Soybean aphids can now be found in many soybean fields. In some fields (but certainly not all fields) in southern Minnesota, soybean aphid populations are approaching levels requiring insecticide application to prevent economic losses. This critical soybean aphid population level, referred to as the economic threshold, is an average of 250 aphids per plant AND aphids on more than 80% of plants AND aphid populations increasing. Many fields are well below this level and do not require insecticide application for aphids at this time. Scouting is required to determine which fields require or may soon require treatment and which fields do not. A guide for soybean aphid scouting in Minnesota was recently posted (http://z.umn.edu/soybeanaphidscouting).
Here, I provide a summary of some key points on aphid scouting, but encourage you to review the full document.
- Scouting requires getting into the field and estimating aphid numbers on plants.
- Aphid populations can vary from field to field, so each field being managed should be scouted.
- Estimate aphids from a representative number of plants spread throughout the field.
- Aphid counts should include winged and wingless aphids, but don't count dead aphids or aphid look-alikes.
- Fields should be scouted on a regular basis (about weekly). Rapidly growing populations may need to be scouted more frequently.
- Scouting should continue until R6.5 (pods and leaves begin to yellow), regardless of calendar date.
- Through R5 (seeds developing, but pod cavity not filled), use the economic threshold of an average of 250 aphids per plant AND aphids on more than 80% of plants AND aphid populations increasing.
- Yield loss can occur into early R6 (pod cavity filled by seed). At this stage consider insecticide application if aphid populations are high and plants are experiencing other stress.
- Fields should be scouted after application of a foliar
insecticide to monitor for resurgence of the aphid population or outbreak of a secondary