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Develop plan to manage aphids

It’s been a decade since soybean aphid populations skyrocketed across the upper Midwest. Managing this pest has become an important part of profitable soybean production.

A good management plan begins with scouting. The unpredictable nature of the aphids during the growing season makes routine scouting the only way to know what is going on in your fields. Experience has taught growers that fields, even as close as across the road from each other, can have very different levels of aphid populations.

Scout early

Begin scouting fields weekly in late June or early July. Scouting in the later parts of the vegetative growth stage will help you prepare for protecting the soybeans, if needed, when they reach the reproductive stage. Count the number of aphids present on 20 to 30 plants per field. When counting, examine the entire plant. Typically, aphids are found in the newer growth; however, environmental conditions can cause them to be found lower in the plant early in the season. Be sure to look at plants throughout the field to get a good picture of the entire field. Also note how much beneficial insect activity is going on.

The University of Wisconsin Nutrient and Pest Management Program has developed a useful tool for speeding up the counting process. To obtain the Visual Guide for Soybean Aphid Scouting, contact your county Extension office, call the NPM Program at 608-265-2660, or go online to ipcm.wisc.edu/Home/
tabid/36/Default.aspx
in the “Publications” section and download one. Another helpful tool for scouting is a magnifying glass.

Regular field visits are critical as aphid populations can reach economically damaging levels within seven days. Keep in mind, however, that the mere presence of aphids does not mean they will necessarily reach damaging levels. A number of factors play a role in their population growth or decline, including beneficial insects, temperature (aphids reproduce fastest between 68 and 77 degrees F), moisture levels, aphid fungal disease presence, plant growth stage and plant stress level.

It is not a sound idea to treat a field at the first discovery of low numbers of aphids. While this practice has been promoted, there is no unbiased research to support it. This situation can actually create an even bigger aphid problem not long after initial treatment. The treatment will clean out all insects in the field, including beneficial insects. After the insecticide residual is gone, this provides a clean and welcoming environment for migratory aphids to quickly repopulate and can result in the need to treat again during the season.

The economic threshold for treating is when 80% of the field has an average of 250 aphids per plant and the population is actively increasing for soybeans between beginning bloom (R1) and full pod (R4). There is about a seven-day cushion built into the economic threshold that allows time to schedule treatment or deal with weather delays.

Later growth stages (R5 and beyond) can tolerate greater aphid numbers; how many more has not been determined as of yet. Once plants reach full seed (R6), it is too late to protect against any yield loss, and treatment would be a waste of money.

Once the threshold is met and the decision to treat a field is made, make sure the insecticide is properly applied for good coverage. Poor coverage can lead to population rebounds and the need to treat again. Follow the product label for best results. Scout fields after re-entry intervals have passed to make sure the desired outcome has been obtained.

Halfman is the Monroe County Extension agriculture agent.

This article published in the July, 2010 edition of WISCONSIN AGRICULTURIST.

All rights reserved. Copyright Farm Progress Cos. 2010.