To best manage for the pest, get to know treatment alternatives and not least, the aphid's natural predators.

Almost every extension group in the Midwest has issued press releases, newsletters and guide sheets concerning the soybean aphid. Most major agronomy meetings this winter referenced the soybean aphid. Here is an effort to summarize the important points.

Soybean aphids are native to Asia and were introduced into North America sometime between 1997 and 2000. They were first discovered in Wisconsin and Illinois during the summer of 2000. They reached economic threshold levels in 2001, crashed in 2002 and regained strength in 2003. Their confirmed range now reaches from the East Coast to the Dakotas and from Canada to Georgia. In 2003, Illinois, Iowa, Minnesota and Wisconsin treated more than 5 million acres for soybean aphids. Economic loss caused by this pest was hard to estimate, but it exceeded $100 million.

Research across the north-central United States has shown an average yield response of 7 to 12 bushels per acre when insecticides are applied to soybeans heavily infested with aphids. This response certainly justifies treatment costs.

Although aphid sightings have been confirmed in most soybean growing regions, only the northern Corn Belt has experienced widespread problems. For example, only 18 to 25 Missouri fields were treated for soybean aphids in 2003. However, this is a mobile, genetically manipulative pest that may eventually cause problems everywhere that soybeans are produced.

Soybean aphids are 1/16 of an inch long and hard to locate until their numbers become extremely high. The aphids are soft bodied, pale yellow (almost translucent) to green. Winged adults have a black head and thorax. The aphids have distinct black cornicles (tailpipes). They are found primarily on upper soybean leaves, stems and petioles early in the season, moving to the lower leaves by mid-August.

The aphids have a complex, but relatively easy-to-understand life cycle. Two host plants are required to complete their life cycle. They over-winter as eggs, with the only known winter host being buckthorn. The eggs are very winter hardy and have survived as far north as Minnesota and parts of Canada.

Soybean aphids produce up to 18 generations annually. Their summer populations consist entirely of reproductive females. These give birth to live young. Nymphs reach reproductive maturity in 5 days. Aphid populations can double every 2 to 3 days.

Soybean aphids hatch in the spring and produce two wingless generations on buckthorn. In response to crowding, poor host quality or seasonal cues, the third generation develops wings. Winged females migrate from buckthorn to soybeans, their dominant summer host. During the summer, the aphids repeat a series of wingless/winged generations, thus dispersing rapidly over wide distances.

As soybeans mature, the aphids migrate back to buckthorn where they lay eggs to continue the cycle.

Soybean aphids thrive in temperatures between 75 and 82 degrees. Their life expectancy is 4 times greater at 68 degrees compared to 95 degrees. Their reproductive rate is one-half at 86 compared to 77 degrees and ceases at 95 degrees. These temperature ranges partially explain why the soybean aphid has not become a major pest in the lower Midwest. However, soybean aphids in Asia tolerate warm temperatures. It is only a matter of time until a population tolerant of warmer temperature develops.

When present in large numbers, aphids can cause severe damage to soybeans. They are most damaging between flowering and late pod set, especially when soybeans are experiencing stressed conditions. Soybean aphids are capable of transmitting several viral diseases. They also induce secondary diseases like sooty mold that thrives in the aphid "honey dew," a sugary substance released during aphid feeding.

Economic threshold
How do you determine when aphid populations require treatment? The most popular threshold for treating aphids is when they exceed 250 non-winged aphids per plant (winged aphids are soon to leave) between first bloom and full pod growth stages. This threshold has a crop injury safety factor of 5 to 7 days. The threshold may need to be lowered under crop stress conditions. After full pod, threshold numbers should be increased.

Field scouting should begin in late June and continue until mid-August, especially when local aphid activity has been reported. Scouting protocol calls for evaluation of five locations for each 20 acres scouted. View five plants per location. Scout whole fields as this aphid is mobile enough that "hot spots" and "edge effect" scouting is not recommended. Do not use preventative insecticides or treat fields when aphid numbers are low. Aphids have the ability to bounce back much quicker than beneficial insects. Beneficial insects include: lady beetles, lacewings and minute pirate bugs. Take special note of winged aphid adults, since once their flight starts, field populations may decline rapidly. Prioritize late-planted, double-crop or stressed soybeans as they are most susceptible to aphid damage.

Consider applying an insecticide when: 1) Soybeans are at first bloom or later; 2) Aphid population is increasing and greater than 250 per plant across most plants; 3) Lower leaves not yet covered with honeydew; 4) Infested plants are not yet stunted; 5) A majority of the aphids are not winged.

Insecticide seed treatments have delayed soybean aphid population growth, but do not depend upon them to provide season-long aphid protection. Most aphid damage occurs late in the season after seed treatment activity has diminished. The migratory nature of soybean aphids allows seed-treated fields to become reinfested.

Several liquid insecticides offer good soybean aphid control. The three major chemistry choices are: synthetic pyrethroids (Asana, Baythroid, Mustang Max, Pounce and Warrior), carbamates (Furadan) and organophosphates (Lorsban, Penncap-M). In general, under hot July/August conditions, the organophosphates and carbamates have quicker knockdown and less residual activity, and are easier on beneficial insects when compared to the pyrethroids. All of these materials are restricted use products.

Most treatments are recommended after flowering when aphids are located low in the canopy. Therefore, spray coverage is critical. For ground applications, use high spray volume and spray pressure.

Hit and miss
Why was 2003 such a bad soybean aphid year? First, 2002 had relatively low aphid numbers (still enough for a decent seed stock) but also had a relatively low number of predators. Early 2003 spring temperatures were warm, with winged adults being initiated in mid-April and early generations being found in soybeans by late May. Much of the early summer was cooler than normal, with temperatures rarely rising above 85 degrees. This combination allowed multiple aphid generations.

Many 2003 soybean aphid management mistakes were made because of our inexperience in dealing with such a late season pest. There was a lack of timely scouting. Many growers and dealers experienced application delays, partially because of limited aerial application services. There were several localized product shortages because the industry was not prepared for the large insecticide demand in mid-August. Many individuals had a difficult time deciding when to quit spraying. Hopefully, we learned from these mistakes and will be prepared for the next serious soybean aphid year.

2004 outlook
What is the 2004 forecast? I am cautiously optimistic that aphid numbers may be low. We had large autumn predator populations in 2003, especially Asian lady beetles. Winter egg counts and early-season aphid captures this year have been low.

The University of Missouri integrated pest management program and MFA Incorporated certified crop advisers will have updated information throughout this field season. Hopefully, widespread treatment of this pest will not be necessary.

Dr. Paul Tracy is director of agronomy technical services for MFA Incorporated.