Insurance Insecticide Treatments For Soybean Aphids

Why we are concerned about this management strategy

By Kenneth Ostlie, Bruce Potter and David Ragsdale

Everyone dreads another pesticide trip across the field. You may be planning or have been encouraged to apply an insecticide with an upcoming glyphosate application, without regard for aphid populations in the field. While this strategy may occasionally work out, there are several potential problems that can arise from this strategy. 1. Cost -- There is no data to suggest that very low aphid populations hurt yield. Early applications are more likely to be re-colonized and require re-treatment. Claims of insecticide residual activity lasting a month, or longer, have little factual basis, particularly when applications are made to rapidly growing soybeans.

2. Resistance -- The more often soybean aphids are exposed to insecticide the more quickly insecticide resistant populations will develop. More than one product could lose effectiveness at once, depending on the mode of resistance. An unpleasant wrinkle to soybean management would be aphids that won’t die.

3. Increasing populations of soybean aphid, or other arthropod pests (e.g. spider mites) by removal of beneficial species -- Removal of beneficials (predators and parasites) can have unexpected consequences. Yes, this really does happen! Imagine how quickly newly arrived aphids reproduce when you’ve already removed the beneficials for them. When we do this with cages that exclude predators, aphid populations go from 10 to more than 1000 in a little more than a week.

4. Compromises leading to poor insect and/or weed control -- Ideal nozzle, water volume and pressure selection for insecticide and glyphosate applications are not the same. Herbicide and insecticide timings should be based on when to apply to the target pest (weed or aphid) to be most successful.

You are responsible for managing your crop for a profit. There is nothing illegal about applying an insecticide labeled for soybean when aphid populations are below threshold. However, insecticide applications do have consequences in the environment. We wish only to point out that there are potential short and long term risks when insecticide applications are made without regard to pest populations.

Epilogue

The following experiment dramatically illustrates one of the reasons we were concerned.

Using a product which claims to have long residual control for soybean aphids, we compared an application that was applied during glyphosate application (July 16, 2008) against an application after the economic threshold (250 soybean aphids/plant) had been reached. The trial was conducted at the Southwest Research and Outreach Center in Lamberton, MN. The economic threshold treatment actually went on well past economic threshold level and was applied at over 700 soybean aphids/plant to R3 stage soybeans on July 30, 2008.

An aphid migration out of the trial occurred in early August and dropped populations in the untreated plots as well as those with an insecticide application. Due to winged aphid immigration, aphid populations rebounded in the early treated plot but remained below threshold in the properly timed treatment. While the early treatment yielded better (43 bushels) than the untreated check (39 bushels), it yielded significantly less than the properly timed application (46 bushels). This plot was not sampled for aphids after August 19 but it appears that the economic injury in early treated plots occurred after that date. These data are reflective of what happened in many fields with many products during the 2008 growing season and often with more serious yield penalties.

Ostlie is Extension Entomologist, University of Minnesota; Bruce Potter is IPM Specialist SW MN, University of Minnesota; and David Ragsdale is Entomologist, University of Minnesota