South Carolina Cotton: Bollworms Demand Attention But Not Necessarily Treatments

Zea Moth, Bollworm. Photo: Kate Harrell, Texas AgriLife

Captures of bollworm moths in pheromone traps have decreased some, as this flight out of corn subsides. The recent widespread rain storms will wet the soil and undoubtedly allow more moths to emerge from corn acres.

The generation that has cycled through early cotton and other hosts after corn will likely make another showing as moths also. So, there might be another round with bollworm in another week or two. We will see.

Continue to watch 2-gene Bt cotton. I observed threshold levels of square and boll damage this week in 2-gene Bt cotton, so the pressure is there with the egg lay and hatched out larvae.

The Bt technology is getting most of them, but some are going to make it through, despite our best efforts. Eggs deposited on blooms that dry in a few days produces bollworms that “size up” on blooms with reduced toxin expression and often make it into boll tips under bloom tags. Control of these caterpillars is difficult, and sprays must be timely.

Any counts of bollworm eggs at or exceeding 20 eggs per 100 plants should get your attention. If 3 or more larvae are found per 100 plants or damage to bolls exceeds 5%, treatment thresholds have been met. Most treatments applied during the first couple of weeks of bloom will likely provide the best control of bollworm escaping Bt toxins.

We are officially almost a week into “stink bug month” – August! I am seeing more stink bugs in the crop, and they should be the focus of insect management efforts in cotton from here to the end of the insect season.

I hope you know in what week of bloom each of your fields are this week. Proper use of our dynamic boll-injury threshold requires you to know what week of bloom you are in for each field. As I have mentioned before, generally, cotton starts blooming about 60 days after planting.

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Drake Perrow, crop consultant in Calhoun County, sent me a text earlier this week with comments about sprays missing some bollworms and stink bugs in cotton. Obvious reasons might be choice and efficacy of products used, resistance development, and others, but he was wondering if folks might be spraying insecticides through tips that produce very coarse droplets designed for new herbicide technologies.

That could certainly be part of the problem – using low-drift spray systems to deliver insecticides. We want small droplets to fog in on insects and penetrate the canopy, but we have to minimize drift of herbicides. This is an issue, for sure, if both herbicides and insecticides are going out on the same pass using coarse droplets.

Because input costs are a real concern this season, I understand minimizing trips across fields, but just realize that you might be putting your insecticide at a disadvantage with these tips. Expect some target insects to be missed.

Fleming McMaster, local crop consultant, asked a great question about non-irrigated cotton that has quit blooming, with several weeks yet to go. The question was about what to do if rains come and “restart” blooming and setting of fruit – should those bolls be protected from stink bugs, and what week of bloom do you call it?

Fantastic question. Under those circumstances, you can likely pick up where you left off on week of bloom, with a pause on the dynamic boll-injury threshold.

In the lower half of the Coastal Plain in SC, we can have blooms through August and into the first week of September turn into bolls that significantly contribute to yield.

This gets pushed back into August some for the Pee Dee Region and Upstate. Termination rules for insecticide use get a little fuzzy when Mother Nature sends hot, dry weather our way…we have to adjust for our dryland fields.

We can keep irrigated fields on track and manage stink bugs without this potential “pause” in the threshold using week of bloom. This is why having a consultant think and worry about your crop is well worth the money.

Soybean Insects

We are still in a pattern in soybeans where we have many different species and no one species is taking over the field. Populations of soybean looper (SBL) are picking up, but there are many green cloverworms (GCW) out there also.

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Small GCW “loop” when they crawl and look just like SBL. You have to look very closely with magification to properly identify the two. Costly materials are needed for control of SBL, but you can control GCW easily with a pyrethroid.

Populations of podworm are low in most soybeans I have checked. We mostly have the defoliating complex of SBL, GCW, and VBC. Watch defoliation, as it is increasing. Don’t let defoliation exceed 30% before mid-bloom or 15% after that. Estimate defoliation at least weekly, as things can change quickly. Remember how quickly VBC defoliated fields last year? It was inside of a week.

Check twice per week, if you can. Use a sweep net or a drop cloth to make counts of insects to see what species you have, as insecticide choice depend on proper identification of species. I am seeing hatch outs of stink bugs now in soybeans that are setting pods.

Eggs deposited by adults moving in recently are hatching. Thresholds for all important species are in the Pest Management Handbook. Be able to recognize larvae and moths! Use the chart here for identifying adults and larvae.




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