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March 17, 2009 A Penton Media Property Vol. 1, No. 1

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Soybean Aphids Again A Threat
Migrating from Asia to the U.S. nine years ago, the soybean aphid has become one of the most feared insects in the Corn Belt and other bean-producing areas. This aphid has been confirmed by USDA in Delaware, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Mississippi, Missouri, Nebraska, New York, North Dakota, Ohio, Pennsylvania, South Dakota, Tennessee, West Virginia and Wisconsin and in the Canadian provinces of Quebec, Ontario and Manitoba.

This newsletter will hopefully help you better understand the threat aphids present, how to spot them on time and how to control them before the damage is done.

Aphids May Be Perennial Pest
Midwestern growers should be prepared for the third straight year of soybean aphid invasions. It’s not a guarantee, but the trend of aphids as perennial pests has certainly been set, says Iowa State University Entomologist Matt O’Neal.

USDA says that in 2008 the soybean aphid “went west,” reaching the western most cultivation of soybeans in the U. S. in Wyoming. More than 8,500 soybean aphids were caught in the Midwest suction trap network in late September (to see the Midwest trap network, click here

Soybean aphids caught in the traps during September and October were migrating from soybeans to buckthorn where the overwintering eggs are deposited. “This number exceeds any seen in previous years during the same time period,” says USDA.

The tiny terrors saw the on-again, off-again cycle broken in 2008.

“In the fall of 2007, we collected very few aphids after the growing season,” says O’Neal. “We thought 2008 would be a quiet year. It wasn’t. I try not to make predictions, but now we may be in a period where soybean aphid outbreaks may be more intense. It may not be a cycle. It may be every year.”

Aphids were first seen in the U.S. in Wisconsin in 2000. “They come from a region of China with a climate much like the Midwest,” says O’Neal.

After emerging here, there was an apparent soybean aphid cycle throughout the Corn Belt. Populations would be low one year, followed by outbreaks well above the economic threshold of 250 aphids per plant the next year. There were small outbreaks in 2006 followed by the large invasion in 2007. So entomologists and growers expected a light year in 2008.

O’Neal says Iowa State research on 36 separate farms is helping researchers and growers learn more about the aphid cycle. It has led to several hypotheses from O’Neal on why 2008 was a bad year for aphids.

“First, the spring of last year was wet and cold and delayed planting,” he says. “That may have disrupted the predator relationship with aphids. We have also found that as the amount of corn and soybean production increases around a soybean field, the amount of predation decreases (as there are fewer predator insects, like lady beetles, that eat aphids due to a lowered number of field borders).”

O’Neal adds that growers who spray insecticides to control other insects may also have reduced predator numbers. He says a well-rounded integrated pest management program is needed for best aphid control. That program could include the use of new aphid-resistant soybean varieties that are coming on line.

O’Neal and others in the North Central Soybean Research Program authored a Soybean Aphid handbook sponsored partially by the Iowa Soybean Association. For more on the publication, go to

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Identifying The Insect
The soybean aphid is a small, yellow aphid with distinct black cornicles that resemble “tailpipes” on the tip of the abdomen, according to the North Central Integrated Pest Management Center (NCIPMC). The aphid can be found on stem apices and young leaves of growing soybean plants and on the undersides of leaves of mature plants.

Because no other aphid species develop colonies on soybean in North America, NCIPMC says it is safe to assume that colonies of tiny yellow aphids on soybean are indeed the soybean aphid. Several other aphid species feed on soybean, but they are migratory and do not colonize soybean.

The aphid has a complex life cycle with as many as 15-18 generations annually. Two different types of host plants are required by the soybean aphid to complete its life cycle. Overwintering eggs survive on species of Rhamnus (buckthorn). Nymphs hatch in the spring and after two generations of wingless females, a generation of winged females is produced that migrates from buckthorn in search of soybean.

On soybean, aphids spend the summer in a repeated series of wingless generations followed by a winged generation that disperses from its crowded hosts in search of other soybean plants. In the fall, they migrate back to buckthorn by winged females that produce a generation of egg-laying wingless females.

Males develop on soybean and search for buckthorn and mate with the wingless females, which lay the overwintering eggs alongside the buds on buckthorn twigs.

For more on the soybean aphid life cycle click here.

Damage Will Hurt Yields
The economic threshold for soybeans aphids is about 250 insects per plant. Aphid infestations that peak at the R1–R2 growth stage of the host may cause stunted plants with reduced pod and seed counts, resulting in lower yields. Damage is worst when 80% of the plants are infested, and natural predators are not controlling aphid population.

The threshold is designed to give about seven days of lead time between scouting and applying insecticides. When that level is reached plants can see stark damage. Leaves may express symptoms of distortion. Later in the growing season, heavily infested plants may have distorted and yellowed leaves. Charcoal-colored residue on stems, leaves and pods is sooty mold that grows on honeydew, a by-product excreted by aphids.

Some of the most severe damage from aphid populations occurred in 2003 when aphid populations exceeded 3,000 per plant in many fields. In parts of Iowa, soybean yields averaged 32 bu./acre, a 16 bu. or a 32% reduction from 2002. This yield reduction was partly due to the soybean aphid coupled with drought conditions.

In low numbers, soybean aphids can feed on soybeans but cause little or no damage. Large populations can result in visual evidence of damage. Honeydew accumulates on the top surface of leaves.

Excessive honeydew permits the growth of sooty mold, turning the leaves dark and interfering with photosynthesis in the plant. Heavily infested plants may be stunted. Plants that become stunted during the early reproductive growth stages of soybean may have reduced pod set and seed counts, resulting in lower yields.

Aphid-transmitted viruses can be an important part of the ecology of the pests. They can transmit numerous viruses to plants, including a number of viruses present in the U.S. that naturally infect soybean. Alfalfa mosaic, soybean mosaic, bean yellow mosaic, peanut mottle, peanut stunt and peanut stripe are among them. (Transmission of these viruses by the soybean aphid has not been documented in the U.S., but is certainly a threat.)

Scouting For Aphids
Tom Hunt, University of Nebraska entomologist, was among those who worked with growers to get soybeans properly scouted for aphids last year. He says growers should begin scouting soybean fields once or twice a week in late June to early July.

He says to check 20-30 plants per field. Aphids are most likely to concentrate at the very top of the plant early in the season, and will move onto stems and within the canopy as populations grow and/or the plant reaches mid to late reproductive stages. As the season progresses, aphid numbers can change rapidly and populations can double in two to three days.

“Soybean aphid populations can grow to extremely high levels under favorable environmental conditions,” says Hunt. “Reproduction and development is fastest when temperatures are in the 70s through the mid 80s (F.). The aphids do not appear to do well when temperatures are in the 90s and are reported to begin to die when temperatures reach 95 degrees.

“When populations reach high levels during the summer, winged females are produced and migrate to other soybean fields. Like a number of other insect species (e.g., potato leafhoppers), these migrants can be caught up in weather patterns, moved great distances, and end up infesting fields far from their origin.”

Brian Kemp, a grower from near Sibley, IA, sees aphids in his soybean fields every year. “I use the speed scouting method to monitor my fields, starting the third week of July,” says Kemp. “My advice, however, is to keep scouting. I think I lost 6-7 bu./acre last year in fields that weren’t sprayed because I didn’t scout later in the season. I have learned that every year is different. I think the fields that I scouted and decided not to spray reached threshold later than normal. The pest was more challenging to manage in 2008.”

The Iowa Soybean Association says a recent survey conducted by researchers at the University of Minnesota measured farmers’ awareness and use of IPM for soybean aphid control. According to this survey, more than 70% of the respondents say the frequency with which aphids should be treated for profitable control depends on aphid counts, weather conditions and plant growth stage.

In addition, more than half of the farmers surveyed consider the lowest aphid density for profitable aphid control to be 250 aphids per plant. Sheila Hebenstreit, a crop consultant with West Central Cooperative near Jefferson, IA, says that a great deal of progress has been made in educating growers about the importance of proper scouting and adherence to economic thresholds.

“Applying insecticides over large areas and indiscriminately killing insects works, but is a problem,” notes Hebenstreit. Ideally, she would like to see multiple modes of action to fight the pests, including biological controls and resistant varieties.

For tips on speed scouting, go to click here (then scroll down to the Soybean Aphid manual)

Sources: University of Nebraska, Iowa Soybean Association

Beneficials Attack Aphids
The biggest enemies of soybeans aphids are their natural ones – predators that feed on aphids and other damaging bugs. The main predators of soybean aphids are lady beetles, green lacewings, brown lacewings, syrphid flies, aphid midges, pirate bugs and damsel bugs.

Entomologists, through funding from USDA and the North Central Soybean Research Program (NCSRP), have been steadily monitoring the soybean aphid and its natural enemies since the pest invaded U.S. soybean fields. Part of its program involves importation biological control, also called classical biological control, of the soybean aphid.

The process identifies, locates, evaluates and releases a pest's natural enemies that evolved with it in its native home, with the intent of significant long-term reduction of pest numbers. Insect predators are generally free-living, very mobile and consume many prey during their lives. Insect predators are usually as large as, or larger than their prey. Predators can be predacious in the larval stage, in the adult stage, or both. Many predators are generalist natural enemies although a few types are specialized.

Spraying Tips
If the soybean aphid economic threshold is apparent, Ron Hammond, Ohio State University entomologist, says the greatest benefit from an insecticide application will likely come in July during flowering or immediately thereafter.

The later in the summer an insecticide application is made, the less the benefit from that spray, he says. Spraying late in the season after pods have filled or following dispersal of winged aphids in mid-to-late summer will be too late to prevent economic yield loss.

Hammond stresses to use care in spraying near apiaries. Most insecticide label directions warn not to apply to blooming or flowering crops if bees are actively working in the target area. Thus, growers should apply most materials for soybean aphid control in early morning or late day when bees would not be active in fields.

Good insecticide coverage and penetration is required for optimal control of soybean aphid, as many aphids feed on the undersides of the leaves and within the canopy. Use high water volume and pressure. Aerial application works well when high water volume is used (5 gal. of water/acre recommended).

Tom Hunt, University of Nebraska entomologist, says that when the current threshold of 250 aphids per plant was developed, it was set so growers had a week to apply insecticide before aphids were likely to increase above the economic injury level (which is much higher than 250 aphids per plant). Midwestern soybean entomologists are recommending that even with higher soybean market values, the economic threshold of 250 aphids per plant should still be used, he says.

Entomologists note that if soybean prices approach the high levels seen in 2008, growers should consider a four- to five-day window to treat rather than a seven-day window. Growers are encouraged to contact their local or regional extension entomologists to determine the best soybean aphid techniques for their region.

For more on soybean aphid control, click here .

Thanks For Viewing This Aphid Alert Newsletter
Thanks For Viewing This Aphid Alert Newsletter
Again, thanks for taking time to read this enewsletter. Hopefully it has helped answer some of your questions on soybean aphids. If you have a particular question you’d like to see answered in our next issue scheduled for early April, contact your Aphid Alert editor at

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