Soybean Aphid

Bryan Jensen, UW Extension and IPM Program

Soybean aphid reports indicate that numbers are still low but isolated hot spots have been found within some fields. Now is the time to initiate field scouting if you haven’t already. I think most people are familiar with the economic threshold of 250/plant on 80% of the plants when soybeans are in the R1-R5 stage of development. However, there is one aspect of that threshold which is often overlooked. That is the aphid population must be increasing. It is important to recognize if the population is trending up, down or stagnant. Measurable yield loss, let alone economic yield loss, does not happen at 250 aphids/plant. Rather the 250/plant is a trigger for people to consider spraying aphid populations to prevent them from reach a much higher population where economic yield loss can occur. Therefore, multiple field visits are necessary to recognize population trends. Making a no spray decision is difficult to say the least. The more information you have to support that decision certainly increases that comfort level and reduces anxiety. Multiple scouting dates will certainly give a clearer picture regarding population trends.

As you are making field visits, keep records on beneficial insects and % white dwarfs. This information can help explain why aphid populations are increasing or decreasing. Beneficial insects can play a big role in aphid management. White dwarfs (blue triangle) are simply smaller versions of the “normal” soybean aphids (red circle) and develop as a result to changing environmental conditions. White dwarfs are important to observe because their life span is approximately ½ that of normal soybean aphids and their reproductive capacity is only 70% of normal aphids.

A final word on use of “insurance” applications. That is to say spraying at below threshold numbers because insecticide costs are cheap. Are they? If you do not have an insect population that is capable of causing economic loss you are adding to the cost of production, especially in a year when margins are tight. You will kill natural insect enemies which can lead to a resurgence of aphids, or the release of a secondary pest (spider mites?) which are normally control by these natural enemies. Driving over soybeans automatically reduces yield because of wheel traffic and if that application does not have economic value to begin with the wheel traffic takes even more money out of the pocket. Finally, overuse of insecticides can lead to resistance. That is not a road we want to go down.

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