On Pest Patrol

February 14, 2009 11:04 AM

Until recently, soybeans pretty much fended for themselves when insects came to dine. Bean leaf beetle, grasshopper, woolly bear caterpillar, green cloverworm and many other pests can do plenty of munching, but seldom does any single species reach population levels that defoliate soybeans enough to cause economic damage. Even sapsucker aphids have been only intermittent pests in most parts of the soybean belt.

Enter 2008. Prices got so hot last summer that you wanted to save every last soybean. Then, in some parts of the soybean belt, aphids abandoned all the rules and showed up for the second year in a row with vengeance.
Southern soybean growers typically battle more bugs than their northern counterparts, but relative newcomers, such as the red-banded stink bug, have been taking a toll on seed quality, resulting in harvest discounts.

"The development of Bt crops changed corn and cotton insect management dramatically, but soybean insect management has remained a scout and treat when warranted
approach," says Kevin Steffey, University of Illinois Extension entomologist. "Bean leaf beetle was probably our No. 1 soybean insect threat over the past three decades," Steffey says. "And it was only a significant threat two or three times in that 30-year span."

That scenario changed when soybean aphids appeared on the scene. "Aphids have quickly become our most important pest in Illinois and possibly the Midwest. Japanese beetle probably comes in second right now, and a distant third is probably two-spotted spider mite outbreaks during drought situations," Steffey says.

"The good thing is these pests don't show up every year, but that fact also puts an emphasis on scouting and timely treatments. Treating at the wrong time—as with aphids—can also backfire and result in the need for multiple sprays."

When you see giant ragged holes appear in leaves, it's easy to forget that soybean plants have a great capacity to compensate for feeding. Research has shown that the key factor driving yield losses from defoliating insects is the degree that defoliation reduces light interception by the soybean canopy. Soybean plants can lose a tremendous amount of leaf area without yield loss if the remaining leaves are still intercepting at least 90% of the incident light, says Tom Hunt, a University of Nebraska Extension entomologist based in Concord, Neb. "Defoliation is notoriously difficult to estimate and is almost always overestimated," Hunt says. "Sometimes the injury seems dramatic, but typically all parts of the canopy are not considered in making estimates.

"Seedling soybean can withstand 50% defoliation before economic damage occurs. Reproductive stages are more sensitive, with full bloom being the most sensitive stage," he adds.

Bt soybeans are still under development, explains Ben Kampelman, Monsanto Company spokesperson. "But they will mostly have application in the South and in South American markets where caterpillar-type insects are an issue," he says. 

One thing that concerns Steffey is the increasing dependence upon neo-nicotinoid-based seed treatments at planting. These products have shown early season control of leaf-feeding and sap-feeding insects because of systemic activity that distributes the insecticide through the plant. However, bean leaf beetles are usually the primary early season insect pest, and it takes a lot of beetle feeding to impact yield.

Farmers who plant earlier than their neighbors and have high beetle numbers and/or bean pod mottle virus are more likely to get an economic benefit from soybean seed treatment. Steffey notes that aphids typically show up too late for seed treatments to have much of an impact, although research suggests that outbreaks can be delayed.

"Virtually all corn is also being planted with a neonicotinoid insecticide, and some foliar insecticides contain the same active ingredients. Resistance is already a known issue in some areas with this family of insecticides," Steffey says. "Soybeans need management that can't be replaced by mere inputs."

Aphid angst. The invasive soybean aphid showed up in North America in 2000 and has rapidly spread throughout the Midwest and southern Canada. High soybean aphid populations reduce soybean yield when their feeding causes stunting, leaf distortion and reduced pod set. An additional threat is the aphid's ability to transmit plant viruses, such as alfalfa mosaic virus, soybean dwarf virus and soybean mosaic virus. Honeydew, excreted by soybean aphids, leads to the development of sooty mold and can contribute to additional losses even if aphids are controlled.

Before 2008, outbreaks of aphids occurred almost exclusively in odd-numbered years. Based on previous experience and the low numbers of winged aphids captured in suction traps during the fall of 2007, no one expected the suckers last summer.

David Ragsdale, University of Minnesota entomologist, says 2008 was a wake-up call. Nebraska, Iowa, Minnesota, Wisconsin, the Dakotas and parts of Illinois saw severe infestations. Many fields were treated with insecticides two or three times. "Most soybean fields in Minnesota received at least one spray," Ragsdale says.

"We knew weather plays a big role in aphid development and we learned exactly how big when the season started out too wet and cold, which delayed soybean planting. Staggered emergence and a patchwork of young plants created a smorgasbord for the aphid." Cool nights also kept the aphid's natural enemies from developing.

The economic threshold for soybean aphids is 250 aphids per plant before stage R5.5. That figure has been painstakingly worked out through collaborative, multistate research during the past few years.

"The soybean aphid economic threshold does not move lower based on soybean price," Ragsdale says. "What changes is how quickly you need to respond once the threshold of 250 is reached. Instead of a week to 10 days, an application needs to be applied in four to five days.

"Using a lower threshold means that your ability to predict whether the pest will reach economic status is about 50%, or no better than the flip of a coin. The 250 threshold is set high enough so that you know a damaging aphid population is almost a certainty. An added benefit is that waiting to reach the 250 threshold before treatment means a single application will control aphids for the entire season.

"One of the biggest issues we face in aphid control is pulling the trigger too early," Ragsdale says. He adds that often growers will add a little insecticide to a fungicide or glyphosate treatment because you are going over the field anyway.

"If you are unlucky and catch a migratory flight of aphids a few days later, there are no natural enemies left to keep aphids under biological control," Ragsdale warns. "It often means that aphid buildup is so rapid that additional spray applications are necessary," he says.

In Wisconsin studies, multiple insecticide applications made to aphid-free plots appear to have flared other pests, such as two-spotted spider mites. As a guideline, sampling for soybean aphid should begin in the late vegetative stage, with emphasis on sampling during flowering (R1) and through seed set (R5.5).

Host plant resistance is the great hope for the future. In 2009, Syngenta plans to introduce aphid-resistant soybeans in Canada and the following year in the U.S. through its Northrup King line. Other soybean seed companies are on its heels. However, biotypes of soybean aphids that can overcome aphid-resistance genes have already been discovered.

"Aphids are our No. 1, 2 and 3 insect problem in Minnesota," Ragsdale says. "Each year, they show us a new trick they learned. They acted differently last year—so much so that we have quit predicting what's
going to happen.

"Scout, be on guard and act in a timely fashion when warranted is the best advice right now," he says.

Japanese invasion. Big, metallic green and mean—the Japanese beetle may be an old-timer (it moved here in 1916), but densities have increased markedly in recent years.

Adults chew on leaves, resulting in a fine network of leaf veins with a lacy appearance. The beetles feed on some 300 plant species, and the larvae can feed on root hairs of seedling soybeans, though the injury is generally not economic. The main threat is defoliation during the reproductive stages of the soybean plant's development.

Japanese beetles are generally part of a complex mix of insects that feed on soybeans. Individually, they may not cause sufficient injury to warrant treatment unless they are combined with other defoliators.

Steffey notes that economic threshold levels for Japanese beetle were developed in the 1970s and 1980s and soybean production has changed a lot since then. "Most entomologists involved with soybean insect management research believe that an overhaul of economic thresholds based upon percentage defoliation is long overdue," he says.

Scout flowering soybean fields and estimate the number of beetles per foot of row. Current recommendations are to treat when defoliation reaches 30% before bloom and 20% during bloom and pod fill. A reduction in economic thresholds for injury may be warranted if plants are suffering from other stress.

Predicting infestations of Japanese beetles is futile. However, you can generally anticipate that they will be worse after a mild winter followed by early planting. Infestations are more likely to be severe in areas where there's a history of problems.

You can e-mail Pam Smith at psmith@farmjournal.com.

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