Cracked Bolls and Heat Units

Howdy yall,

We are back to hot and dry weather this week, and grain harvest is in full swing. We are seeing a good bit of separation in maturity between our irrigated cotton and our dryland now. Some of the earlier planted irrigated cotton I check is at cutout this week, and the dryland fields planted within a day or two with the same variety have cracked bolls already. 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 as 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. 

I've included two bolded lines at the cutoff for bollworms and plant bugs, and for stink bugs. Bolls that will reach maturity by harvest will be too hard for the respective pests to feed on at this stage. A good bit of our dryland cotton is no longer susceptible to insect damage now.

Below is my scouting information for the week of 7/1/2022. We are getting close to being out of danger for insect damage in all of my fields.

Cotton Aphid
Kate Crumley

The threshold for cotton aphids is 50 aphids per leaf, unless you have cracked bolls. Once we have cracked bolls the threshold for cotton aphids is 10 aphids per leaf. If you see aphid mummies in the field (tan or black dry and unmoving aphids), that's a good thing. Parasitoid wasps lay eggs in the aphids, and the aphid forms a mummy while the wasp larvae is pupating inside. These wasps, lady beetles, and lacewings can knock back aphid populations. Treatment for aphids is rarely justified, but I have seen some cotton at threshold this year. If you do decide to treat for aphids, do not use a pyrethroid. Pyrethroids and organophosphates are broad spectrum, and kill beneficial insects as well as your target insect, but pests like aphids bounce back much quicker than their predators do. Their high reproductive rate will allow their numbers to soar after a broad spectrum insecticide application kills all their predators.

Aphid Mummies on Cotton

Kate Crumley

For those of you with cotton still susceptible to bollworm damage, they 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.

H. zea Larvae
Kate Crumley

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. Below is a chart showing the overlap of Bt traits between crops and technologies.

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 plant terminals, checking the upper third of the plant for caterpillars and eggs. I also check lower down the plant, and count eggs while making fruit counts. 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).

H. zea Eggs on Cotton
Kate Crumley

We are still seeing very little stink bug damage in the cotton still susceptible, and few stink bugs (still mostly brown), as well as Lygus and Verde plant bugs (Creontiades). The threshold for verde plant bugs (Creontides) is 20-25 insects per 100 plants, but have not found verde plant bugs or lygus to reach threshold on their own anywhere, so if cotton is squaring I count them as two fleahoppers, and if its after bloom is going, I count them as either one or half a stink bug depending on the boll size. Verdes can feed on bolls up to an inch in diameter. Below are the action thresholds for both as well as photos of the insects.

Verde Plant Bug
Ben Crumley

Lygus Bug Adult
Kate Crumley

Lygus Bug Nymph
Kate Crumley

Evidence of Sucking Insect Damage on Cotton Boll
Kate Crumley

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.

Green Stink Bug adult
Photo: Kate Crumley

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

Sugarcane Aphids in Sorghum
Photo: Kate Crumley

In sorghum we need to be scouting for sugarcane aphid before harvest. I've picked it up in sorghum in all three counties, but right now they populations I've seen have not been blowing up. The locations I've seen treated this year have not had rebounding populations. Here are some links for just sugarcane aphid, and a more recently updated guide for sorghum insect management. Here are links to the threshold calculators for sorghum midge, rice stink bug, and sorghum headworm.

There is also a Women in Agriculture Ranch Tour and Workshop coming up on July 9 with our Prairie View Ag Agent, Braxton Mitchell. Please call the Wharton County Extension Office if you have any questions or check out this link to register.

Please check out our weekly IPM Audio Updates, the website to sign up to receive those is listed below. If you have any questions feel free to contact me either by email or calling the office. Have a good weekend, everyone!

Sincerely,

Kate Crumley

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Cotton Insect Management Guide

Development and Growth Monitoring of the Cotton Plant