Manage Bollworms and Stink Bugs in Cotton

Cotton growers should be scouting for stink bugs and bollworms and applying insecticides when either of these pests reach thresholds. Cotton is particularly susceptible to injury and loss of yield potential from stink bugs during the third through the fifth week of bloom. Scouting for bollworm eggs and larvae should continue at least weekly from first bloom and as long as young bolls have the potential to mature and produce lint.

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As long as bollworms are identified and controlled before they exceed 1/4 inch in length, they should not have an impact on cotton yield potential. However, small worms can develop rapidly. Once a bollworm enters a square or a boll, they can be difficult to nearly impossible to control with insecticides. This makes early scouting, bollworm identification, and insecticide applications, if bollworm numbers exceed thresholds, extremely important. Scouting thresholds vary somewhat from state to state. However, all now recommend beginning scouting by first bloom.

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"We use a modified whole-plant search in Georgia," says Phillip Roberts, Extension entomologist, University of Georgia. “Scouts should search the top 12 inches of the plants for eggs, larvae, and damage. They should include one bloom, one bloom tag boll and an additional boll lower in the canopy on the same plant. If any damage is observed, the entire plant should be searched. We recommend treating if we see eight 1/4 inch larvae per 100 plants," Roberts says. Roberts, P., 07/13/17)

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Dominic Reisig, Extension entomologist, North Carolina State University, encourages two scouting options in North Carolina, preventative and reactive. "Begin scouting cotton leaves and squares (focusing on bracts) for eggs starting in mid-July," says Reisig. "Eggs can be laid on any plant part, but are most often found on leaves and squares, especially near blooms. If there are 25 eggs on 100 terminals, stems or fruit (squares, bloom, bloom tags, and bolls) and if stink bugs or plant bugs are not an issue, apply Prevathon® insecticide. If stink bugs or plant bugs are an issue, apply Besiege® insecticide. Do not use this strategy if eggs have hatched and second instar larvae are present." (Reisig, D. 07/13/17)

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"Our reactive strategy is focused on finding second instar larvae," Reisig says. "Growers should only apply an insecticide when they find three live larvae on 100 plant parts (squares, blooms, bolls, and boll tags) in one scouting trip, or two second stage bollworms or larger in 100 fruiting tissues on two consecutive scouting trips, or one second stage bollworm or larger in 100 fruiting tissues on three consecutive scouting trips."

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Week of BloomThreshold % Internal Boll Damage
1 50
2 30
3 10
4 10
5 10
6 20
7 30
8 50
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Table 1: Stink Bug Treatment Thresholds113;10;

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"The advantage of this strategy is that it allows B.t. cotton to do its job by killing all the tobacco budworm larvae that are newly hatched from eggs. The disadvantage is that it has the potential to let bollworms get a foothold. Once bollworm larvae obtain some size (third instar or larger) and move into squares and bolls, they are extremely difficult to control. There are a number of insecticides noted for bollworm control in state recommendations," Reisig says.

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"Bollworm eggs will begin hatching about three days after they are laid," says Jeremy Greene, Extension entomologist, Clemson University. "By mid-July or later in South Carolina, moths may deposit a higher percentage of their eggs lower on the plants. Scouts should check whole plants for bollworm eggs and larvae. They should look at a white bloom, a pink bloom, and the two smallest bolls. They should also remove bloom tags to look for damage on the tips of small bolls where bollworm larvae often gain entry. An insecticide should be applied when three or more worms (larger than 1/4 inch) are found per 100 plants or if 5% of small bolls are damaged," Greene says. (Greene, J. 07/13/17)

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To scout for stink bugs, pull a random sample of quarter-sized diameter bolls (one boll/acre and no less than 25/field). Sort bolls into two piles: those with and those without obvious external lesions. Crack and inspect bolls with external lesions for internal damage (boll warts, stained seed or lint). If the threshold is not met for that week of bloom, check the remaining bolls for damage. Treat the field only if the threshold is met for that week.

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Source:1Bacheler, J., Herbert, D.A., Greene, J., Roberts, P., Toews, M., Blink, E., and Smith, R. Scouting for stink bug damage in cotton: description and use of a pocket scouting decision aid. www.cottoninc.com. Web source verified 07/27/2017. 170731114525

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