Though mites are best seen with magnification, they can be seen with the unaided eye. A arrows point to mites. B arrows point to eggs - Photo: Whitney Cranshaw, Colorado State University
Mites on the underside of the leaf - Photo: Phillip Glogoza and Ian MacRae, University of Minnesota Extension
The best time to scout is in July and August, especially during periods of drought. Spider mite populations can build rapidly, especially if the weather gets hot.
As you scout soybeans for other pests and problems, always give a look to the plants near the edges of the fields or where soybeans are stressed. If spider mite populations are increasing, they will probably be noticed there first.
Soybean foliage infested with spider mites will initially appear speckled or "stippled". As plants become heavily infested, the foliage turns yellow, then bronze, and finally the leaves drop off the plants from the bottom up. The effect of heavy feeding by mites leads to dehydration and death of the plant.
For farmers and crop advisors not familiar with spider mites, the progression of symptoms from silvering, yellowing, browning, loss of lower leaves, and plant death may be mistaken for drought symptoms. Heavily stippled upper leaves may also exhibit deformations that look like herbicide injury.
Checking for mites
Look for yellowing plants that appear sandblasted, and then look for the mites and webbing on the undersides of the leaves. Hold a clipboard with a white sheet of paper under the leaves and tap the leaves to dislodge the mites. Yellow, brown or black specks that move across the paper are probably spider mites. Crushing the mites will leave a small, reddish-brown spot on the white surface.
A 10X hand lens is necessary to clearly see the mites, although with good eyesight mites can be seen unaided, once your eyes are adjusted.
Determining treatment levels
Sample both injured and healthy plants to determine if the infestation is spreading. Also, be certain that the yellowed soybeans are caused by spider mites. Other factors can cause soybeans to turn yellow.
If mite presence is verified, it’s time to progress into the field. Move at least 100 feet into the field before making your first stop. Walk a “U” pattern checking at least 2 plants at each 20 locations. Assess mite damage using the following scale:
- 0 — No spider mites or injury observed.
- 1 — Minor stipling on lower leaves, no premature yellowing observed.
- 2 — Stipling common on lower leaves, small areas or scattered plants with yellowing
- 3 — Heavy stipling on lower leaves with some stipling progressing into middle canopy. Mites present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common. Small areas with lower leaf loss. (Spray Threshold)
- 4 — Lower leaf yellowing readily apparent. Leaf drop common. Stipling, webbing and mites common in middle canopy. Mites and minor stipling present in upper canopy. (Economic Loss)
- 5 — Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stipling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy.
No specific threshold levels have been established for the two-spotted spider mite. Most Extension entomologists recommend treatment only if damage and mites are detected throughout the field.
Use the previous scale following guide, consider treatment when injury progresses to a rating of 3. Fields with ratings of 5 or worse may not be salvageable. Check fields every 4-5 days if drought persists since damaging infestations can develop quickly.