Widespread field-evolved resistance of bollworm [Helicoverpa zea (Boddie)] to Cry1 and Cry2 Bt proteins has threatened the utility of Bt cotton for managing bollworm. Consequently, foliar insecticide applications have been widely adopted to provide necessary additional control. Field experiments were conducted across the Mid-South and in Texas to devise economic thresholds for foliar insecticide applications targeting bollworm in cotton. Bt cotton technologies including TwinLink (TL; Cry1Ab+Cry2Ae), TwinLink Plus (TLP; Cry1Ab+Cry2Ae+Vip3Aa), Bollgard II (BG2; Cry1Ac+Cry2Ab), Bollgard 3 (BG3; Cry1Ac+Cry2Ab+Vip3Aa), WideStrike (WS; Cry1Ac+Cry1F), WideStrike 3 (WS3; Cry1Ac+Cry1F+Vip3Aa), and a non-Bt (NBT) variety were evaluated. Gain threshold, economic injury level, and economic thresholds were determined. A 6% fruiting form injury threshold was selected and compared with preventive treatments utilizing chlorantraniliprole. Additionally, the differences in yield from spraying bollworms was compared among Bt cotton technologies. The 6% fruiting form injury threshold resulted in a 25 and 75% reduction in insecticide applications relative to preventive sprays for WS and BG2, respectively. All Bt technologies tested in the current study exhibited a positive increase in yield from insecticide application. The frequency of yield increase from spraying WS was comparable to that of NBT. Significant yield increases due to insecticide application occurred less frequently in triple-gene Bt cotton. However, their frequencies were close to the dual-gene Bt cotton, except for WS. The results of our study suggest that 6% fruiting form injury is a viable threshold, and incorporating a vetted economic threshold into an Integrated Pest Management program targeting bollworm should improve the sustainability of cotton production.

During 2021 and 2022, eight field-collected and five laboratory Helicoverpa zea strains with varying susceptibility to different Bt proteins were evaluated for their responses against HearNPV using diet-overlay bioassays. The five laboratory strains included SS (susceptible to all Bt proteins), CRY-RR (resistant to Cry1 and Cry2), VIP-RR-70 (resistant to Vip3Aa), VIP-RR-15 (resistant to Vip3Aa), and TRE-RR (resistant to Cry1, Cry2, and Vip3Aa). Our findings showed that the susceptibility of TRE-RR, VIP-RR-70, and VIP-RR-15 strains to HearNPV was similar to that of the SS strain. However, the field and Cry-RR strains were more resistant to HearNPV compared to the SS strain. Because most feral H. zea strains in the southern U.S. have developed practical resistance to Cry Bt proteins but remain susceptible to Vip3Aa, the results suggest that the reduced susceptibility to HearNPV in H. zea may be associated with the resistance to Cry Bt proteins but not with the resistance to Vip3Aa. Correlation analysis confirmed that there was a significant positive relationship between Cry resistance and HearNPV resistance, but not between the Vip3Aa resistance and HearNPV resistance in H. zea. Our findings provide valuable insights into the relationship between susceptibility to HearNPV and resistance to Bt proteins in H. zea.

The diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), is a major pest affecting Brassica crops worldwide. In North America, P. xylostella outbreaks have caused substantial economic losses in canola (Brassica napus L. and Brassica rapa L.) and brassicaceous vegetable (Brassica oleracea L.) crops. Natural enemies, including the parasitoid Diadegma insulare (Hymenoptera: Ichneumonidae), play a key role in controlling P. xylostella populations. To better understand the distribution and impact of Diadegma spp., we adopted Diadegma-specific primers, developed originally for haplotyping, and used a molecular screening approach to detect parasitism of P. xylostella larvae collected across Canada and the United States. The primer set was first tested on D. insulare adults and laboratory parasitized P. xylostella to confirm amplification and specificity and determine sensitivity. Screening was then conducted on 421 field-collected larvae from 45 localities to detect and estimate parasitism rates and assess geographic variability. The results revealed notable variation across collection sites in parasitism of P. xylostella larvae by Diadegma spp., with more parasitized larvae collected from field sites in Canada than from field sites in the United States. Variation in parasitism across collection sites could be the result of differences in climate, agricultural practices, and insecticide use between agricultural systems. This research demonstrates the utility of molecular screening to detect parasitism of P. xylostella larvae and highlights potential differences in the impact of Diadegma spp. on the population dynamics of P. xylostella in Canada and the United States.