The weather has been ideal for soybean aphids and has been dry enough in areas to also see mites starting up. Some fields are starting to see one or both of these pests increasing in numbers. Here is just a reminder of important things to consider when assessing fields:
Soybean Aphids – The threshold of “250 aphids per plant and increasing on 80% of the plants” works for up to the end of the R5 stage. Most fields have been in pretty healthy condition and can tolerate a few more aphids than 250 per plant, though make sure not to let it increase past 660 aphids per plant which is the economic injury level where yield is lost. Once into R6, you need more aphids per plant before yield is impacted. And you have to start worrying about pre-harvest intervals for the products registered. If you are having trouble deciding if a spray is necessary, try the free Aphid Advisor when scouting. It will help you sort out if a spray is required.
Spider mites – Some fields are starting to see the symptoms of spider mite injury. Plants are starting to look bronze in colour or leaves are starting to drop off the plant. 4 mites per leaflet or one severely infested plant indicates control is needed. A spot spray in problem areas might be enough, as long as the mites have not moved beyond these hot pockets. Dimethoate (Cygon or Lagon) is the only active registered on soybeans that would work on spider mites. The pre-harvest interval for Cygon or Lagon is 30 days so control can really only take place up to early R6. DO NOT use Matador on spider mites as it kills the predatory mites that feed on spider mites and can cause the spider mite populations to increase.