ABSTRACT
The diamondback moth, Plutella xylostella (Linnaeus), is one of the most important insect pests of Brassica crops worldwide. In October 2016, outbreaks of an invasive P. xylostella population and unexpected control failures occurred on broccoli and cauliflower crops throughout all vegetable-growing regions in Arizona. Nineteen populations of Plutella xylostella were collected from 2016 to 2021 from various commercial cauliflower fields in Yuma and Scottsdale, Arizona, and from experimental broccoli plots at the University of Arizona, Yuma Agricultural Center (UAYAC), Yuma, Arizona. Populations collected from the commercial cauliflower fields had been transplanted with seedlings produced in a local Yuma nursery in 2016 and Salinas, CA in 2017 to 2018, whereas experimental broccoli plots were direct seeded. These populations were evaluated for their susceptibility to chlorantraniliprole, spinetoram, emamectin benzoate, and cyantraniliprole. In this study, field rate laboratory bioassays, serial dilution laboratory bioassays, and field efficacy spray experiments were performed. The field rate laboratory bioassay results showed that spinetoram, emamectin benzoate, and cyantraniliprole remained effective at controlling P. xylostella, but chlorantraniliprole did not control P. xylostella at the field rate. Additionally, serial dilution bioassays confirmed significant levels of cyantraniliprole and chlorantraniliprole resistance in the P. xylostella populations collected from transplanted cauliflower fields. However, the results of the multiyear/growing-seasons study monitoring the susceptibility of P. xylostella populations collected from direct-seeded broccoli and field efficacy trials conducted at the UAYAC indicated that the resistance to diamide insecticides was neither uniform nor persistent following the 2016 outbreak. Nevertheless, the risk for P. xylostella resistance in Arizona vegetable-growing regions exists, particularly in Brassica transplants. Therefore, we recommend that Arizona Brassica growers remain vigilant and practice rigorous insecticide resistance management to offset the development of resistance.
ABSTRACT
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.
ABSTRACT
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.