This week I’ve been seeing the bollworm (H. zea, corn ear worm, etc.) egg lay picking up. The most this week was in a field near Egypt, 24% of the plants had eggs.
When eggs are laid in multiples like in the photo below, only count it as one egg. Bollworms are highly cannibalistic, and only one caterpillar is going to survive being that close together. I’ve seen a few small worms, and a few 3 to 4 day old larvae.
Most of our Bt technology is not working as well as it has been in the past, as we saw last year. It still has some activity on worms, but when we tested last year in our area, the bollworms have resistance to all but the new Viptera trait.
The chart below shows the current and past technologies, and the overlap between them. This overlap has helped select for bollworms that are no longer effectively controlled by the technology.
Even if you have traited cotton, it’s important to be checking for bollworms. There is no threshold set for egg lay (since they have to feed for Bt to effect them and many other insects consume eggs) , but our adapted threshold for caterpillars after last year is 6% damaged bolls or squares with live caterpillars present.
I am using the whole plant inspection method when scouting, and have been looking at 50 to 100 plants in 4 or 5 different places in the field, depending on the field size. I look over the entire plant and count the number of undamaged bolls or squares, and the number of damaged ones.
I am also pulling 10-15 bolls at each spot I check to look for stink bug damage. Damage can entail carpal warts on the inside of the bolls, or brown and damaged lint and seed in the boll. The threshold for stink bugs is 20% of bolls damaged with stink bugs present.
I have been seeing some stink bugs, and a few of the consultants I’ve talked to have picked them up at above threshold levels.
I’ve been picking up aphids in cotton as well, the heaviest population I picked up this week was near Tidehaven school in Matagorda county. Treatment for aphids in cotton is rarely warranted at this growth stage. The threshold is at 40-70 aphids per leaf until bolls begin to crack.
Once we have open bolls, the threshold drops to 10 per leaf, as the honeydew can cause mold to grow on lint and cause problems with harvest. If you know you have aphids and need to spray for either bollworms or stink bugs, try to either choose a more selective chemical for treating your pest to avoid knocking back the beneficial populations and flaring the aphids, or look for an insecticide with aphid activity as well.
A few folks have contacted me about problems with sugarcane aphids in sorghum. At the bottom of the article I’ve included a link to the scouting card for them. Make sure to keep an eye on the pre-harvest interval for insecticide applications on those.