Texas Upper Coast IPM: Bollworms and Stinkbugs

Bollworm moth - corn earworm cotton square. Photo: Kate Harrell, County Extension Agent- Integrated Pest Management

Several stink bug species feed on bolls in Texas cotton fields, and I’ve seen scattered stink bugs in all of my counties. Our primary stink bug species are the southern green stink bug, followed by the green stink bug, and brown stink bug.

They are strong flyers and can move into cotton from corn, grain sorghum, soybeans, and various alternate hosts. Stink bugs have piercing-sucking mouthparts and damage cotton by piercing the bolls and feeding on the developing seeds. Stink bug infestations can cause substantial economic losses through reduced yield, loss of fiber quality, and increased control costs.

Stink bugs favor medium-sized bolls, but they can feed on any size boll. Stink bugs may feed on bolls 25 or more days old, but bolls of this maturity are relatively safe from yield loss. Their feeding on young bolls (less than 10 days old) usually causes the bolls to shed. In larger bolls, stink bug feeding often results in dark spots about 1/16 inch in diameter on the outside of bolls.

These dark spots may not always correlate well with the internal damage—callus growths (or warts) and stained lint. There may be several spots on the outside of a boll without internal feeding damage being present. Damage to the internal boll wall is a good indication that lint and seed are affected.

Excessive stink bug feeding causes reduced yield, stained lint, poor color grades, and reduced fiber quality. In addition to direct damage, stink bug feeding can transmit plant pathogens that cause boll rot.

Stink bugs are difficult to scout, especially in tall, vigorous cotton. Adults tend to group together, and the distribution of stink bugs within a field may be highly concentrated, particularly along field margins.

Use any of the sampling techniques such as visual inspection, drop cloth, and sweep net for scouting. Recent research by entomologists at the University of Georgia and Clemson University suggests that decisions to treat for stink bug infestations are best made based on the percentage of bolls with evidence of internal damage (warts or stained lint associated with feeding punctures).

To use this technique, remove about 10 to 20 bolls, one inch in diameter (about the size of a quarter), from each of four parts of the field, avoiding field edges, and break open the bolls by hand or cut them with a knife. Look for internal warts on the boll walls and stained lint on the cotton locks. 

Click Image to Enlarge

Click Image to Enlarge

Check bolls with visible external lesions first to determine if the internal damage threshold has been met, since bolls with external lesions are more likely to also be damaged internally. The action threshold is 20 percent or more damaged quarter sized bolls with stinkbugs present.

I want everyone to be on the lookout for bollworms once your cotton starts to bloom. My field scout found 2nd instar larvae this week, and I have seen moths moving around more in cotton.

The egg lay has begun picking up in all three counties this week as well. Last week the bollworm eggs were fewer and farther between, but this week they are picking up, especially in Jackson county. The egg lay near La Salle was at 17 eggs per 100 plants.

To scout for bollworms in Bt cotton, search the entire plant for larvae and injury. A proper sample includes squares, white blooms, pink blooms, bloom tags, and bolls. Reduce the scouting intervals to 3 to 4 days during periods of increasing bollworm egg-laying, especially during peak bloom.

The presence of eggs alone should not trigger treatment since hatching larvae must first feed on the cotton plant to receive a toxic dose.

To use the terminal and square inspection method, divide the cotton field into four or more manageable sections, depending on the size of the field. Examine 25 plant terminals (on the upper 1/3) of the plant), selected at random from each quadrant, for small larvae and eggs. Examine 25 half-grown and larger green squares, small, medium, and large bolls for bollworms and their damage as well.

Keep track of the number of undamaged and damaged squares and bolls, and select fruit at random, not deliberately choosing flared or yellow squares in the sample. Pay attention to bloom tags and petals stuck to small bolls, as they can hide larvae burrowed into the tip of the boll.

For the whole plant inspection method, once again divide the cotton field into four or more manageable sections, dependent on field size. Make whole plant inspections of five randomly chosen groups of three adjacent cotton plants in each section. Look in every square, bloom, and boll.

Thoroughly inspect dried blooms and bloom tags attached to small bolls. Count the number of undamaged and damaged fruit and calculate the percentage of damaged fruit.

Managing Cotton Insects in Texas, pg 14.  Click Image to Enlarge

Thresholds in Bt cotton fields are based on how many worms survive to late first or second instar larval stage, not on the newly hatched larvae or on the presence of eggs. Since newly hatched larvae must feed on the plant for the Bt toxin to be effective, base treatment decisions are made on damaged fruit and the presence of larvae.

Insecticides in the diamide, oxadiazine, and spinosyn classes are more selective than the pyrethroid and carbamate classes. See the tables below for suggested insecticide options.

Managing Cotton Insects in Texas, pg 31.  Click Image to Enlarge

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