Heads Up! Pest Alerts

It is the season to begin thinking about management of soybean aphids in our soybeans. Although we usually do not see much activity until the end of June, it is the growth stage of the plant that is the important indicator. Let's review the situation, look at a management plan, and then, if necessary, apply appropriate controls.

The soybean aphid has been in Kentucky since at least 2000. In those years the pest has been largely a curiosity and hopefully, it will remain so.

Since first discovery of the insect we have conducted informal surveys of where it has occurred. From this information it is obvious that the pest can reach all of Kentucky's soybean producing regions every year. However, for the most part, the numbers of aphids remain very low.

The generally low numbers are probably due to two reasons. First, the soybean aphid can not over winter in Kentucky. This pest has a complex biology, but part of its needs is the presence of a specific plant on which to lay over-wintering eggs. The common buckthorn (Rhamus cathartica) is the most important host but there are at least two other species that may also serve as over-wintering hosts. These plants either do not occur in Kentucky or are very rare. The result is soybean aphids must travel from more northern areas (N. Illinois, Wisconsin and Michigan) to get to us. Secondly, research in Wisconsin indicates that the soybean aphid is a "cool" weather insect. It does best at moderate temperatures. For example, the optimum temperature for growth and reproduction is from 68-77^o F. At 86^o F reproduction decreases by 60% and at 95^o F the aphids die.

The number of aphids in the 2004 crop depends, at least in part, on how many aphids survived the winter. Winter survival depends, in part, on how many aphids migrated back to the over-wintering trees in the fall of 2003. Observations by the research group at the Illinois Natural History Survey indicate that the fall 2003 migration was smaller than expected. Hopefully, this will result in fewer aphids in 2004.

Early detection is of major importance with this pest. Generally, in Kentucky we need to start looking at the end of June / beginning of July. However, it is the plant growth stage, not the calendar date, that is important. Plants should be examined beginning AT LEAST by the final vegetative stage.

Late planted and/or double-crop plants are probably at the greatest risk. Their reproductive stages occur later in the season, which allows more time for the aphids to migrate to Kentucky and become established. Additionally, any plant stress, but especially drought stress, increases the risk of yield loss at any particular aphid population level. Check these fields first.

When you examine plants, check the whole plant. Though aphids are most often found on the newest leaves, their intolerance to heat will cause them to move about the plant searching for the least stressful environment.

Aphid build-up generally occurs during the reproductive stages. Current research indicates that aphid density during the early reproductive stages (R1-R3) is definitely related to yield loss. Plants in the R4, and later stages, can tolerate greater aphid numbers, but just how many is a bit of a guess and undoubtedly depends upon environmental conditions. Current research may indicate why this is the case. It appears that yield reduction is based on the number of pods per plant. Seed weight and number of seed per pod do not seem to be affected.

Research and experience has been used to develop a set of action thresholds for soybean aphid. These thresholds were developed by consensus of extension/research entomologists working in the infested states. Though they are likely to become more refined and robust as more work is completed, they are certainly serviceable at this point. However, many questions remain to be answered.

At present it is suggested that insecticidal control be applied if soybean aphid populations are: 1.) increasing and 2.) reach the level of 250 aphids per plant during growth stages R1 through R4. At R5 – R6, levels would need to be between 800 and 1000 aphids per plant. After R6, data indicates that nothing is gained by applying control.

It is important not to "pull the trigger" too soon. Many predators and parasitoids feed on these aphids, keeping their populations low. Application of insecticide removes this "Natural Control" but will not control all the aphids. Those that live have a predator-free environment in which to feed and reproduce.

In the unlikely event that insecticidal control is warranted, producers will have several products from which to choose. The synthetic pyrethroid products - Asana XL (esfenvalerate), Fury (zeta-cypermethrin), Mustang Max (zeta-cypermethrin), Warrior (lambda-cyhalothrin) and the organophosphate products - Lorsban (chlorpyrifos) and Penncap-M (methyl parathion) currently have federal labels.

In Kentucky, marketers of two additional products have issued 2(ee) recommendations. They are for Baythroid 2 (cyfluthrin), a synthetic pyrethroid, and Chlorpyrifos (chlorpyrifos), an organophosphate, that can be used within the state. If you choose to use one of these products make sure you have a copy of the 2(ee) recommendation sheet.

Regional research has shown that good return on investment can be made if insecticides are used in a timely manner when the appropriate populations of aphids are present. As timing (plant stage) changes and population size decreases from the recommended numbers, likelihood and size of return on control cost decreases.

Review

Status in 2004.

• The soybean aphid was first discovered in KY in 2000 and has been seen all subsequent years.

• The soybean aphid can be found in the complete KY production region every year.

• Generally aphid numbers remain low.

• Soybean aphid can not over-winter in KY. They must migrate to KY each year.

• Soybean aphid does not appear to be well adapted to hot temperatures.

• Fall 2003 migration to over-wintering hosts was reduced. Hopefully, this will result in fewer aphids in 2004.

Management

• Early detection is important. Begin examining plants AT LEAST by the final vegetative stage. Generally, about the end of June is a good time to start.

• Late planted / or double crop beans are at greater risk than early planted plants.

• Drought stress increases risk of yield loss.

• When checking plants, check the whole plant.

• Protecting plants during the R1 to R3 stages is the most important time.

• Plants in the R4-R6 stages can tolerate more aphids.

• After R6 it is unlikely that controlling aphids has any impact on the yield.

• Thresholds have been developed by entomologists across the soybean infested states.

• With an actively increasing population of soybean aphid, use an action threshold of 250 aphids per plant at R1 through R4 and 1000 aphids per plant from R4 through R5. This threshold incorporates an approximate 7-day lead-time between scouting and treatment. After R6 no control is warranted.

• Do NOT apply insecticides too early. You may end up with more aphids than you had!!

Insecticidal Control

• Several insecticides are available for use if needed. See INSECT MANAGEMENT RECOMMENDATIONS FOR FIELD CROPS AND LIVESTOCK at: http://www.uky.edu/Agriculture/PAT/recs/rechome. htm

• Print copies are available at your Kentucky County Extension office

(first published in Kentucky Pest News, Number 1025,June 14, 2004)

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	Original document: 16 June 2004 Last updated: 16 June 2004