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Soybean aphid a formidable foe

Over the past decade the soybean aphid has proven to be an adaptable insect pest. Severe infestations can reduce yields by as much as 40%, but it is a sporadic pest. You need to scout fields regularly to keep an eye on populations. They can explode in a matter of days to damaging levels.

Aphids cause damage when high numbers of them populate a field, sucking the plant sugars from leaves. Also, they excrete honeydew onto soybean leaves from which airborne molds can land, grow and produce sooty mold on the leaves that blocks photosynthesis. The time and expense to scout, and if necessary to spray an insecticide, are well worth it. New and improved tools to make management of this pest even easier include the recent development of resistant soybean varieties and the use of “speed scouting.”

Soybean varieties that have resistance to aphids are the newest management tool. Several seed companies have released soybean varieties that feature the non-genetically engineered Rag 1 or Rag 2 traits.

Aphids feeding on soybean plants that have either the Rag 1 or Rag 2 genes do not live as long or produce as many offspring compared to aphids feeding on susceptible soybean plants. In small plot evaluations of aphid-resistant soybeans, there is a dramatic decrease in the seasonal accumulations of aphids compared to susceptible soybean varieties. If aphid outbreaks occur, aphid-resistant varieties can yield more than susceptible varieties even without the use of insecticides.

Try ‘speed scouting’ aphids

Even resistant soybean varieties should be scouted for soybean aphid. Resistant plants will have fewer aphids than susceptible plants, but will not be aphid-free.

Resistant varieties can fit into soybean aphid management plans by allowing growers to buy some time, staving off insecticide applications for weeks, or even for a whole season, if the population never reaches the economic threshold.

Speed scouting is a time-saving alternative to standard scouting. It can be used with resistant or non-resistant varieties, with insecticide treated seed or not. It fits with any situation during the general scouting window starting around mid- to late June through R5.5 growth stage.

Speed scouting simplifies scouting by classifying plants as either “infested” or “non-infested.” If a plant has fewer than 40 aphids, it’s considered non-infested. But if the plant has 40 or more aphids, it’s considered infested. There is no need to count any higher than 40 on a given plant.

Making treatment decisions

The speed scouting card from ISU Extension shows steps to take and how to make treatment decisions. For example, following speed scouting instructions, if 23 of 31 randomly selected plants have more than 40 aphids on each, it’s time to treat. Speed scouting is not a new economic threshold; it’s just a different mathematical approach to get to the same kind of results. The economic threshold is still 250 aphids per plant with increasing aphid populations through R5.5 stage of soybean growth.

Iowa State University recently revised its handy pocket-size, fold-out card titled “Speed Scouting for Soybean Aphid.” It goes by publication number CSI 0015, revised May 2012, and is available from the ISU Extension Online Store and at most county Extension offices. Authors are Erin Hodgson, ISU Extension entomologist, and Adam Sisson, Integrated Pest Management specialist.

ISU has also released the publication “Soybean Aphid-resistant Varieties for Iowa.” It has helpful information for considering whether to plant aphid-resistant soybeans, and an explanation of how to use the varieties properly. And for more soybean aphid management information, consult “Soybean Aphid Management Field Guide,” second edition.

Lang is the ISU Extension field agronomist at Decorah in northeast Iowa.

This article published in the July, 2012 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.