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Angie Peltier

Angie Peltier
Former Extension Educator, Commercial Agriculture

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Hill and Furrow

Current topics about crop production in Western Illinois, including field crops research at the NWIARDC in Monmouth.
Figure. Stippling (small yellow or white spots) caused by the two-spotted spider mite on soybean.
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Two-spotted spider mites abundant in Western Illinois soybean fields

At a recent farm visit right outside of Alexis, soybean producers and I scouted a field for the two-spotted spider mite. The infestation in this field was wide-spread and had likely reached threshold days or weeks prior.

Two-spotted spider mites are very small, eight-legged, spider-like soybean pests. Spider mites can attack soybeans between May and September and can result in irreversible leaf damage that may lead to defoliation and yield loss.

When approaching a spider mite infested field the first thing that you can see from 10 or more feet away is the discoloration of the leave s- they have a more yellow -silvery sheen than non-infested plants. As you get closer, you may be able to see symptoms called stippling, or very small white or yellow specks on the leaf tissue (Figure). When you look even closer you may see small webs. Webs can be seen best when looking at the leaf surface from the side and not directly at the planar surface. You also may be able to see adults with the naked eye if they move, but a handlens is almost essential. Adult two-spotted spider mites are yellowish/clear in color with a conspicuous black spot on each side of the body and have no wings. Under a microscope you can see both adults and eggs (Figure).

Two-spotted spider mite infestations are favored by very hot, dry weather which can allow the arachnid to complete its life cycle from egg to adult in as little as 5 to 7 days. This feature can allow the spider mite to spread very quickly throughout a field if hot-dry weather persists throughout the season.

To consider whether you should treat your soybeans for spider mites, scout your field beginning at the edge and moving into the field, checking plants at least five different spots. Look for leaf stippling, webbing, and mites on lower leaf surfaces.

For detailed information about scouting your field for spider mites, take a look at the University of Minnesota Extension publication – Managing Two-spotted Spider Mites on Soybean. This publication talks about scouting specifics and the 0 to 5 spider mite infestation scale which can help you to determine both where your infestation falls on the scale and whether you have reached the economic threshold to consider a miticide spray.

The Purdue University Field Crops IPM Extension publication – Two-spotted Spider Mite, has very good close-up pictures of spider mites and also discusses treatment options.

One caution: some traditional insecticides, such as pyrethroids, can be ineffective against spider mites and may actually result in heightened spider mite populations by decimating natural insect enemies. Effective miticidal products are listed in- or linked to- in both publications listed above.



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