Hodgson

Soybean aphids should be on a soybean grower’s radar every year, says Dr. Erin Hodgson, Iowa State University Extension entomologist. Now that the insects have broken the on-off, every-other-year cycle, experts can no longer consistently predict when, where, or how heavy infestations may be anymore.

“We don’t know what has changed, whether it’s overwintering, places to overwinter, if they just get a good start in the spring in some areas, or something else,” Hodgson explains.

Potassium (K) may even be a factor. A recent survey of field samples in Wisconsin and Michigan, reported in the Better Crops With Plant Feed publication from the International Plant Nutrition Institute, leads to some concern that low soil K levels in soybeans may leave the crop susceptible to soybean aphid. In their conclusion, the study’s authors stated that “aphid infestations can still occur when K nutrition is adequate, but preventing K deficiencies provides at least one degree of protection or insurance against yield loss from soybean aphids.”

Choosing soybean varieties with resistant genes adapted to a specific growing region can lessen the pressure, but soybean fields may still need a foliar insecticide application. And biological predators, in particular lady beetles, may help keep soybean aphid populations from escalating into an infestation warranting insecticide application.

Strong Opponent

If the original cycle were still in play, 2009 should have been a year of heavy infestation. Instead, while some areas required spraying, it was not as bad as previous years, Hodgson says.

The soybean aphid first appeared in the U.S. in the Great Lakes states around 2000. A decade later, it covers nearly all soybean-growing areas in the U.S. and Canada. After overwintering as several generations of eggs and nymphs in the invasive, woody buckthorn plant, winged generations begin moving into soybean fields in June when beans are at the R1 reproductive stage. That’s when your dealership’s scouts and your grower-customers need to get into the fields to scout.

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Scouting early and often may prevent an explosive expansion later in the season. Aphids are known for their long-distance summer migration, and for their ability to reproduce. Those that survive a seed treatment or insecticide application just keep multiplying. “It’s important to know that even if a grower uses a seed treatment, it’s not a silver bullet,” Hodgson warns.

A high kill rate with a foliar insecticide treatment — if the threshold warrants the application — also is crucial to success. Aphids tend to rebound from broadspectrum insecticide applications much better than the beneficial insects, and can increase their numbers rapidly in favorable conditions.

“Last year we saw high aphid infestations in late-planted regions and also where the growing season turned relatively cool,” says Paula Davis, Pioneer Hi-Bred senior manager for insect and disease traits. “The buildup time for aphid populations typically is late June through July, but last year it occurred later, from July into September in some places. Illinois especially got hit with later flights of aphids.”

Keep On Scouting

Soybean growers should develop a scouting plan to monitor the crop’s condition throughout the season, whether they employ your staff’s services or do it themselves. Examining multiple places in the field to get an overall analysis and identify problem areas is critical every year, says Travis Belt, Mycogen Seeds customer agronomist.

Hodgson recommends that retailers and growers get out into the field to scout at flowering and continue through seed set. Scout the undersides of new leaflets on a regular basis, preferably weekly but at least every 15 days, to determine how fast the aphid numbers are climbing.

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University entomologists have determined that foliar application of an insecticide is needed when the field has reached a threshold of an average of 250 aphids per plant — but not before. “It’s the best way to protect yield and reduce cost,” Hodgson says. “Applying anything below the threshold is a waste of money. Some growers have limited access to equipment, so they get scared and schedule the application early so they get on the schedule — before determining if they’ve reached the treatment threshold. The infestation could be at 100 and never go any higher, for example. That’s very expensive; the cost of fuel, equipment, and product adds up.”

What should your scouts and grower-customers look for? Usually, the aphids will form colonies on the undersides of the plant leaf, especially the newest trifoliates. Hodgson recommends walking through each field, checking 30 plants randomly per 50 acres. “It should take 20 minutes, in and out,” she says. “Count every aphid on the plant and take the average of the 30 plants to determine your count.”

Spray What?

Some of the insecticides approved by EPA for use in controlling soybean aphids this season include:

■ Endigo ZC (lambda-cyhalothrin + thiamethoxam) from Syngenta Crop Protection is formulated with Zeon technology and uses dual modes of action for fast and effective knockdown of key insect pests, including soybean aphids, and long residual control against re-establishing insect populations, yet has minimal impact on beneficial insects.

■ Belay (clothianidin) from Valent U.S.A. Corp. is a third-generation neonicotinoid with a novel mode of action. The highly systemic chemistry provides initial knockdown of soybean aphids after a few hours, and also quickly translocates through the plant for control from 14 days to 21 days.

■ Leverage 2.7 (imidacloprid + cyfluthrin) from Bayer CropScience, now available at a new lower price, provides fast knockdown, extended residual control, and Stress Shield (which helps defend plants from environmental stresses). Two modes of action provide both contact and in-plant protection from a broad spectrum of sucking and chewing pests, including soybean aphids.

■ Cobalt (chlorpyrifos + gamma-cyhalothrin) and Lorsban Advanced (chlorpyrifos) from Dow AgroSciences LLC provide broadspectrum insect control in soybeans.

■ BASF recently signed an agreement with Meiji Seika Kaisah, Ltd. for the co-development of a new insecticide, currently called ME5343. The new class of chemistry and new mode of action show significant efficacy for the control of aphids and certain other insects, especially those that have developed resistance to other insecticides. BASF will submit it for regulatory approval in the U.S. in 2013, with a commercial launch planned for 2015.