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Learn to Navigate the Pest Triangle

November 12, 2011
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
Learn to Navigate the Pest Triangle 1
Hot and dry or cool and wet? Ken Ferrie (right) says that understanding the environment preferred by each pest makes scouting more efficient.   

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.

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FEATURED IN: Farm Journal - Mid-November 2011

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