Scouting for Bollworms in Cotton

Growers should scout fields as soon as cotton begins to bloom and apply an insecticide if bollworms are observed to help maintain yield potential.

Identification

Larvae are 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 1).

In corn-growing regions, bollworm moths will typically deposit eggs in cotton fields after emergence in nearby cornfields. Eggs are deposited on upper leaves and hatch after three days. Eggs and larvae may be found anywhere on the plant, especially on blooms or young bolls.


stinkbug

Figure 1. Bollworm on a cotton boll. Peggy Greb, 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 they can hide larvae.

Threshold Levels

Cotton Stage Action Threshold
(Bt and non-Bt Cotton)
Before Bloom 8 or more worms per 100 plants
After Boll Formation 6 or more damaged squares and/or bolls and worms present
Fields with 350 DD60s beyond 5 NAWF are no longer susceptible to first or second instar bollworm larvae. Action threshold should be adjusted according to yield potential and production system (dryland vs irrigated).