Scout Early for Bollworms in Cotton

KEY POINTS

  • Fields should be scouted as soon as cotton begins to bloom and an insecticide application may be necessary if bollworms are at or above local thresholds.
  • Transgenic cotton varieties with two or more B.t. genes have increased efficacy against bollworms; however, under very heavy pressure, significant injury and loss of yield potential can occur if the crop is not protected with timely insecticide applications.
  • Bollworm treatment thresholds vary from state to state. Review local state recommendations before applying insecticides.

Causes

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.

Management

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.


Bollworm

Figure 1. Bollworm on a cotton square. Source: Scott Bauer, USDA Agricultural Research Service, Bugwood.org.

 

Terminal and Square Inspection Method

  • Separate the field into four or more sections.
  • Inspect 25 plant terminals for small larvae, 25 green squares and 25 bolls for bollworms and bollworm damage.
  • Make a note of how many squares and bolls were damaged or undamaged.

Whole Plant Inspection Method

  • Separate the field into four or more sections.
  • Inspect the whole plant of three side-by-side plants five times per section.
  • Calculate the percentage of damaged fruit.

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.


Bollworm

Figure 2. Bollworm on a white bloom. Source: Russ Ottens, University of Georgia, Bugwood.org.

 

Identification

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.


Bollworm

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