Thursday, June 18, 2015

Soybean aphids on Minnesota soybean: They’re out there, but don’t panic

by Robert Koch, Extension Entomologist

Soybean aphids can now be found in soybean fields in southern Minnesota. There are also reports of soybean aphids from west central Minnesota. However, there is no need to panic. The percentage of plants infested and number of aphids per plant are still low (far below economic levels). Furthermore, aphid predators, such as lady beetles, have been observed feeding on soybean aphids in some fields.  As we get into late June, you may want to begin checking soybean fields for aphids.

Early in the season, pay particular attention to fields in areas with abundant buckthorn, smaller fields with wooded borders, and/or early-planted fields. At this time of year, soybean aphids will typically be found on the new growth. Also, the presence of lady beetles or ants on soybean plants is often an indicator of the presence of early-season aphid colonies.

Last year, Bruce Potter and I created a guide for soybean aphid scouting in Minnesota (http://z.umn.edu/soybeanaphidscouting). Here’s a summary of some key points on aphid scouting and management that you should keep in mind throughout this growing season.
  • 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.
  • Infested 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 pest.

Again, the soybean aphid infestations that I have seen and heard of this year are currently far below economic levels. It is too early to tell what will become of these infestations as the season progresses. However, if you implement a regular scouting program, you can stay abreast of the infestations in your fields.