South Carolina Cotton: Bollworms on the Rise

Captures of bollworm moths in my pheromone traps increased again this week and might continue up for next week. We will keep reporting the numbers. Moths are in the air, and larvae are easy to spot in some of my plots of non-Bt cotton. There has been reported injury to squares, and bolls.

Caterpillars can be found under the bloom tag feeding on the top of the small boll, which happens frequently in Bt cotton, as the larvae can feed and survive on blooms because of lower concentrations of toxins there. When the bloom dries down, they move into the top of the boll with some size that often allows them to overcome the toxins in the boll.

Take note of the moths flying around in fields you walk, and look for eggs and injury to squares, bolls, and blooms. Use our treatment thresholds of 3 or more large (>0.25 inch) larvae per 100 plants or 5% damage to squares or bolls.

On 2-gene Bt cotton, you can also trigger when egg density pushes past 20 towards 50 eggs per 100 plants. Use 50+ eggs per 100 plants for 3-gene Bt cotton. More detailed recommendations for bollworm can be found in our 2021 Pest Management Handbook.

Most of our cotton is now in the 3rd, 4th, or 5th week of bloom, and stink bugs should be the primary focus of scouting efforts. You should know the first week of bloom for all of your cotton fields. That is when every other plant has its initial flower. That is generally around 60 days after planting, depending on planting date, variety, temperature, accumulation of heat units, etc.

The dynamic boll-injury thresholds for stink bugs are specified by week of bloom. Examine the same size bolls each week – I recommend pulling the largest but softest bolls you can find that will easily mash between your thumb and index finger to open. Examine 25 bolls, at a minimum, for each field, regardless of field size, and add a boll for each acre over 25 acres.

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Aphids can be found in low numbers, but most of the populations have crashed due to the fungus. If you have yet to spray insecticides in your fields, the aphids were there to build up populations of natural enemies that are there now waiting on other pests to show up. This natural control helps us tremendously with bollworm, stink bugs, etc.

Spider mites continue to be held back by the frequent rains.

Plant bugs are becoming less important now as we transition into monitoring for and addressing stink bugs. If your fields have yet to bloom or they are still at first bloom or into the 1st or 2nd week of bloom, you still need to monitor for plant bugs.

Keep plant bugs below 8 per 100 sweeps (or roughly 1 per 10 sweeps to keep the math simple) for pre-bloom sampling or 3 per 5-6 rowft using a black drop cloth post-bloom. If numbers exceed either of these thresholds AND square retention drops below 75%, you probably need to treat for plant bugs.




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