Check your Bt for Bollworms

Howdy everyone,

Bollworm Egg
Photo: Kate Harrell

  I've looked at cotton past cutout (5 nodes above white flower) in all three of my counties this week, this season has been flying by. I've been seeing the bollworm (H. zea, corn ear worm, etc.) egg lay and some larvae. The most larvae we found this week was near Elm Grove, with 5 small caterpillars on 100 plants. I spent a good bit of time in a field trial location this week, and was finding a few live caterpillars in everything but the Viptera traited cotton.

  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 Bt 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. When cotton reaches cutout (5 NAWF), it has 350 heat units remaining before the bollworms can no longer cause yield loss.

Large Cotton Bollworm in Cry 1Ab + Cry 2Ac Cotton
Photo: Kate Harrell

Treatment options for Cotton Bollworm in Cotton
Managing Cotton Insects - Page 31

  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. Once cotton has reached 450 heat units past cutout, treatment for stink bugs may no longer be necessary, as the bolls they can still feed on will not mature in time to contribute to the overall yield. Fields that experienced drought stress this year may have a longer time frame for yield loss to occur, and stink bugs may still need to be a consideration until 450 heatu units past 3 or 4 NAWF.

Carpal warts from stink bug feeding
Photo: Kate Harrell

  I'm still seeing some aphids in cotton as well. 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.

Minute Pirate Bug eating an Aphid
Photo: Kate Harrell

 I hope everyone has a safe grain harvest!

Sincerely,

Kate Harrell

For more information check out:

Texas Row Crop Newsletter

Managing Cotton Insects in Texas

Sugarcane Aphid Scouting Card