Soybeans

(APG file photo)

The recent hot weather and heavy rainfalls have slowed soybean aphid populations in some of the fields in southern Minnesota. Some of the aphid populations have left indicator spots at the University of Minnesota South West Research and Outreach Center according to Bruce Potter, Extension Integrated Pest Management Specialist.

Although new vegetation is still being produced at the top of the plants, some aphids are now found lower in the canopy, perhaps a response to heat and storms. Scouting soybeans however is still recommended as temperatures fluctuate throughout the day and temperatures within the soybean canopy may differ from the above-canopy air temperature. Therefore, you should not assume that high summer temperatures will stop aphid population growth.

When scouting for soybean aphids the number of aphids per plant should be estimated from a representative number of plants. Plants should be selected from throughout the area being scouted. The average number of aphids per plant should then be calculated. Through R5 (seeds developing, but pod cavity not filled), use the economic threshold of an average of 250 aphids/plant AND more than 80% of plants having aphids AND aphid populations increasing. Count both adults and nymphs, including any white dwarves. However, avoid counting soybean aphid look-a-likes, such as potato leafhoppers and whiteflies, as well as cast soybean aphid skins and dead aphids.

Soybean aphids reproduction most rapid R2-R4 and late R5-early R6. Under ideal environmental conditions, soybean aphid populations can double every 2 days (usually 3 days or longer). Detectable soybean yield loss does not occur at densities of aphids below the economic threshold of 250 aphids per plant (for a short period).

Application of insecticide at aphid densities below this threshold, such as 5, 50 or 100 aphids per plant, are unlikely to save yield and may result in treating a large percentage of fields where aphids would never develop into yield threatening problems. Treatment below the economic threshold also increases the risk of negative impacts on beneficial insects (predators, pollinators) and the risk of aphids developing resistance to insecticides

The economic threshold of 250 aphids per plant remains valid despite changes in soybean and input prices. These changes in prices adjust the economic injury level (EIL), which is the point at which losses are sufficient to justify insecticide application. However, the economic threshold of 250 aphids per plant is still sufficiently below the EIL to allow several days to make an insecticide application before losses occur. A lower economic threshold for soybean aphid is not justified at this time.

Dave Nicolai is extension educator for crops at the University of Minnesota Extension Office.

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