Open Bolls and Stink Bugs

Open Cotton
Ben Crumley

Howdy,

Our grain crops are mostly out of the field. There are spots we've been delayed by rain, but for the most part combines are running and we are getting corn and sorghum out of the field. Sorghum yields have mostly been poor due to sprouting and the wet conditions we had earlier, but our corn yields have mostly been fair to good.

This week most of our cotton is starting to crack, bloomed out, or around 2-5 nodes above white flower (NAWF), and our later replanted cotton is at or just past cutout. Cutout occurs when carbohydrate supply equals demand, and vegetative growth ceases. When the plant reaches cutout, no more harvestable fruit is set. This is normally at 4 to 5 NAWF. I've included heat unit charts below to help with spray decisions for our insect pests our fields reach cutout. Temperature data for each county location is based on a field close to the center of each county. Cotton is no longer susceptible to economic damage by plant bugs and bollworms at 350 degree days (DD60), or heat units, past cutout, and is no longer susceptible to economic damage from stink bugs at 450 DD60 past cutout. Bolls that will reach maturity by harvest will be too hard for the respective pests to feed on at this stage.

The fields I check are almost all out of risk of damage by bollworms and stink bugs, we have one still susceptible to bollworms, and two to stink bugs. We're still mostly seeing brown stink bugs if we find any, but we are seeing some green stink bugs, leaf-footed bugs, lygus bugs, and tarnished plant bugs, all below threshold or in places that are no longer susceptible to damage. This week's scouting results are below. Fields marked in black are no longer susceptible to damage from that pest.

Helicoverpa zea is our cotton bollworm and corn earworm. These are caterpillars that feed on multiple crops and vegetables. In cotton they feed on squares and bolls, causing fruit loss. The last few years we had high numbers of this insect in our Bt cotton. As the corn matures, the next generation of bollworm eggs will be laid in cotton. I start looking for bollworm eggs in cotton when that field starts blooming. We are finding a few larvae now, but our survivorship in cotton has been low compared to previous years.

Small H. zea Larva on a Cotton Boll
Kate Crumley

Large H. zea Larva
Kate Crumley

Our scouting guide recommends checking in the top 1/3 of the plant, but it's important to also pull bolls and flowers from lower to check as well. Egg lay has been occurring low in the plant as well, and it's not uncommon to find small larvae or eggs in flowers and bloom tags. We also have seen small larvae feeding and entering bolls either on the very tip of the boll, or along the seams of the bolls. This makes the entry wounds more difficult to find, but the damage is easy to see if you pop open the bolls.

Our Bt traits overlap across corn and cotton. If the caterpillars survive the traits on corn then as adults fly to cotton to lay eggs, it's likely their offspring will survive the same traits on cotton. The chart above shows the overlap of Bt traits between crops and technologies.

H. zea Eggs on Cotton
Kate Crumley

To scout for cotton bollworms I use the terminal and square inspection method. I make about four stops in a field, more if the field is larger than 100 acres. At each stop, I look at 25 plants, checking the entire plant, including blooms and under bloom tags, for caterpillars and eggs. I also pull 25 half grown or larger green squares to bolls and look for bollworm damage. When documenting egg lay, if I find more than one on a leaf, I only count it as one. This caterpillar is highly cannibalistic, and generally only one caterpillar will result from eggs too near each other. The economic threshold for bollworms is 6% damaged bolls with live caterpillars present. In areas like ours on the upper gulf coast with documented Bt failures, the threshold for eggs on single and dual gene cotton is 20% (20 plants out of 100 with at least one egg). We can stop scouting for bollworms at 350 DD60. Bolls with slight dark indentations like the photo below could be chewing damage from bollworms. Look closely at dark spots to see what they're from. Early superficial damage like the photo below is unlikely to cause fruit drop, but if the caterpillars survive or get through the carpal walls, it can quickly become a problem.

Chewing Damage on Cotton Boll
Kate Crumley

Evidence of Sucking Insect Damage on Cotton Boll
Kate Crumley

When scouting for stink bugs, check the inside of the bolls for warts, lesions, and stained lint. Above is a photo of a boll with potential stink bug feeding damage from the outside, note the slightly raised look of the dark spots. Be sure to open the bolls to confirm it is damaged, other sucking plant bugs may be unable to get through the carpal walls, and the inside will be clean. The economic threshold can be found below, depending on how long the field has been blooming. It is based on the percent damaged bolls with live bugs present. This year we've mostly seen brown stink bugs in cotton so far, and some of the brown stink bug populations in our area have been shown to have some resistance to pyrethroids. We can stop scouting for plant bugs at 350 DD60 past cutout, and stink bugs at 450 DD60.

Green Stink Bug adult
Photo: Kate Crumley

Carpal Wall Warts from Stink Bug Feeding Damage
Photo: Kate Crumley

If you've got soybeans, we need to be scouting for stink bugs right now. The red banded stink bug thresholds are now 4 bugs per 25 sweeps for R2-R6, and 10 bugs per 25 sweeps for R6.5-R7, unless we have rainy and humid conditions, then we should continue checking through R8. The threshold for red-shouldered, brown, green, and southern green stink bugs is 9 bugs per 25 sweeps for R2-R6, and 20-25 sweeps after R6, and we can stop checking at R6.5. Since color can vary on stink bugs, the best way to check if you are unsure if you are finding red banded stink bugs is by looking at the underneath of the insect. A red banded stink bug has a large spine just below the legs, green stink bug has a short one, and the other species do not have a spine at all. I've seen mostly brown stink bugs, but some of the consultants I've spoken to have picked up greens and red banded stink bugs as well.

Red Banded Stink Bug, showing ventral spine
David Kerns

Green Stink Bug, showing short spine
David Kerns

Soybean Looper

If you've got later planted soybeans, they can handle 30-35% defoliation prior to bloom, but most of our beans are farther along. During bloom and pod set, defoliation should not exceed 20-25%. The threshold for caterpillars in soybeans is 300 worms per 100 sweeps, but count soybean loopers as 2 worms if they're present. 

Our office is still getting calls on fall armyworms in pasture and hay. Fall armyworms are at treatable levels in pasture and hay fields at 2-3 worms 1/2 an inch long or larger per square foot. You can apply pyrethroids to control caterpillars, and these will work within 2-3 days, but they also have a very short residual control period. You can add a diflubenzuron (Dimilin) to a pyrethroid to extend the residual to 10-12 days. Methoxyfenozide (Intrepid) is another product available, and it can provide up to 7 days residual control.  However, these options are not rainfast, and will lose effect if rained on. Products containing chlorantraniliprole (Prevathon, Vantacor, Besiege, Elevest) are rainfast at about 2 hours after application, since they are absorbed by the plant. These are more expensive products, but will provide longer residual control. For example, a 20 oz application with Prevathon will provide 20-21 days of control. As always, when selecting and making pesticide applications, read the label before making a decision.

There have also been some questions recently on bermudagrass stem maggots. We saw this species last year as well, and one of my ag agents put together a publication on those covering their biology and control options. Check out the publication here if you have questions on those.

Check out our weekly IPM Audio Updates, the link is below. As always, if you have any questions feel free to contact me either by email or calling the office. Have a good weekend everyone!

Check out our weekly IPM Audio Updates

PGR Management Considerations

Plant Growth Regulators as Tools for Challenges

Cotton Insect Management Guide

Development and Growth Monitoring of the Cotton Plant

Fall Armyworms in Pasture

Bermudagrass Stem Maggot