Populations of Helicoverpa zea (Boddie) in the southeastern United States are commonly resistant to Cry1Ab, but still susceptible to Vip3Aa20 expressed in MIR 162 corn

Authors: NIU, YING - Louisiana State University Agcenter; OYEDIRAN, ISAAC - Syngenta Crop Protection; YU, WENBO - Louisiana State University Agcenter; LIN, SHUCONG - Louisiana State University Agcenter; DIMASE, MARCELO - Louisiana State University Agcenter; BROWN, SEBE - Louisiana State University Agcenter; REAY-JONES, FRANCIS - Clemson University; COOK, DON - Mississippi State University; REISIG, DOMINIC - North Carolina State University; THRASH, BEN - University Of Arkansas; Ni, Xinzhi; PAULA-MORAES, SILVANA - University Of Florida; ZHANG, YAN - Syngenta Crop Protection; CHEN, JENG - Syngenta Crop Protection; WEN, ZHIMOU - Syngenta Crop Protection; HUANG, FANGNENG - Louisiana State University Agcenter

Submitted to: Toxins
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2021
Publication Date: 1/15/2021
Citation: Niu, Y., Oyediran, I., Yu, W., Lin, S., Dimase, M., Brown, S., Reay-Jones, F.P., Cook, D., Reisig, D., Thrash, B., Ni, X., Paula-Moraes, S.V., Zhang, Y., Chen, J., Wen, Z., Huang, F. 2021. Populations of Helicoverpa zea (Boddie) in the southeastern United States are commonly resistant to Cry1Ab, but still susceptible to Vip3Aa20 expressed in MIR 162 corn. Toxins. 13(1):63. https://doi.org/10.3390/toxins13010063.
DOI: https://doi.org/10.3390/toxins13010063

Interpretive Summary: The corn earworm, also known as the cotton bollworm, is a cross-crop target pest of both transgenic maize and cotton in the U.S. Maize and cotton are usually planted adjacently in the southern U.S. (often called the U.S. Cotton Belt) and predominant areas of the two crops currently in the Cotton Belt are planted with varieties containing bacterial transgenes. The cropping system plus the similar transgenic bacterial proteins expressed in maize and cotton in the Cotton Belt creates an ideal environment for the insect to develop resistance to certain bacterial toxins in transgenic crops. In recent years, corn earworm resistance to a group of bacterial toxins (i.e., crystal inclusions during bacterial sporulation phase of bacterial growth, also known as Cry toxins) in transgenic crops have been documented in the U.S. At the same time, a group of vegetative (form of) insecticidal protein (Vip) from a bacterium (such as Vip3A) has been confirmed to have maintained its corn earworm susceptibility in transgenic crops to date. Therefore, preservation the corn earworm susceptibility to the vegetative form of insecticidal proteins is critical for the continued success of transgenic crop technology. During 2018 and 2019, 32 corn earworm populations were collected from non-transgenic maize (19 populations), transgenic maize with only Cry proteins (12 populations), and cotton with both (Cry/Vip3A) toxins (1 population) across major maize production areas in seven southeastern U.S. states. Compared to a known susceptible insect strain, 80% of the field populations were 13- to >150-fold resistant to Cry toxins, while their susceptibility to Vip toxins ranged from >11-fold more sensitive to 9-fold more tolerant. The results showed that resistance to Cry toxins in the corn earworm populations has been widely distributed in the Southeastern U.S. region. Also, corn earworm population resistant to Cry toxin are not cross-resistant to the Vip toxin and the insect in the region is still susceptible to Vip toxin. In conclusion, the information generated from this study will be useful in developing appropriate mitigation measures for insect resistance to Cry proteins and improving resistance management programs to sustain Vip3A susceptibility in managing corn earworm damage on corn and cotton.

Technical Abstract: The corn earworm, Helicoverpa zea (Boddie), is a major pest targeted by pyramided Bacillus thuringiensis (Bt) traits in maize and cotton in the U.S. Cry1Ab is one of the first insecticidal proteins used in Bt crops, while Vip3A is a relatively new protein that has recently been incorporated into transgenic maize and cotton varieties to generate pyramided Bt traits targeting lepidopteran pests including H. zea. The objectives of this study were to determine the current status and distribution of the Cry1Ab resistance, and evaluate susceptibility to Vip3Aa20 expressed in MIR 162 maize in H. zea in the Southeastern region of the U.S. During 2018 and 2019, 32 H. zea populations were collected from non-Bt maize (19 populations), Cry maize (12 populations), and Cry/Vip3A cotton (1 population) across major maize production areas in seven southeastern U.S. states. Susceptibility of these populations to Cry1Ab and Vip3Aa20 was determined using a diet over-laying bioassay. Compared to a known susceptible insect strain, 80% of the field populations were 13- to >150-fold resistant to Cry1Ab, while their susceptibility to Vip3Aa20 ranged from >11-fold more sensitive to 9-fold more tolerant. Mean susceptibility to each Bt protein was not significantly different between populations collected from non-Bt and Bt fields, as well as between 2018 and 2019. The results showed that resistance to Cry1Ab in H. zea has been widely distributed in the southeastern U.S. region. Cry1Ab-resistant populations of H. zea are not cross-resistant to Vip3Aa20 and the insect in the region is still susceptible to Vip3Aa20. Vip3Aa20 concentrations between 5 and 10 µg/cm2 could be used as diagnostic concentrations for susceptibility monitoring in future. In addition, information generated from this study will be useful in developing appropriate mitigation measures for insect resistance to Cry proteins and improving resistance management programs to sustain Vip3A susceptibility in H. zea.