Protect yield and quality from cotton insect pests.
By Carroll Smith
The boll weevil was a legendary cotton pest that tortured farmers for years. “Wanted” posters featuring the insect’s “mug shot” adorned the walls of many farm offices in South Texas. And songs were even written about the insect, including Brook Benton’s version in 1961. In this adaptation, the boll weevil taunts the farmer, “You better sell your old machines. ‘Cause when I’m through with your cotton, you can’t even buy gasoline.” The boll weevil wreaked destruction of epic proportions on a cotton crop.
Thanks to the Boll Weevil Eradication Program, this pest is not longer the nightmare that it once was. Many younger-generation cotton farmers have never seen a live boll weevil. Although this pest has faded away, others have stepped in and must be monitored and controlled to protect cotton yields and quality.
In the Southeast, thrips are the primary pests that attack young cotton early in the season. Jeremy Greene, Clemson University professor of entomology, says, “This pest group is becoming more of a concern in cotton because we don’t have a lot of options to manage these insects, particularly if we lose the neonicotinoids that are delivered as a seed treatment. However, aldicarb is coming back under a different trade name, so we may have that control option next year.”
In looking beyond this early season pest, Greene says the next insects to watch for in the Southeast are aphids and plant bugs, although their presence infrequently requires action in the region. As the cotton begins to bloom and set bolls, fields are monitored for spider mites, bollworm and stink bugs.
“Spider mites are becoming more problematic every year,” Greene says. “Although we have miticides to manage those arthropods, treating for spider mites represents an unplanned expense for most of our growers. We continue to monitor for bollworm to make sure the Bt technologies are still efficacious. Bt proteins don’t control all lepidopterans (caterpillars), but they get most of them.”
The No. 1 group of insect pests in the Southeast after the early threat from thrips is the stink bug complex – predominantly brown, green and southern green stink bugs.
“Just about every acre is sprayed or needs to be sprayed for stink bugs,” Greene says. But the trick is determining when to spray. “We developed a dynamic threshold that changes by week of bloom, refining the static threshold used for years. In the first week of bloom, the threshold is 50 percent of bolls that show at least one feeding symptom from stink bugs. Symptoms appear as warts or callus growths or some type of associated feeding puncture in which there is associated damaged or stained lint. Because there are subtle differences in the region, some states don’t have a threshold for the first week of bloom.
“The second week of bloom is 30 percent, the third, fourth and fifth week drops to 10 percent, then goes back up to 20 percent in the sixth week. The threshold returns to 30 percent in the seventh week and 50 percent or higher in the eighth week of bloom. These are the numbers that we recommend for use in South Carolina each week to determine whether to treat for stink bugs.”
Stink bug scouting recommendations include pulling the appropriate size bolls and opening them by hand to assess feeding symptoms on the internal boll walls.
“Refer to our guidelines for determining the appropriate size boll so you won’t be looking at old damage each week,” Greene says. “We also recommend pulling at least 25 bolls per field for a field that goes up to 25 acres. If you have a 10-acre field, you still need to pull 25 bolls. We recommend adding a boll for every acre that goes above 25. So if you have a 50-acre field, look at 50 bolls minimum. You can monitor the effectiveness of insecticide applications over time based on feeding symptoms that show up in the bolls. To control stink bugs in South Carolina, we recommend spraying a pyrethroid instead of an organophosphate (OP) or carbamate.”
Although pyrethroids also are harsh on beneficials, Greene points out that these insecticides do provide residual control of any bollworms that escape the Bt technology.
“The pyrethroid application can also be spiked with an organophosphate, such as acephate or Bidrin or even the carbamate Vydate, but I wouldn’t recommend an OP or carbamate by itself,” he says.
Texas South Plains
Insect pests and pressure vary across the cotton-producing regions of Texas. In the South Plains, Suhas Vyavhare, cotton Extension entomologist, Texas A&M, Lubbock, says the major early season threat comes from thrips. Although most farmers use seed treatments that last about two to three weeks from planting, they sometimes have to come back with foliar insecticide applications to treat for thrips. Thrips populations vary year to year and location to location, depending upon the weather and availability of alternate hosts. Frequent scouting for thrips early in the season is crucial to make decisions about foliar insecticide applications.
As the plant begins to produce squares, cotton fleahoppers may be seen feeding on tender portions of the plant, including developing squares. Fleahopper feeding causes squares to turn brown and die, resulting in a “blasted” appearance. The action threshold in the South Plains during the first three weeks of squaring is 25 to 30 fleahoppers per 100 terminals, depending on the square set.
“If we have good square set, the plant can often compensate for fleahopper damage,” Vyavhare says. “But if there is less than 90 percent square set in the first week of squaring and we see 25 to 30 fleahoppers per 100 terminal buds, then we need to spray. After first bloom, treatment is rarely justified for fleahopper control.
Later in the season when bolls are forming, both adult and immature lygus bugs feed on the squares and small bolls. Lygus bugs, or Western tarnished plant bugs, are one of the predominant insect species in Texas cotton. The threshold during the first week of squaring is eight lygus bugs per 100 sweeps.
“Using broad-spectrum insecticides, such as organophosphates and pyrethroids, can flare secondary pests, so we recommend applying softer chemistries to control lygus,” Vyavhare says.
Spider mites may be found on the undersides of leaves later in the season, but typically don’t appear in high enough numbers to treat. Spider mites turn the plant a reddish color. If more than 50 percent of the plants in a field are turning red, an insecticide spray of abemectin or proparagite is recommended.
“We don’t see a lot of insects crossing the economic threshold in the South Plains later in the season,” Vyavhare says. “And the adoption of Bt technology has eliminated the threat of bollworms and other caterpillar pests here.”
For more information about insect management in Texas cotton, visit http://cottonbugs.tamu.edu/.
Key California Pests
The most important mid-season pest in the California cotton system is lygus, followed by aphid and whitefly, whose populations may begin to build during July and August. Lygus attack the squares, causing them to drop off the cotton plant.
“When the squares drop off, the plant has to take more time to make them up,” says Pete Goodell, Cooperative Extension adviser, Integrated Pest Management, University of California Statewide IPM Program. “This takes time, and we don’t have a lot of time in our system because we are still in a water-limited situation this year. The thresholds change as we move through the season, which is why the UC IPM Cotton Pest Management Guidelines should be consulted.”
The landscape around cotton fields has changed, too. Other crops, such as alfalfa hay, serve as hosts for lygus and are planted next to cotton fields in many places but can be managed by leaving strips of uncut alfalfa to provide habitat. Lygus can be managed with insecticides in seed alfalfa and safflower, which reduces the number of insecticide applications in cotton and allows beneficial populations to remain intact to fight aphid and whitefly.
Spider mites haven’t been much of a problem “in terms of blowing up,” Goodell says, because of effective miticides being used as preventatives. These selective miticides also spare beneficial insects.
In July and August, producers are urged to watch for whitefly and aphid. Both pests secrete honeydew that affects cotton quality. “We definitely want to avoid sticky cotton,” Goodell says. “Whitefly can be managed in early July with insect growth regulators. Later in the season, we apply more broad-spectrum insecticides to clean them up before harvest. Before open bolls are present, look for 50 aphids per infested leaf. Once the bolls open, the threshold drops to 10. We recommend managing whitefly and aphid populations with insect growth regulators and more selective materials. If you wait on economic thresholds, the density of the crop makes it difficult to get adequate coverage when cotton is most susceptible.”
IPM Tool Gains Popularity
UC’s IPM Decision Support Tool is now available to make navigating the UC IPM website more manageable and provide IPM adoption documentation to professional crop advisers. It’s a live interaction that allows you to choose a crop and a pest. Then the tool goes through the UC IPM guidelines and returns pertinent information. For example, it shows pest identification, sampling techniques, biological and chemical control recommendations, and cultural control options if there are any. It also shows the impact on surface water.
The report can be saved as a PDF with hyperlinks to the UC IPM website. Notes can be added to the PDF for future reference.
“The pests on UC’s IPM Decision Support Tool are the ‘rogue’s gallery’ of pests we need to worry about in the California cotton system,” Goodell says. “People will be looking at them for management options, as well as yield, quality and environmental protection. This tool is not a field app as much as it is a strategic app that shows tactics you may want to use this year. When the time comes, you can go back and review the report. In my opinion, the tool provides the “IDEA”: Identify the pest, determine the population, evaluate the risk and assess what management options you have – chemical and non-chemical. It also shows up well on a tablet or smartphone.”
Access UC’s IPM Decision Support Tool at http://www2.ipm.ucanr.edu/decisionsupport/.
Insecticide Options For Plant Bugs In Cotton
[highlight]Editor’s Note: The tarnished plant bug is one of the most damaging pests in Mid-South cotton. When Transform WG insecticide entered the market, it provided good control of plant bugs, but its labeled was revoked at the end of last year. Today, the industry is assessing its options for controlling this destructive pest. Integrated Pest Management Extension Specialist Scott Stewart, University of Tennessee, offers his suggestions for managing plant bugs in the absence of Transform.[/highlight]
So what is the plan to manage tarnished plant bugs in cotton if Transform is not available? My strategy remains similar to previous years.
I suggest Centric (1.75-2 ounces/acre) or maximum labeled rates of imidacloprid for applications made prior to bloom when mostly adults are present. Once nymphs become more common, and definitely after first bloom, we need to switch to other chemistries. This would normally be the window when I would suggest using Transform. Now, our options are limited. We will be relying heavily on Orthene/acephate (0.75 pound ai/acre is generally an adequate rate) or Bidrin (6-8 ounces/acre).
The first couple of weeks of bloom is a good time to get Diamond in the mix. Diamond at 4-6 ounces/acre can and generally should be tank mixed with acephate or Bidrin. In this tankmix, you can probably back off the rate of acephate and Bidrin (but just a little bit). As we get closer to August, the best option is to stick with acephate or Bidrin. I often recommend tankmixing with a pyrethroid insecticide in this window to pick up stray stink bugs and bollworms (and it adds a little kick to plant bug control).
A good crop consultant is needed to help make these treatment decisions. It is concerning how few insecticide classes are available to effectively manage plant bugs, which makes resistance management difficult. Because neonicotinoid resistance in cotton aphids is now fairly widespread and because we may not be able to use Transform, you might expect more than usual problems with this pest.
Fundamental management practices that promote crop earliness will also help. Do not over fertilize. Data show that 80 pounds of nitrogen is optimum for most fields. Select early maturing varieties and use plant growth regulators appropriately for these varieties. Irrigation should be adequate but not excessive.
– Scott Stewart, IPM Extension Specialist,
University of Tennessee