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Bt Cotton Needs Careful Scouting

Crop and Soil Environmental News, July 1997

Charles Hagedorn
Extension Specialist, Biotechnology

As many cotton growers are still gaining experience with Bt cotton, one thing seems clear from past experience - traditional insecticide treatments should not be overlooked, whether planting transgenic varieties or not. Some research tests have shown that Bt cotton nets the highest yields when oversprayed with a pyrethroid. For example, test plots conducted by Agricultural Management Services in Clayton, LA, showed that over-sprayed Bt cotton produced l00 pounds more lint per acre than standard varieties.

Because of the experimental nature of Bt cotton, many agricultural consultants from the Southeast, Southwest and Mid-South conducted test plots last year (1996) to learn how to most effectively manage Bt cotton. Overall, the consultants discovered that while Bt cotton was an effective choice, it still required careful scouting and follow up treatments - especially late in the season. Plant bugs, fall armyworms, boll weevils and stink bugs were an early to late-season problem in Bt cotton in 1996 that was not treated on a regular basis. Although threshold levels were not reached for most of these insects, the additive effect of having small amounts of damage from each of them disrupted the fruiting pattern and therefore reduced yields at several test plot locations.

When the Bt was over-sprayed with an appropriate pyrethroid, the insects were not a problem and the cotton often yielded over l,000 pounds. The combined results from several locations suggest over-spraying with a pyrethroid on a seven to ten-day schedule based on egg and/or small worm thresholds can be very beneficial. The best time to begin sprays on Bt cotton appears to be at first bloom, but if plant bug pressures are severe, other product sprays in late May or early June may be needed.

The best current advice for managing insects on cotton (transgenic or not) is to remember that regardless of what kind of cotton is planted, there will be insect problems. Consultants should scout Bt cotton fields even more closely than conventional cotton because no one can be sure if insects will survive the Bt, when insects will survive during the growing season, or exactly which insects may survive. As consultants and growers continue to work with the transgenic cotton varieties, they should realize that a many different tools are needed to control the entire spectrum of cotton pests. Even when using tools such as Bt cotton, pyrethroids and other traditional control aids can still play a vital role in stopping pests and boosting yields.

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