Integrated Crop Management

Soybean aphids -- next two weeks critical

Soybean aphid populations continue to climb in northeastern Iowa, as reported by Brian Lang, extension field crops specialist. As of July 6, almost all of the plants in a sentinel field were infested with aphids. The average soybean aphid population was 179 aphids per plant, still below the economic threshold of 250 aphids per plant.

Near Ames in Story County, populations also are building. I scouted a field on July 9 and 83 percent of plants were infested, but 90 percent of plants had fewer than 40 aphids per plant. However, several plants (less than 1% of the total plant population) had at least 750 aphids per plant. Heavily infested plants were stunted by 4--6 inches and had ants tending the aphids for honeydew and lady beetles feeding on the aphids. Fortunately, many counties throughout Iowa have very low soybean aphid populations and may not experience economically damaging populations.

The next two weeks will be a critical time for scouting for soybean aphids and making management decisions. Populations will likely increase and plants may be under additional stress from insufficient soil moisture. Populations may continue to increase, but only field scouting can provide the information necessary to make an informed decision. The economic threshold for soybean aphids is still 250 aphids per plant. I have heard of fields being sprayed with populations at 50--100 aphids per plant because the grower feared that the population would reach 250 per plant. Remember that 250 is the economic threshold--the population level at which a pest population should be controlled to prevent it from reaching the economic injury level. The economic injury level is 1,000 aphids per plant--this is the population you do not want to reach. The goal is to not spray until the population reaches 250 per plant but before it reaches 1,000 per plant--these are the boundaries.

Additional information on where aphids have been reported in Iowa and management recommendations can be found at

Decorah, Iowa: Percent plants with aphids
2002 2003 2004 2005
June 1 -- -- 0 6
June 7 -- -- 0 15
June 15 -- 40 0 33
June 22 7 90 0 31
June 29 15 100 0 85
July 6 70 100 4 99
July 13 93 100 8
Decorah, Iowa: Average aphids per plant
2002 2003 2004 2005
June 1 -- -- 0 0.1
June 7 -- -- 0 1.4
June 15 -- 10 0 2.5
June 22 1 115 0 4.0
June 29 2 341 0 48
July 6 14 745 0.5 179
July 13 25 2,803 1
Ants and aphids [1]
Ants on a soybean plant often indicate the presence of aphids. The dark-colored ants can be used as 'crop scouts' to help in locating small colonies of aphids that are difficult to see with the naked eye. (Marlin E. Rice)

This article originally appeared on pages 141-142 of the IC-494(18) -- July 11, 2005 issue.

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