Bryan Jensen, UW-Madison Dept. of Entomology and Integrated Pest Management Program
The calls about soybean aphids have started to come in but let me put this comment in perspective. Those calls have usually been a curiosity and prompted because of a gradual increase in aphid number. Certainly, high numbers have not been reported. However, a little spraying has started.
We have been in a weather pattern that could be conducive to soybean aphid buildup so continue to keep scouting. We have a few things on our side to help with your aphid management decisions. First, a lot of our soybean were planted earlier than the previous two years. Which means as each day goes by, we have banked more yield so there is less yield to protect. We are also getting reports of summer dwarfs which are shorter lived and have a lower reproductive rate.
Soybean aphid populations have been relatively low all season. Certainly, we need to continue to use and respect the long-established threshold of 250/plant (80% of plants infested and the aphid populations must be increasing) but another way to look at it is how long have the aphids been feeding. My point is a population of 250/plant for 3 days is less problematic than the same populations that was present for 3 weeks. That scenario is a bit simplistic, but it does indicate my point. A population that spikes to 250/plant may not need immediate spraying if, for example you are close to R6 (when little if any yield response would be expected) and/or natural enemies are building and/or hot weather is expected that should slow reproduction.
In addition to using the 250/plant threshold also consider looking at cumulative aphid days. One aphid day equals 1 aphid feeding on a plant for one day. Ten aphids feeding on one plant for one day equals 10 cumulative aphid days. That same 10 aphids feeding for 10 days equals 100 cumulative aphid days. The economic threshold for cumulative aphid days is 5,500 days. I am hopeful this alternative method of using the same threshold will help you put the need for control in perspective and is especially useful if you have weekly scouting data.
The closer we get to the end of season soybean aphid population crash the lower your probability of a return on investment that your spray decision should provide. Keep in mind if using ground application equipment wheel traffic will slight reduce yield. With that in mind, I do think you want to be relatively confident that an insecticide application is necessary. These and other reasons make late season economic spray decision very difficult. I guess at this time of the growing season I try to come up with reasons not to apply an insecticide. If I can’t? I feel more confident I made the right decision. Also spray decisions usually do not have to be made on the spot. Often decision can be delay for a few days to allow more time for observations and to build confidence.
When deciding to make a spray decision be sure to check for the presence of economic populations of two-spotted spider mites. Some areas of Wisconsin have been dry and spider mites may be a concern. If present in low numbers, it puts more pressure on making a correct spray decision. Insecticides used to control soybean aphids will kill beneficial arthropods which could easily flare spider mite (and aphid) populations. If spider mites are present in economic numbers it should also impact your spray decision to choose a product with aphid and spider mite activity.