Computer vision for detecting field‐evolved lepidopteran resistance to Bt maize

Abstract

BACKGROUND Resistance evolution of lepidopteran pests to Bacillus thuringiensis (Bt) toxins produced in maize and cotton is a significant issue worldwide. Effective toxin stewardship requires reliable detection of field‐evolved resistance to enable the implementation of mitigation strategies. Currently, visual estimates of maize injury are used to document changing susceptibility. In this study, we evaluated an existing maize injury monitoring protocol used to estimate Bt resistance levels in Helicoverpa zea (Lepidoptera: Noctuidae). RESULTS We detected high interobserver variability across multiple injury metrics, suggesting that the precision and accuracy of maize injury detection could be improved. To do this, we developed a computer vision‐based algorithm to measure H. zea injury. Algorithm estimates were more accurate and precise than a sample of human observers. Moreover, observer estimates tended to overpredict H. zea injury, which may increase the false‐positive rate, leading to prophylactic insecticide application and unnecessary regulatory action. CONCLUSIONS Automated detection and tracking of lepidopteran resistance evolution to Bt toxins are critical for genetically engineered crop stewardship to prevent the use of additional insecticides to combat resistant pests. Advantages of this computerized screening are: (i) standardized Bt injury metrics in space and time, (ii) preservation of digital data for cross‐referencing when thresholds are reached, and (iii) the ability to increase sample sizes significantly. This technological solution represents a significant step toward improving confidence in resistance monitoring efforts among researchers, regulators and the agricultural biotechnology industry.

Practical resistance to Cry toxins and efficacy of Vip3Aa in Bt cotton against Helicoverpa zea

“…The generally high efficacy of Bt plants producing Vip3Aa in combination with Cry toxins seen here and previously [32][33][34][35][36][37][38][39][40] is encouraging. Nonetheless, several other factors and recent findings have raised concerns.…”

“…Nonetheless, several other factors and recent findings have raised concerns. With widespread resistance to Cry toxins, Vip3Aa is the only toxin produced by Bt crops that remains fully effective against many populations of H. zea 32–40 . The US EPA concluded that cotton producing Vip3Aa alone met the high‐dose standard against H. zea in one test but not another, whereas corn producing Vip3Aa alone did not meet the high‐dose standard 7,49 .…”

“…With widespread resistance to Cry toxins, Vip3Aa is the only toxin produced by Bt crops that remains fully effective against many populations of H. zea. [32][33][34][35][36][37][38][39][40] The US EPA concluded that cotton producing Vip3Aa alone met the high-dose standard against H. zea in one test but not another, whereas corn producing Vip3Aa alone did not meet the high-dose standard. 7,49 As noted earlier, bioassay data from the region studied here show a small but significant decrease in susceptibility to Vip3Aa that provided an early warning of resistance.…”

“…This article focuses on practical resistance to Cry toxins in Helicoverpa zea (also known as bollworm and corn earworm), one of the most economically important pests of cotton and other crops in the United States 3,12,13 . Many previous studies have documented widespread H. zea resistance to Cry toxins in the United States 14–37 . By contrast, Bt corn and cotton producing the vegetative insecticidal protein Vip3Aa in combination with Cry toxins have been highly effective against nearly all populations of H. zea tested, despite some decreases in susceptibility to Vip3Aa that have been categorized as an early warning of resistance 32–41 …”

“…Many previous studies have documented widespread H. zea resistance to Cry toxins in the United States 14–37 . By contrast, Bt corn and cotton producing the vegetative insecticidal protein Vip3Aa in combination with Cry toxins have been highly effective against nearly all populations of H. zea tested, despite some decreases in susceptibility to Vip3Aa that have been categorized as an early warning of resistance 32–41 …”