NDSU Extension - Ramsey County

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July 16, 2012 Agriculture Column

Howdy!!!

I have included information pertaining to spider mites in soybeans.  I have not seen nor heard of any spider mites yet but wan to aware you of things to look for.  Make sure in scouting to be patient and wait for thresholds.  I know of many that have made an insecticide application already and had only found 15-20 aphids on a small handful of plants.  Remember when scouting for insects that there are beneficial insects, as well, so know your bugs.

How do you scout for mites in soybeans?

Infestations typically are first noted near field edges or where soybeans are stressed. If lower leaf loss, yellowed or browning spots are noted at the field edge or in patches within the fields, its time for some detective work. Examine plants at the field edge first, especially adjacent to roadside ditches or alfalfa fields. Pull plants and examine the leaves from the bottom upwards. Look at the underside of leaves. Note stipling, webbing, and examine for mites with a hand lens or magnifying lens. Examine how far up the plant mites and symptoms have progressed. Tap selected soybean leaves with symptoms over a white sheet of paper or cardboard and examine for mites. Note the abundance of moving dark spots (mites) highlighted against the white paper. 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.

Check fields every 4-5 days if drought persists since damaging infestations can develop quickly.

When should spider mites be sprayed in soybean?

Full Pod (R4) and Beginning Seed (R5) stages are critical in determining soybean yield.  Spider mite feeding reduces photosynthetic area and accentuates drought stress. The result is reduced pod set, seed number, and seed size. If leaves drop or plants are killed, pod fill is stopped in its tracks. Pods on mite-stressed plants are more likely to shatter, which compounds yield loss. Only a 10-15% reduction in effective leaf area, yield losses will justify an insecticide / miticide application.  Unfortunately it’s not easy to estimate a 15% reduction in effective leaf area.

When the soybean field reaches a 3 on the scale above, spray to protect middle and upper canopy leaves.

Treatment is recommended only if damage and mites are detected throughout the field.  Edge treatments are not effective since mites are usually spreading throughout the field before any visual symptoms are noted.  During the severe outbreak in 1988, fields progressed from symptoms visible at the field edge to severe leaf loss throughout the field in only 2 or 3 days.

 

When should spider mites be sprayed in soybean?

Full Pod (R4) and Beginning Seed (R5) stages are critical in determining soybean yield.  Spider mite feeding reduces photosynthetic area and accentuates drought stress. The result is reduced pod set, seed number, and seed size. If leaves drop or plants are killed, pod fill is stopped in its tracks. Pods on mite-stressed plants are more likely to shatter, which compounds yield loss. Only a 10-15% reduction in effective leaf area, yield losses will justify an insecticide / miticide application.  Unfortunately it’s not easy to estimate a 15% reduction in effective leaf area.

 

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