 2023), which is supported by infestation levels in Bt corn presented here, resistance management is critical for prolonging the use of the technology. Helicoverpa zea resistance to Bt toxins is primarily managed in corn using non-Bt refuge plantings (Bates et al 2005).…”

“…The majority of corn grown in the United States has been genetically engineered to express toxins produced by the soil-borne bacterium Bacillus thuringiensis (Bt). Transgenic corn expressing Bt toxins can provide varying levels of control of H. zea , with resistance reported to all Cry toxins, and Vip3A currently being the only Bt toxin providing very good levels of control (Bilbo et al 2019b, Reay-Jones et al 2020, Yang et al 2021, Reisig et al 2023). While H. zea is not considered an economic pest in corn (Reay-Jones and Reisig 2014, Bibb et al 2018, Olivi et al 2019), it is a major pest of cotton, a crop that expresses many of the same or similar Bt toxins found in corn (Kanel et al 2016).…”