You can do everything right early in the season to establish a strong cotton crop, but cotton bollworm pressure in July and August can quickly undo a lot of that effort.
Increased pest pressure and reduced susceptibility to Bt traits in some areas have complicated cotton bollworm control, requiring a more active approach as the season progresses.
While Bt cotton still provides important protection, it works best as part of a broader strategy. Consistent scouting, awareness of economic thresholds and timely interventions all play a role in protecting yield and fiber quality during this critical window.
Cotton bollworm identification and life cycle
Cotton bollworm (Helicoverpa armigera) is an economically important caterpillar pest in cotton, with 1% to 1.5% yield losses in some regions.1,2 Accurate, early identification is critical. Distinguishing bollworm eggs and small larvae from other caterpillar pests, like budworms, ensures targeted management.
The adult cotton bollworm moth lays eggs on various parts of the plant, and once larvae emerge, they can quickly move into squares and small bolls where feeding begins. Eggs can hatch in as little as three days, and the full life cycle can be completed in about a month under favorable conditions.
Larvae vary in color from green to brown as they develop, but size is the more important factor for control decisions. Early, less destructive instar larvae are easier to control. As they grow, larvae can move deeper into fruiting structures, where they are less exposed to treatments.3
Scouting and economic thresholds
To stay ahead of cotton bollworms, consistent scouting and adherence to economic thresholds are essential. During peak summer pressure, scout at least weekly, increasing to every two to three days as activity builds. Focus on plant terminals, squares and developing bolls where eggs are laid and early feeding begins.4
Before bloom, the economic threshold for non-Bt cotton is ten eggs or five small worms per 100 plants between July and August. For Bt cotton, like TwinLink, it’s ten worms per 100 plants.5 However, economic thresholds can vary by region, crop stage and trait package.
Thresholds for bollworm before bloom are often based on higher larval counts, while during bloom and boll development, treatment is typically justified at around 4 larvae (≥1/8 inch) per 100 plants or 6% fruit injury in Bt cotton.6 In some systems, egg thresholds near 10–20 eggs per 100 plants may also be used during early bloom to anticipate pressure. These stage-dependent thresholds reflect the need to target damaging larvae while preserving beneficial insects and delaying resistance.
Bt resistance in transgenic cotton
Bt traits in cotton have significantly reduced reliance on insecticide applications and continue to provide an important foundation for bollworm control. However, in some regions, reduced susceptibility to Bt traits has been observed, resulting in surviving or “escaped,” larvae.
Bt cotton should still be scouted regularly to confirm performance. Monitoring for live larvae in squares and bolls helps determine whether additional control measures are needed. Observing nearby refuge areas, such as non-Bt cotton or alternate host plants, can also provide insight into local pressure.7
Insecticide selection for cotton bollworm control
When economic thresholds are reached, insecticide effectiveness depends on both product selection and targeting the correct life stage. Applications aimed at early instar larvae are generally more effective and help prevent damage before larvae move deeper into squares and bolls.
When selecting products for cotton bollworm control, consider efficacy, residual control and impact on beneficial insects. A range of insecticide options are available, including products such as Fastac® and other labeled solutions.
Cultural practices and integrated pest management
Cultural practices such as managing nitrogen levels and consistent irrigation to avoid excessive vegetative growth can make fields less attractive to an egg-laying bollworm moth. These practices support, but do not replace, scouting and treatment decisions. When combined with thresholds and timely applications, they contribute to a broader integrated pest management strategy that helps you maintain cotton bollworm control throughout the season.
Experts are available to help you make your insecticide decisions. Reach out to your seed retailer, a nearby extension office agent or a professional like your regional BASF representative.
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Endnotes
- https://ipmdata.ipmcenters.org/documents/pmsps/PMSP%20Southern%20Plains%202024%20October%2031.pdf, https://www.cotton.org/beltwide/proceedings/2005-2022/data/conferences/2006/pdfs/1135-1150.pdf
- Williams, Michael R. Cotton Insect Loss Estimates—2005. Proceedings of the Beltwide Cotton Conferences, National Cotton Council of America, 2006, pp. 1135–1150. https://ipmdata.ipmcenters.org/documents/pmsps/PMSP%20Southern%20Plains%202024%20October%2031.pdf
- Barlow, V. M., et al. Cotton Bollworm. University of California Agriculture and Natural Resources, UC IPM Pest Management Guidelines: Cotton, UC ANR Publication 3444, revised continuously, https://ipm.ucanr.edu/agriculture/cotton/cotton-bollworm/#gsc.tab=0
- Zukoff, Anthony, and Logan Simon. Mid-Season Insect Management for Cotton Production. Kansas State University Agronomy eUpdate, 11 July 2024,https://eupdate.agronomy.ksu.edu/article/mid-season-insect-management-for-cotton-production-599-5.
- Zukoff, Anthony, and Logan Simon. Mid-Season Insect Management for Cotton Production.
- Towles, Tyler, Don Cook, and Whitney Crow. “Bollworm Management in Soybeans and Cotton.” Mississippi Crops, 15 July 2025,Bollworm Management in Soybeans and Cotton.
- Stewart, Scott D. Bt Cotton. University of Tennessee Extension,https://utcrops.com/cotton/insects-and-mites/biological-control/bt-cotton/.
