Scout Early for Bollworms in Cotton

  • Cotton growers should scout fields as soon as cotton begins to bloom and be prepared to spray an insecticide to protect yield potential if bollworms are observed. 
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  • Transgenic cotton varieties that have two or more B.t. genes have increased efficacies against bollworms; however, under situations of very heavy pressure from bollworm, some B.t. technologies can incur significant injury and loss of yield potential if they are not protected with timely insecticide applications.
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  • Bollworm treatment thresholds vary from state to state. Cotton growers should review their state recommendations before applying insecticides.
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Topic

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Figure 1. Bollworm egg on square.
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A relatively mild winter followed by plenty of rain and good growing conditions have been good for cotton growth and development. These same conditions have been ideal for insect development. Entomologists across the South are encouraging growers to scout early to identify bollworm eggs and larvae as cotton begins to flower. Scouting should continue, at least weekly, as long as bolls have the potential to mature and produce lint.

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"Growers should be scouting for bollworms when they see the first bloom in each field," says Jeremy Greene, Extension entomologist, Clemson University. "Scouts should follow up at least weekly and after any insecticide application to make sure bollworms are being controlled." (Greene, J., 07/15/17) 

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Dominic Reisig, Extension entomologist, North Carolina State University, cautions growers that corn earworms (bollworms) will move from corn into cotton. "There are going to be a lot of (bollworm) moths moving from corn into cotton during July and possibly even persisting into September. Growers should begin checking for eggs once the plant starts blooming and continue scouting as long as they have fruit they want to keep. We detected bollworms early in 2017. Some must have survived over the winter. With some late-planted cotton this year, we have the potential for a fifth generation of bollworms in North Carolina. I want to raise awareness among growers that they need to scout early, and if they find bollworms, they need to be ready to spray to preserve yield potential." (Reisig, D., 07/13/17)

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"The B.t. technology has worked so well that we sometimes tend to take for granted that it will always work," says Phillip Roberts, Extension entomologist, University of Georgia. "We have learned that it is absolutely worth our time to scout, primarily looking for eggs and small worms. If we see bollworms more than 1/4 inch long, we believe they will survive the technology. If we see damage anywhere, we need to search the entire plant. The technologies are still very good, but Bt. cotton is not immune to bollworms and never has been. We've been fortunate in Georgia that we have not had any major issues since we started planting two-gene cotton. But we still need to scout. We do believe the three-gene cotton will be even more active against bollworms. It is also important that we recognize that B.t. cottons still provide excellent control of tobacco budworm." (Roberts, P., 07/13/17)

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Effect

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Figure 2. Small bollworm in white flower.
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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.

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Action

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Figure 3. Small bollworm in square.
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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 Roberts. Scouts should search the top 12 inches of the plants for eggs, larvae, and damage. They should including 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.

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Reisig 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®. If stink bugs or plant bugs are an issue, apply Besiege®. Do not use this strategy if eggs have hatched and second instar larvae are present."

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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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"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 Greene. "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.

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