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A relatively mild winter followed by regular precipitation and good growing conditions encourage a cotton crop produce lush young growth. These same conditions are ideal for increased bollworm populations. In corn-growing regions, bollworm moths will typically deposit eggs in cotton fields after emergence in nearby cornfields. Bollworm moths can deposit eggs on either side of young leaves in the top third of a cotton plant. In Bt cotton, moths may deposit eggs more often on blooms or bloom tags, as these plant tissues may have a lower concentration of the Bt toxin.
Entomologists across the cotton-growing region recommend scouting cotton for bollworm eggs and larvae at the beginning of flowering. Scouting should continue, at least weekly, as long as bolls have the potential to mature and produce lint.
For Bt cotton, examine the whole plant for larvae and damage. Include squares (Figure 1), white blooms (Figure 2), pink blooms, bloom tags, and bolls (Figure 3). Treatment may not be necessary if only eggs are found as the larvae must feed on cotton to receive a toxic dose.
Figure 1. Bollworm on a cotton square. Source: Scott Bauer, USDA Agricultural Research Service, Bugwood.org.
Terminal and Square Inspection Method
Whole Plant Inspection Method
For both methods, be sure to remove and check bloom tags as larvae may be hidden from view.
Chemical control may be necessary if local thresholds are met. Check extension recommendations for action thresholds based on cotton growth stage. Use a more selective insecticide like those in the diamide, oxadiazine, and spinosyn classes to prevent a secondary pest outbreak. Control is more effective when worms are young and found on the upper third of the plant.
Figure 2. Bollworm on a white bloom. Source: Russ Ottens, University of Georgia, Bugwood.org.
Moths. Cream in color and medium in size. Under heavy infestations, moths can be found frequently during the day. If cotton is slow to produce new leaf tissue, like in stressful conditions, moths may deposit eggs on the lower portion of the plant or during high temperatures and low humidity.
Eggs. Round, pearly white to cream in color and roughly half the size of a pinhead. Eggs will turn light brown prior to hatching in three to four days.
Larvae. Roughly 1.5 inches long and can vary in color from light green, pink, or brown to black with stripes running along their back (Figure 3). Larvae begin feeding on young leaves, leaf buds, and small squares at the top of the plant before moving down the plant to feed on larger squares and bolls.
Figure 3. Bollworm damaging a boll. Source: Clemson University— USDA Cooperative Extension Slide Series, Bugwood.org.
Sources: 1 Bollworm and tobacco budworm. Texas A&M AgriLife Extension. Cotton Insect Management Guide. https://cottonbugs.tamu.edu/. 2 Experts: Cotton farmers may face increased bollworm pressure this season. 2018. Southwest Farm Press. http://www.southwestfarmpress.com/. 3 Michaud, J.P. 2013. Cotton insects. Kansas State University. https://entomology.k-state.edu/. Web sources verified 07/12/18. 170709153118