Learn to Navigate the Pest Triangle

November 12, 2011 06:45 AM
Learn to Navigate the Pest Triangle

Spray only the fields where diseases, insects are present

Quick quiz: The pest triangle is (a) an alarm you ring when you spot bugs in your soybeans; (b) a romantic conflict involving three Japanese beetles; or (c) a set of factors that helps farmers determine whether an insect or disease is reaching problem levels.

The answer, of course, is (c), and the triangle consists of the host, the pest and the environment. The pest must be present and the environment must allow it to reach a damaging level before treatment is justified.

Anecdotal evidence suggests that a lot of farmers could have saved a lot of money if they had considered the triangle in 2011.

"Conversations with my colleagues, and the number of aerial applicators I see spraying fields, convince me the frequency of fungicide applications is increasing," says University of Illinois Extension entomologist Mike Gray.

"With current soybean prices, some growers are inclined to spray preventively, rather than basing their decision on scouting and economic return," says Mark Baer of Sun Ag Supply in Tremont, Ill.

Basis for treatment. Prevention without evidence of a threat is not a sound basis for treating, agree Baer and Gray. "At current crop price levels, you can understand farmers’ thinking," Baer says. "But we feel there should be a demonstrated need, based on threshold levels of a pest."

"Farmers have more acres to take care of and they are looking at high commodity prices," Gray says. "Many believe just a few more bushels per acre will at least cover the cost of the treatment. But they are not considering the potential negative long-term consequences of unneeded pesticide applications: the reduction of natural predators and the development of resistance by the pests."

In the Midwest, most soybean insect and disease problems in 2011 were found to be localized.

"In Iowa, soybean aphids were the most prominent soybean insect in 2011," explains Iowa State University Extension entomologist Erin Hodgson. "They showed up first in Minnesota, and then were found in most north-central states. But not every field needed to be treated—although I hope every field was scouted."

Some Iowa growers had to replant fields because black cutworms reduced their soybean stands, Hodgson con-tinues, and some growers had to spray for spider mites. A few Iowa growers had to make late-season applications to control bean leaf beetles attacking soybean pods.

The presence of black cutworms in any field is governed by weather patterns as the moths migrate northward (along with weedy habitat in those fields). But whether the other insects reach problem levels depends on local weather conditions—the third link in the pest triangle.

"Being cold-hardy, soybean aphids can overwinter almost anywhere," Hodgson explains. "But population growth during the summer is determined by temperature. In 2011, temperatures above 90°F in July kept populations from building up. Insecticide seed treatments and host-plant resistance also helped slow aphid development and spread during the spring and early summer."

In some fields, cooler temperatures in August helped aphids build beyond economic thresholds and required treatment, Hodgson adds. Drought stress, along with heat, led to problems with spider mites, which love hot, dry weather. The relatively mild Iowa winter of 2010-2011 allowed more bean leaf beetles to survive.

Targeted scouting. In each case, understanding the pest environment helps farmers know which insects to worry about and when to start scouting for them. "If you don’t have the environmental conditions for an insect, it won’t be there," says Farm Journal Field Agronomist Ken Ferrie.

Ferrie adopted the term "pest triangle" from "disease triangle," a phrase that was coined by plant pathologists because the concept applies to both insects and disease.

Learn to Navigate the Pest Triangle 3

"Along with varietal resistance or susceptibility, environment is the overriding factor for most soybean diseases," says University of Illinois Extension plant pathologist Carl Bradley. "For instance, charcoal rot generally is seen in hot, dry conditions—which is why it showed up in the lower two-thirds of Illinois in 2011. We usually see sudden death syndrome [SDS] in wetter conditions, so most of the state saw less SDS last season."

"In central Illinois, we expected 2011 to be a big year for white mold [Sclerotinia stem rot]," Ferrie says. "The disease can survive in soil for many years—and fields that got hammered by white mold two years ago were headed back to soybeans, so many growers made preventive fungicide applications. But those applications were wasted because white mold requires cold, wet conditions—just the opposite of the weather we got."

Besides potentially contributing to long-term resistance buildup, unneeded pesticide applications might also kill beneficial fungi or insects, which might lead to other pest problems later in
the season. "In 2010, some Iowa growers who used a pyrethroid insecticide to control aphids noticed a flareup of spider mites later in the season," Hodgson says.

In Illinois, Baer says, "Some of our early planted soybean fields, which were the first ones treated with fungicides, had to be treated later, after it turned hot and dry, for spider mites. On later-planted fields, we did not apply fungicides because, by that time, the weather had changed and the disease triangle was broken.

"Some growers spray fungicides routinely because they think there is value in improved plant health," Baer adds. "But, just as with insects, we feel there has to be a need. Why spend money if there is no obvious return?"

Big payoff. The spotty nature of pests in 2011 made scouting, as opposed to preventive treatment, pay dividends, Ferrie says. "We have seen years when spider mites took over the state," he says. "But this year, despite hot, dry conditions, they were only present in certain fields. Targeting those fields is what integrated pest management is all about."

If you farm a lot of acres, it’s natural to worry about staying ahead of pests. But applying preventive treatments is not the best solution, for a variety of reasons, Ferrie concludes.

"Better options include hiring a consultant to scout your fields or appointing yourself or a staff member as pest boss," he says. "Then use the pest triangle to plan and target your scouting and make it more effective. Make sure you continue scouting for insects through pod fill."

Of course, in-season scouting and, when it is required, applying pesticides is only one aspect of pest management. Although you can’t predict what weather next season will bring, you can choose to plant varieties that spread your risk.

"Since weather determines which pathogens are troublesome, choosing varieties with resistance to several diseases will help maintain yields," says Ohio State University plant pathologist Anne Dorrance.

In the spring, the environment leg of the pest triangle can help you decide whether to apply a seed treatment to protect seedlings from diseases such as phytophthora, pythium and fusarium.

"Consider a seed treatment especially if you are planting early, in cool, wet conditions," Bradley advises.

Which Pest to Expect?

Some pests like it hot, some like it cool. Understanding how the environment influences insects and diseases will help you anticipate problems. Here are a few examples for the pests mentioned in the story.


  • Spider mites like hot, dry weather. "Development speeds up with temperatures above 90°F, and reproduction slows down at cooler temperatures," says Farm Journal Associate Field Agronomist Missy Bauer. "It’s common to identify heavy spider mite pressure in fields that were densely populated with chickweed in the spring. Weeds provide a food source for mites early in the season."
  • Learn to Navigate the Pest Triangle 2
    Bean leaf beetle
  • Bean leaf beetles might become a problem if the adults overwinter successfully. Following a mild winter, "watch for seedling injury in early planted soybean fields, and especially in the first fields to emerge in an area," Bauer advises.
  • Japanese beetle adults like warm, sunny days. Watch for them when the temperature gets above 70°F, but especially if it hits 85°F to 95°F. If the relative humidity reaches 60% or higher, they do less flying and more feeding in localized areas.
  • Soybean aphids can overwinter almost anywhere in the U.S. But summer temperatures determine whether aphids will become a problem for soybean growers. Temperatures above 90°F will help keep populations from building up to damaging levels.


  • Sudden death syndrome flourishes with warm, wet weather, especially with saturated soils during the R1 to R3 growth stage.
  • Frogeye leaf spot is favored by warm, humid weather. Wind and splashing rain spread spores for short distances. "Dry weather severely limits disease development," Bauer says.
  • Anthracnose likes warm, wet weather, especially when soybean plants are blooming or beginning to develop pods. The fungus can be transmitted on seeds, and it can overwinter on crop residue.
  • Charcoal rot shows up during hot, dry weather, which is why it was common over the southern two-thirds of Illinois in 2011.
  • White mold loves moist, humid weather during the time that soybeans are flowering. "This past season’s hot, dry conditions during flowering limited the disease in many areas," Bauer says.

Learn more about crop diseases.

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