Understanding the Risks Associated with Vip3A Resistance in Helicoverpa zea
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Since 1996, Bt has been utilized in cotton and corn to control caterpillar pests. Bt refers to a protein toxin that is produced by the soil bacterium, Bacillus thuringiensis. The bacteria produces crystalline proteins that are toxic to many caterpillars. There are two types of these toxins, Cry proteins and Vip proteins. These proteins act by binding to the midgut of the caterpillar, resulting in ulcers that allow gut contents to move into the body of the insect resulting in death. There are several different kinds of Cry proteins, Cry1As, Cry1F and Cry2As, used for caterpillar control. All of these proteins differ slightly in how they bind to the midgut. Currently there is only one Vip protein used, Vip3A, and this protein binds very differently than the Cry proteins, but causes the same type of injury. Cotton and corn has been genetically modified (GMO) to produce these toxins by inserting the gene from the bacterium that expresses the various Bt proteins, into the plant. These crops are termed, Bt corn or Bt cotton. To help prevent Bt resistance, these Bt proteins are used together within a single plant; this is called pyramiding. Corn earworm or cotton bollworm, Helicoverpa zea is a major target of Bt corn and Bt cotton in the U.S. Over the past few years, growers, agricultural consultants and state extension professionals have been reporting increasing incidences of H. zea surviving and injuring Bt corn and cotton in the Southern U.S., and field-evolved resistance has been reported to all of the Cry proteins in much of the southern U.S. Thus, there is a great deal of dependence on Vip3A for maintaining control of H. zea in Bt corn and cotton.In 2018 and 2019 there have been unexpected occurrence and damage of H. zea on Bt corn and cotton expressing the Vip3Aa protein in some fields of Texas, Louisiana and Mississippi. Additionally, in 2019 we detected a Vip3A resistant gene in H. zea in Texas, which essentially renders the insect immune to the Vip3A protein. These incidences suggest that the risk of evolution of Vip3Aa resistance in H. zea could be high, and that the Vip3A resistant H. zea strain we have should be further studied to determine the risk it poses to corn and cotton expressing Vip3A.In our project we will conduct mating studies using the Vip3A resistant and susceptible H.........
