Scouting soybean fields for pests

By Scout Nelson

Crop scouts are actively surveying western Minnesota soybean fields this summer to detect pests and diseases as part of a Minnesota Soybean Research & Promotion Council program. Recent field visits have revealed the increasing prevalence of various pests and diseases in these areas.

However, because planting dates and soybean varieties differ across the state, issues observed in scouted fields may not match those in your own fields. Therefore, it is essential to scout your own soybean fields.

Maps from joint efforts by the University of Minnesota and North Dakota State University scouts are available for viewing soybean and wheat pests from previous weeks and into August.

The percentage of plants infested with soybean aphids has varied widely, ranging from 0 to 100%, with some fields in northwest and southwest Minnesota and southeast North Dakota experiencing complete infestation. Early planted fields with larger plants are particularly susceptible to early aphid infestations as winged soybean aphids migrate from buckthorn to soybean plants in spring.

Although the number of aphids per plant was relatively low as of last Friday, soybean aphids can double in population in as few as three days under moderate temperatures (70-77 degrees) and low relative humidity.

Thus, regular scouting is crucial to avoid significant yield losses and unnecessary treatments. Understanding the population dynamics in your field is also essential. For example, if your field is near the treatment threshold later in the season and you observe more winged aphids, a follow-up scouting a few days later may indicate whether these aphids have migrated.

Soybean aphid populations reach the treatment threshold when the following criteria are met:

• The average aphid count exceeds 250 per plant.
• More than 80% of plants are infested.
• The aphid population is increasing.

The 250-aphid threshold is conservative, as detectable plant damage has not yet occurred. Typically, growers have about 5-7 days before aphids reach the economic injury level of approximately 670 aphids, the break-even point for insecticide treatment.

Photo Credit: istock-ds70