By Robert Koch, Ken Ostlie and Ian MacRae, Extension Entomologists; Bruce Potter, IPM Specialist
Soybean aphid populations in many areas of Minnesota are increasing. This year, there are a number of factors making population development and management less predictable than in the previous couple of years:
- Late summer dispersal of soybean aphids is currently occurring, bringing high numbers of winged aphids to colonize fields — sometimes those that were previously treated.
- Forecasted weather conditions for the upcoming week look favorable for aphid population growth.
- A number of fields in southwestern Minnesota have reported unexplained failure (poor performance) of recent insecticide treatments and will require additional applications to control existing populations.
All of these factors point to the importance of weekly scouting for soybean aphids and treating when populations reach the threshold of 250 aphids per plant when 80% of plants have aphids (University of Minnesota guide to soybean aphid scouting). At the threshold, yield loss is not yet occurring, but will if aphids are not soon controlled.
If a field needs to be treated more than once in the same year, remember the potential for development of insecticide resistance. Do not reapply the same insecticide mode of action (insecticide group).
For example, if a field was treated with an organophosphate insecticide and needs to be treated again for aphids or some other pest, such as spider mites, avoid using organophosphates for the second application. Instead, use a different insecticide group, such as a pyrethroid. The mode of action (or group) is on most insecticide labels.
Of particular concern this week is a series of reports from southwestern Minnesota of performance issues related to a pyrethroid insecticide. These failures are being evaluated and at this point appear restricted to a narrow area in southwestern Minnesota. Failure of an insecticide does not necessarily mean the pest is resistant to the insecticide. Several factors can contribute to poor performance of an insecticide:
- misapplication of the insecticide (incorrect insecticide or rate, poor coverage),
- unfavorable weather conditions (wind, rain, temperature), and
- recolonization by the pest.
If these factors can be ruled out, insecticide resistance may be the cause. Resistance is not necessarily immunity to the insecticide. Resistance may be expressed as reduced susceptibility, where higher rates of insecticide are required to kill the pest (rate creep). However, do not respond with higher rates of the same insecticide. Doing so can increase the rate at which resistance will develop.
If aphids were treated with a pyrethroid and need to be retreated, use a labeled insecticide that is not a pyrethroid for the second application. If a third application is needed, consider a third insecticide mode of action, or if another is not available, consider a pyrethroid other than what was previously used. Do not "spike" (tank-mix) with a below-label rate of another insecticide. Use of rates below that recommended by the label can increase chances of development of resistance.
As the season progresses, be aware of the pre-harvest intervals of the various insecticides.