By Suhas Vyavhare, Extension Cotton Entomologist, Texas A&M AgriLife Extension Service
Cotton will benefit from some of the timely rainfall we been receiving recently. However, it is going to make conditions better for survival and multiplication of several pest species as well. In general, the hot and dry weather of West Texas helps to keep bollworm numbers in check (through desiccation of eggs). Humid and cloudy conditions over the last couple of weeks, however, may increase egg hatch rate and worm survival in cotton. In addition, added new growth on plant terminals will help both bollworms and aphids thrive better.
Over the last 10 days, we have spotted many fields with cotton aphid infestations. Aphid colonies are mainly
concentrated to plant terminals but as the numbers build-up, they may move on to the leaves. Overall, beneficial numbers seem to be lower compared to the previous year, but they are present. Isolated showers will also help wash out honeydew and some of the aphids from the plants. If aphid colonies are spotty and mostly restricted to plant terminals, I would wait and monitor the situation over the next few days. Often aphid populations crash out in response to beneficials and rain. Click the link below to access more detailed information on cotton aphids: http://lubbock.tamu.edu/files/2017/07/Cotton-aphid_ENTO074.pdf
I have also come across a few reports of bollworm damage to non-Bt cotton. In our research trials, bollworm damage ranges from 5-6% boll injury in non-Bt cotton and <1% in Bt cotton. The threshold is 6% fruit injury with the presence of live worms in both Bt and non-Bt crop. Among various insecticide options for bollworm control, Diamide insecticides (Prevathon and Besiege) are the most reliable choices. Remember, Besiege contains both a diamide and a pyrethroid so it would be a better choice if stink bugs are present too. However, if aphid colonies are present in the field, the pyrethroid component may flare-up aphids. If a field needs to be treated for both aphids and bollworms, Prevathon can be tank-mixed with any of the commonly used aphidicides such as acetamiprid (Intruder).
Along with the proper insecticide selection, coverage is also important getting the desired level of worm control. In fields with dense plant canopy, it is important to get material down in the lower canopy where worms are in protected places. Air induction nozzles recommended for newer herbicide technologies produce coarser spray which may not penetrate through the dense plant canopy and provide thorough coverage. Penetration through plant canopy can be improved with flat fans or hollow cone tips and by increasing final volume (no less than 10 GPA with a preference of 15 GPA for ground rig). If using an airplane, use at least 5 GPA.