North Carolina Cotton: Plant Bug Scouting, Thresholds Change with Bloom

Tarnished plant bug nymph on cotton bloom. Photo: Virginia Cooperative Extension

When cotton blooms, it’s time to switch sampling and thresholds for plant bugs. This previous article covered why you should scout differently in pre-bloom and post-bloom cotton. There is no magic switch point, but once cotton blooms for a couple weeks, monitoring square retention becomes a less reliable way to make treatment decisions, as does the sweep net.

For this reason, we recommend a threshold of 2-3 plant bugs per drop cloth sample (~0.5 per row foot) during the bloom. During early bloom, consider using both a sweep net and a drop cloth for sampling.

You can visit the cotton scouting guide for detailed information and a video for how to use the drop cloth correctly. Recently published work done in North Carolina supports the fact that net profits will be higher when these thresholds are used.

Remember that plant bugs will feed on larger squares and small bolls during bloom. The feeding on the large squares can result in dirty blooms. You should never treat based on the presence of dirty blooms, but they are an indicator that you should use your drop cloth in the field and scout.

Once threshold is reached during bloom, it would be a good idea to switch away from a straight neonicotinoid product. Options are many, but you could pick up plant bugs and stink bugs with pyrethroids, Othene, and Bidrin. In the Midsouth, they have found that mixing Diamond with pyrethroids has lengthened their spray intervals.

Note we have documented pyrethroid resistance in both the Midsouth and North Carolina; therefore pyrethroids alone should be used with caution, but they will be more effective when tank-mixed with other insecticide classes. Researchers in the Midsouth have also found that back to back sprays within 4-5 days of the first spray has improved control over back to back sprays made one week apart.

The take home for us is to rotate insecticides, tank mix multiple modes of actions among insecticides, and scout more frequently, even twice a week. If you chose to apply Diamond, keep in mind that it is an insect growth regulator and that only immature insects grow. So it will only be active on nymphs, not adults.

If you want to kill adults, you will need to mix in some other knock down product with Diamond. Finally, keep in mind that many of these products are harsh on beneficials. That could mean more spider mites, aphids, or bollworms later in the season.

We have a Section 18 for Transform this year, again. This would be a great insecticide to try in your rotation to manage plant bugs, while preserving beneficial insects. We sprayed Transform (at 2.25 oz) during a bollworm flight during 2018 and had significantly higher yields than untreated plots, while other insecticides did not.

My vote is to save this insecticide for late in the season and to use it at 2-2.5 oz, depending on pressure.

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