Abstract
The corn earworm, Helicoverpa zea, is a major target pest of the insecticidal Vip3Aa protein used in pyramided transgenic Bt corn and cotton with Cry1 and Cry2 proteins in the U.S. The widespread resistance to Cry1 and Cry2 proteins in H. zea will challenge the long-term efficacy of Vip3Aa technology. Determining the frequency of resistant alleles to Vip3Aa in field populations of H. zea is critically important for resistance management. Here, we provided the first F2 screen study to estimate the resistance allele frequency for Vip3Aa in H. zea populations in Texas, U.S. In 2019, 128 H. zea neonates per isofamily for a total of 114 F2 families were screened with a diagnostic concentration of 3.0 μg/cm2 of Vip3Aa39 protein in diet-overlay bioassays. The F2 screen detected two families carrying a major Vip3Aa resistance allele. The estimated frequency of major resistance alleles against Vip3Aa39 in H. zea in Texas from this study was 0.0065 with a 95% CI of 0.0014–0.0157. A Vip3Aa-resistant strain (RR) derived from the F2 screen showed a high level of resistance to Vip3Aa39 protein, with a resistance ratio of >588.0-fold relative to a susceptible population (SS) based on diet-overlay bioassays. We provide the first documentation of a major resistance allele conferring high levels of Vip3Aa resistance in a field-derived strain of H. zea in the U.S. Data generated from this study contribute to development of management strategies for the sustainable use of the Vip3Aa technology to control H. zea in the U.S.
Highlights
The corn earworm/cotton bollworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), is a major target pest of both Bt cotton and Bt corn in North America
Studies have shown that Vip3Aa is highly effective for control of H. zea in the field[24,25] and Vip3Aa is currently used in combination with Cry1/Cry[2] proteins in almost all Bt corn and Bt cotton products in the U.S17
The Vip3Aa proteins produced by transgenic Bt corn and cotton plants are >99% identical, which places strong selection pressure on H. zea that feed on both crops in the Southern U.S All these factors greatly increase the risk of resistance to Vip3Aa in H. zea
Summary
The corn earworm/cotton bollworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), is a major target pest of both Bt cotton and Bt corn in North America. Yang et al.[15] and Kaur et al.[21] documented field-evolved practical resistance of H. zea populations in Texas and Louisiana to Cry1A.105/Cry2Ab2 corn, respectively. It is important to determine current resistance allele frequency for Vip3Aa in field H. zea populations so that appropriate management strategies can be developed for the sustainable use of Bt crops including the Vip3Aa technology. Further studies to estimate resistance allele frequency have been hindered by extremely low mating frequency in single pair of male and female H. zea moths[29,30,31]. We conducted the first F2 screen study using light-trapped female moths to estimate the resistance allele frequency to Vip3Aa in H. zea in Texas. We provide the first documentation of a resistance allele conferring high levels of resistance to Vip3Aa in a field-derived H. zea strain in the U.S Data generated from this study greatly contributes to improving IRM practices to increase sustainability of Bt crops producing Vip3Aa for management of H. zea in the U.S
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