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Good Guys, Bad Guys

Consider The Use Of ‘Natural Enemies’ To Help Control Western Cotton Pests

• By Carroll Smith,
Editor •

silverleaf whitefly

Photos courtesy USDA-ARS

In the Arizona cotton insect arena, the most notorious villains are Lygus bugs and sweetpotato or silverleaf whiteflies.

Lygus damage squares with their piercing-sucking mouthparts, which can adversely affect potential yield. Whiteflies are also piercing, sucking pests. But they primarily cause cotton quality loss by depositing a sticky, sugary secretion called “honeydew” on the cotton leaves that ultimately drips onto the fiber in open bolls.

According to the University of Arizona Cooperative Extension, “Effective control of whiteflies is absolutely essential to produce high quality cotton.” The first step is to sample the number of whiteflies present in a field, and then compare it to established action thresholds to determine if a spray is warranted.

Over the years, the trend has shifted from using broad-spectrum insecticides to ones that are more selective against target pests and safer for beneficials (predators) or “natural enemies.”

Integrated pest management experts around the world have consistently repeated the mantra, “Preserve your beneficials.”

Until now, farmers and pest control advisers did not have any guidelines to follow about “which ones” and “how many is a good number to be considered a threshold.”

Predator Thresholds Developed

Dr. Peter Ellsworth, director, Arizona Pest Management Center at the University of Arizona, says it’s important to be able to identify the six key players in the ecosystem service of biocontrol and enlist their help in reining in the “bad guys” that attack cotton fields.

The natural enemy roster for whitefly control includes lacewing larvae, minute pirate bugs, big-eyed bugs, Collops beetles, crab spiders and Drapetis flies or “dance flies.”

He describes the predator thresholds that have been developed to help control whiteflies as a “career-long culmination of research that I and my close collaborator, Steven Naranjo from the U.S. Department of Agriculture, started 25 years ago. Our goal isn’t just to remove sprays but make them in a timely manner according to what we understand the economic need to be, which is to preserve the quality of the crop.

“Anytime you can defer a spray in our system is another opportunity for biocontrol to resolve the problem or for a monsoon storm to come through and remove a lot of whiteflies from your field. We think considering predator thresholds does two things:

» “Tells you when you can defer a spray for at least a week because biocontrol is working quite well.

» “About one-third of the time, tells you to advance a spray ahead of the regular whitefly threshold because, for whatever reason or reasons, your field does not have enough biological control potential to function on its own.”

The Arizona entomologist says he believes incorporating the biological control with the use of insecticides safe (or selective) to natural enemies is the path to sustainability.

“It’s not just about the pests anymore,” Ellsworth says.

natural enemies

A Field Man’s Perspective

Arizona pest control adviser Gordon Goodwin with Fertizona-Yuma says cotton is one of the most interesting crops to watch, and there are a lot of variables to consider. Some of these include varieties, plant growth regulators, nutritional requirements and disease and insect control.

He samples cotton fields for insect pest counts to determine if they have reached action threshold levels.

“We have established whitefly and lygus thresholds,” he says. “Several years ago, Peter Ellsworth asked us to also consider watching our beneficials, which made a lot of sense. Crab spiders and Collops beetle really help keep the whitefly down.

“With that being said, in the past seven or eight years, it’s been important to only use selective insecticides or insect growth regulators on cotton to preserve our predators and reduce the number of applications we need to make to complete a successful cotton crop. If you introduce an organophosphate or a pyrethroid into the crop after bloom, beneficials are destroyed, which makes it tougher to control whiteflies.

“When we started paying attention to the natural enemies, we actually postponed a whitefly treatment by about two to three weeks. We eventually did have to treat the field for whitefly, but we cut down by one application.

“This system may not save money every season but using established thresholds and selective chemistries and keeping an eye on beneficials, especially for whitefly control, are the big things.

counting predators

‘Think A Little Harder’

“If farmers can control whiteflies, they get better cotton quality in the end because the honeydew secretions are not there. I always try to do the best job I can for my growers.”

When asked if incorporating the new natural enemy threshold system into his pest control program creates more work for him as a PCA, Goodwin says it just makes him think a little harder.

“I inspect cotton fields twice a week, and I have to keep my mind in the game,” he says. “As a field man, I maintain an accurate count of the pests and have also started paying attention to the predators in my net. I may not document an exact count of their numbers, but I know in general they are present or that their numbers have gone up.”

Goodwin recalls an instance several years ago involving the crab spider’s relationship to the sweetpotato whitefly. After observing whitefly numbers going way up, he noticed it took about three or four days for the crab spider numbers to increase and whitefly numbers to go down.

“It was interesting to watch,” he says, “but you have to be patient.”

natural enemy thresholds

Natural Enemy Thresholds Evolve

In central Arizona, Karl Button grows several different crops on his 4,000-acre operation, including 800-plus acres of cotton this year. Over time, he has been a farmer cooperator with Ellsworth and other researchers on various projects.

He says when Bt cottons were introduced, producers finally had a selective approach to control lepidopteran pests. But broad spectrum insecticides were still in use for other pests, making sweetpotato whiteflies and “sticky cotton” still an issue.

“Peter spent a lot of time developing guidelines for when to spray, when the whitefly populations could be tolerated and when it was necessary to spray to avoid population increase,” Button says.

At this time, beneficials still were not being taken into account because farmers were using organophosphates, which also took out the predator populations.

As selective insecticides that only worked on sucking, piercing insects came on the scene, the predators began to return to the field, but no one knew what they were actually accomplishing out there.

Ellsworth, Naranjo and their students began to turn their attention to identifying and counting the natural enemies in cotton fields to determine at what thresholds they might be a legitimate criterion to include in the decision-making process of whether to spray or defer a spray.

A Subjective Decision

Button says he attends a lot of the UA Extension meetings, is familiar with the predator threshold system and has agreed to test it on one of his commercial cotton fields for usability if he is needed for the project.

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“We have occasionally deferred sprays for a couple weeks because of a privately held suspicion that the pest numbers weren’t high enough, and something seemed to be going on in the field that we didn’t want to mess with right now,” he says.

“When the numbers started increasing after a long plateau, we felt that whatever was in there holding the equilibrium had shifted.

“At that point, it becomes an economic determination of risk, which is always a subjective decision on the part of the grower. How much risk will I tolerate and how much damage will I tolerate? It’s the individual farmer’s call.

“But Peter’s predator thresholds do give us another component to consider when making that subjective decision.”