Strategies to control sweet corn pests

Sweet corn producers must rely on timely pest monitoring and effective insecticide sprays to minimize ear damage by corn earworm, European corn borer and fall armyworm. The fresh-market can tolerate only minimal damage to the ears. Pyrethroids are the popular choice for worm control, but efficacy has declined in recent years due to resistance in corn earworm populations.

Based on insecticide trials in the mid-Atlantic area, pyrethroids have lost about one-third of their efficacy since 2002, although efficacy varies from year to year depending on the relative susceptibility of migrant moths from the south. Spray mixtures of Lannate or Larvin plus a pyrethroid have become a common practice to circumvent a potential resistance problem. Also, rotations and mixtures with the newer insecticides such as Coragen, Belt and Radiant, as well as some premix products (VoliamXpress, Hero EC) are recommended options and provide excellent control.

However, despite the insecticide used, timing the first spray at the first signs of silking followed by a prescribed schedule based on moth pressure and adequate spray coverage of the ear zone are critical steps to achieve effective control. For example, most corn earworm eggs are oviposited directly on sweet corn silks. Once larvae hatch, they quickly move down the silk channel and begin feeding on the ear tip, where they are protected from insecticidal sprays. It is thus important to maintain a residual level of insecticide on freshly emerged silk tissue at all times.

As an alternative, the most potent bioinsecticide for sweet corn insect control is provided by transgenic hybrids expressing one or more insect-active toxins from the bacterium, Bacillus thuringiensis (Bt sweet corn). Attribute Insect Protected hybrids from Syngenta Seeds have been commercially available since 1996. Acreage of Bt sweet corn has increased significantly in recent years with the introduction of improved Rogers Brand fresh-market hybrids. The availability of seed in 25,000 units has also made it easier for the small producer to use the Bt technology.

Attribute Bt hybrids express a single Cry1Ab toxin that is highly effective against European corn borer, but this toxin alone does not provide 100 percent control of corn earworm or fall armyworm. Based on multiple-year field trials in Maryland, Attribute hybrids eliminated all whorl treatments and reduced silk sprays by at least four applications. Although these hybrids provide excellent protection against the caterpillar complex, two and sometimes three supplemental insecticide sprays may be needed to ensure fresh-market-quality ears, especially during high moth activity.

The development and commercialization of new Bt field corn events by Syngenta Seeds has isolated a novel vegetative insecticidal protein from B. thuringiensis – Vip3A. This toxin is highly effective against a range of agriculturally important lepidopteran larvae including black cutworm, fall armyworm, corn earworm and western bean cutworm. In field studies conducted in Maryland and Minnesota, hybrids expressing the Vip3A trait and pyramided with the Cry1Ab Bt protein were compared to near isogenic non-Bt hybrids.

Over all years and locations, the non-Bt hybrids, without insecticide protection, averaged between 43 and 100 percent ears infested with a range of 0.24 to 1.74 H. zealarvae per ear. By comparison, no larvae were found in the pyramided Vip3A x Cry1Ab hybrids, indicating virtually 100 percent of all lepidopteran larvae. Syngenta Seeds plans to commercialize its next generation Attribute II technology in sweet corn in 2013.

Monsanto’s Seminis Seeds also has developed and is marketing pyramided Bt sweet corn seed under the Performance Series trade name. Bt hybrids available are Temptation II, Obsession II and Passion II, and these hybrids express three insecticidal proteins: Cry1A.105 and Cry2Ab to control lepidopteran larvae, and Cry3Bb1 to control rootworms; and herbicide-tolerant traits. Maryland studies have shown that the Performance Series hybrids provide virtually 100 percent control of corn borers and fall armyworms, and more than 95 percent control of corn earworms, of which all surviving larvae were small and caused very minor injury on the ear tip.

The Attribute II and Performance Series sweet corn hybrids ideally fit the IPM philosophy by combining host plant resistance traits, different modes of action to prevent resistance and a reduced-risk bioinsecticide, and by providing an environmental safe option to conserve beneficial insects. Clearly, the Bt technology can significantly reduce pesticide use and control costs, but control efficacy may vary under adverse growing conditions or very high insect pressure. And finally, the Bt hybrids will not be insect pest free, so regular monitoring of insect pests not affected by the expressed proteins will be essential for successful IPM.

By Galen P. Dively, University of Maryland