With field corn planting rapidly approaching in Louisiana, it is important to keep in mind the value of planting refuge. The vast majority of the field corn acreage planted in Louisiana express Bt proteins for protection against lepidopteran larval such as corn borers and corn earworms.
A refuge serves as a method of insecticidal resistance management or IRM. A refuge can be deployed in several ways but should be within ½ mile of the Bt expressing field corn and should mature concurrently.
The EPA mandates that for every 80 acres of Bt field corn, there should be 20 complimentary acres of non-Bt field corn. Refuge deployment strategies can be seen below.
To better understand the theory behind utilizing a refuge in field corn, I will use corn borers for an example. Since insecticide resistance is a naturally occurring phenomenon, there is a low possibility that some corn borers may survive after feeding in Bt field corn.
Simply put, if two Bt resistant corn borer moths were to mate, the subsequent offspring would be more likely to survive Bt in the future. A refuge is an area of non-Bt field corn planted alongside Bt expressing field corn and it acts as a source of Bt-susceptible insects.
In the rare instance that a corn borer larvae were to survive to adulthood in Bt field corn, it would likely mate with one or several of the many Bt-susceptible moths produced by the refuge, ultimately passing on Bt-susceptible offspring. However, in an instance where field corn refuge isn’t utilized, there would be fewer Bt-susceptible insects in the landscape, increasing the likelihood of two resistant insects mating.
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Historically, field corn refuge compliance has been relatively low across the midsouth for various reasons. Due to this, we have seen a decrease in Bt protein efficacy primarily in corn earworm populations over the last several years. Field corn producers are less likely to realize this decrease in product efficacy because corn earworm isn’t a yield limiting pest in field corn.
However, the corn earworm is a yield limiting pest in cotton on an annual basis. Crop consultants and cotton producers have noticed this decrease in Bt efficacy because additional chemical control measures are being warranted for corn earworm control in Bt expressing cotton.
Going forward, I am recommending that two-gene Bt field corn hybrids be utilized because it still provides excellent control of corn borers, which can be yield limiting pests in field corn. A newer Bt protein, Vip3A, is being marketed in field corn hybrids and cotton varieties.
To prevent the selection of Vip3A resistant corn earworm, planting field corn hybrids expressing this protein should be avoided. There is data indicating that field corn hybrids expressing 2nd and 3rd generation Bt trait packages yield similarly signifying that there is little to no economic benefit to planting Vip3A expressing hybrids in the midsouthern US.
In Louisiana and across the midsouth, the true value of the Vip3A protein lies in cotton and should be utilized in cotton only.