Thursday, July 31, 2008

Soybean Growth Stage Does Not Matter: Soybean Aphid Threshold Should Remain at 250 Aphids per Plant

High commodity prices coupled with receding flood waters have some growers pushing their soybean planting dates well into late July. As these soybeans emerge and develop they will be greeted by an increasing population of soybean aphids. Dr. Eileen Cullen UW Field Crops Entolomogist suggests the following plan of action to manage the soybean aphid on late planted soybean.
From Dr. Eileen Cullen:

For vegetative stage soybeans, if aphids are colonizing, do not treat the soybeans below 250 aphids/plant. Data from yield loss experiments in WI and throughout the North Central region show that there is no yield benefit to decreasing the threshold, even on small soybeans (Journal of Economic Entomology 100: 1258-1267). Keep in mind also that 250 aphids/plant is not the Economic Injury Level (EIL). The EIL is the number of aphids that need to be present for the value of the lost yield to equal the costs of control. There is lead time of 7 days to reach that +/- 674 aphids per plant EIL.

Now that the market value for soybeans has risen, a lowered EIL can be calculated. David Ragsdale, the lead author on the paper describing the existing EIL for soybean aphids, calculated a new EIL for soybeans selling at $15 per bushel, with $8 per acre control cost, and an anticipated yield of 50 bushels per acre. With these values the EIL is lowered to 452 aphids per plant. However, this doe not lead to a decrease in the Economic Threshold of 250 aphids/plant.

Entomological research in six states (including WI) from 19 yield-loss experiments conducted over a 3-yr. period found no detectable yield differences attributed to treating soybean aphids at numbers below the Economic Threshold of 250/plant. Additionally, treating soybean aphids below the ET of 250 aphids/plant increases risk to producers by treating an aphid population that is growing too slowly to exceed the Economic Injury Level, eliminates generalist predator insects, and exposes a larger portion of the soybean aphid population to selection by insecticides, which could lead to development of insecticide resistance.

If the ET of 250 is reached/exceeded on vegetative stage soybeans, treatment is warranted on the double-cropped beans, due to winged soybean aphid dispersal flights and potential outbreak.conditions in affected areas such as Western Wisconsin. Continue to scout soybeans following treatment to watch for recolonization and soybean aphid resurgence.

Economic threshold is being exceeded in many fields in Western Wisconsin. Winged soybean aphids are dispersing from southern MN and westerly, northwesterly winds have the potential to move dispersal flights toward WI. (This is largely happening in parts of Western WI now). For full information on Soybean Aphid Scouting and management recommendations, please vist the Wisconsin Crop Manager Newsletter article “Soybean Aphid Increasing and Variable – Scout Fields Now” at
Image 1. Winged soybean aphid colonizing V2 soybean plant.Image 2. Soybean aphids feeding on V2 soybean plant.
Photos by John Gaska