As we approach the end of August, one question that has been received lately more often than any other is at what stage do I stop worrying about insects in cotton?
The major insect activity that I am noticing in South Plains cotton at this stage is around lygus bug and stink bug.
Stop sampling and treating for lygus bug when cotton accumulates 350 DD60 (degree days 60) beyond five nodes above white flower (NAWF). The window of vulnerability for developing cotton bolls to lygus bug damage is significantly less after bolls accumulate 250–300 heat units or 8 days post-anthesis when the carpel wall has become sufficiently thick and mature that probing activity is less damaging.
Lygus bugs will continue to feed on squares and small bolls in the plant terminals even beyond 5NAWF+350 DD60 but those fruiting structures will not contribute significantly towards the yield (not to the extent to offset the cost of an insecticide application). Same applies to bollworms. Fields that have reached 5NAWF+350 DD60 are no longer susceptible to small (≤ 1/4 inch) larvae.
This time frame may widen for larger worms but if you are not picking any bollworm activity at this stage, it is very unlikely they will appear from somewhere and reach damaging levels anymore.
Stink bugs, on the other hand, will have to be scouted for a bit longer. Although they favor medium-sized bolls, they can feed on any size bolls. Stink bugs may feed on bolls 25 or more days old; bolls of this maturity are relatively safe from yield loss. In larger bolls, stink bug feeding often results in dark spots about 1/16 inch in diameter on the outside of bolls.
These dark spots do not always correlate well with the internal damage—callus growths or warts. Once the cotton reaches 450 DD60 beyond 5NAWF, sampling and treating for stink bugs may no longer be necessary since bolls produced after this point will not become fully mature or contribute significantly to the crop yield.
Now, it is possible that this value may shift slightly due to factors such as boll shedding, variety, irrigation, presence of pathogens (e.g., boll rot). Action threshold for stink bugs is based on percentage of bolls with internal injury. For instance, the stink bug threshold during 5th, 6th, 7th, and 8th week of bloom is 10% 20%, 30%, and 50% internal boll damage (lint staining, boll warts), respectively.
Link to updated cotton insects guide: here.
