By: Suhas S. Vyavhare, Assistant Professor and Extension Specialist; and David Kerns, Professor, IPM Coordinator and Extension Specialist
What is Bt cotton?
Bt cotton is genetically altered by the insertion of genes from a common soil bacterium, Bacillus thuringiensis, to produce certain proteins that are toxic to specific insects. Currently available Bt cotton varieties produce either or both crystal (Cry) and vegetative insecticidal proteins (Vip) that target specific caterpillar pests such as beet armyworm (Fig. 1), cotton bollworm (Fig. 2), and tobacco budworm.
How does Bt cotton help manage pests?
Initially, Bt cotton provided a means to effectively manage pests, such as tobacco budworm and pink bollworm, that were difficult to control or had developed resistance to commonly used insecticides. Since then, Bt cotton has been instrumental in providing control of other pests, including beet armyworm, bollworm, fall armyworm, and loopers. The use of Bt cotton has reduced the need for foliar insecticides targeting these pests and has also reduced outbreaks of secondary pests.
What Bt traits are available?
Since its introduction in 1996 into US agriculture, Bt technology has developed from a single-gene trait to multi-gene trait packages. The first-generation Bt cotton (Bollgard) had a single Bt gene that expressed (produced) Cry1Ac. The second-generation Bt technologies, such as Bollgard 2, TwinLink, and WideStrike, produce two Bt toxins, and the most recent third-generation Bt technologies (WideStrike 3, Bollgard 3, and TwinLink Plus) are three-gene trait products.
Which Bt traits are the most effective and what factors affect the efficacy of these traits?
Currently, the third-generation Bt technologies with three genes are expected to be most effective for controlling worms. Biotic (living organisms) and abiotic (physical factors such as prolonged cool and wet soil, soil salinity, extreme drought) stress negatively affect plant growth and, subsequently, protein expression in Bt crops.
Can insect populations develop resistance to Bt?
Yes. Research shows the reduced efficacy of some of the Bt traits against cotton bollworms at many locations across the Southern United States. Newer technologies with two or more relatively dissimilar toxins should slow down the rate of resistance development as it is less likely that any one insect will be simultaneously resistant to more than one or two toxins.
Does a Bt cotton field need to be scouted for caterpillar pests?
Yes. None of the current technologies provide 100 percent insect control. Cotton varieties with third generation. Bt have excellent activity against tobacco budworm and bollworm. However, under heavy infestation pressure, supplemental insecticide treatment for bollworms may be necessary. Also, field monitoring is essential because bollworm or fall armyworm populations may develop, particularly on blooms, and also appear late in the season on stressed cotton where the Bt toxin production may be compromised.
When is the best time to begin insecticide treatment?
Thresholds in Bt cotton fields are based on how many caterpillar pests survive to late first or second instar larvae stage, not on newly hatched larvae or the presence of eggs (Fig. 3). Since newly hatched larvae must feed on the plant for the Bt toxin to be effective, delay decision making until you can determine the survivorship of larger larvae (worms). For example, the threshold level before bloom is 30 percent or more damaged squares with worms present. After boll formation, the threshold is 6 percent or more damaged fruit with worms present (Fig. 4).
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