By Dr. Suhas Vyavhare, Extension Entomology Specialist, Lubbock 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 at this stage is around lygus bug and stink bugs.
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 ~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 (d•1/ 4 inch) larvae. This time frame may widen for larger worms but if you are not picking up on 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. They prefer larger bolls (about quarter size diameter) and may need treatment up to 450 DD60 after 5NAWF. 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 shading, variety, irrigation, presence of pathogen (e.g., boll rot) but it shouldn’t change to the extent that we need to keep scouting and spraying going into mid late September or October. As per the recommended threshold, we will have to hit 50% boll damage going into 8th week of bloom to trigger a spray and the 8th week of bloom wouldn’t go beyond August.
We have had instances in the past where stink bug damage ( hard locks) was found in harvestable bolls in October—this scenario would be most likely from the earlier infestations that went undetected.