New research from the University of Maryland indicates that corn earworm, also known as tomato fruitworm, has developed resistance to the Cry1 Bt toxins present in some sweet corn hybrids. Based upon Galen Dively’s twenty-one years of research comparing non-Bt sweet corn varieties with Bt varieties in plot studies, the proportion of damaged ears in Bt varieties has dramatically increased over that time.
Syngenta sweet corn hybrids with Attribute® technology expressing Cry1 Ab toxins in 1996 showed less than 10% ear damage. Those hybrids with Attribute® in 2016 averaged 84% ear damage. Similarly, the amount of damaged area per ear had tripled.
Seminis produced sweet corn hybrids with Performance SeriesTM containing Cry1 A.105 and Cry2 ab for worm control. When some of these hybrids were first evaluated in 2010 they provided 100% control of fall armyworm and 95% control of corn earworm. In 2016, six plantings at research farm sites of these hybrids averaged 67% damaged ears.
Syngenta also developed Attribute II® hybrids, incorporating the Cry1 Ab toxin with a new vegetative insecticidal toxin, Vip3A, which was released commercially in 2013. In thirteen field trials, Attribute II® hybrids had no live larvae (worms) in the ears and no ear damage was found, indicating 100% control of the larvae of corn earworm, fall armyworm, and European corn borer.
What does this mean for our sweet corn farmers? Effective, almost 100% control of European corn borer can be achieved with any of the Bt sweet corn hybrids. Bt hybrids that express only the Cry1 Ab toxin will not control corn earworm and fall armyworm sufficiently so that silking sprays will be needed to maximize marketable yields. Those hybrids expressing Cry1 A.105 and Cry2 ab will probably require some silking sprays, as well. The hybrids expressing the Cry1 Ab toxin and Vip3 A toxins may currently provide nearly 100% control of corn earworm and fall armyworm and silking sprays may not be necessary.
However, farmers have to look at the seed cost of these different hybrids and what the yield potentials are. If a particular variety has the Bt traits that you want but is expensive and doesn’t yield well, then you should look at other varieties. If fall armyworm is not a concern for you, and you are willing to use silking sprays for corn earworm, then a variety expressing only the Cry1 Ab toxin may economically work for you.
Lastly, the trend over the past twenty years has been a gradual decline in corn earworm populations. This has likely been the result of the planting of Bt hybrids in field corn and cotton. Even though the moths are becoming resistant to the Bt toxins, it does weaken the moths so that they are less fit, that is, fewer eggs are laid, fewer eggs hatch, and fewer larvae survive to adulthood. But these moths have greater impact on the sweet corn crop because they are increasing in their Bt resistance, so that fewer moths may cause greater damage on Bt hybrids.
By Joseph Ingerson-Mahar and Kristian Holmstrom