Twospotted Spider Mite Potential on Soybean in Dry Areas
Eileen Cullen, Extension Entomologist
On July 22 Phil Pellitteri, UW-Madison Insect Diagnostic Clinic, received a report (confirmed with a plant sample) of twospotted spider mite on soybean in Green County near Monroe. While some areas of the state have ample rainfall, other areas are dry. For the remainder of the season, be aware of the potential for twospotted spider mite in soybean.
Spider mites are associated with dry, drought stressed plants. Drought stress accelerates spider mite movement to soybean from surrounding vegetation (grasses, field margins, other crops). Drought stress also diminishes spider mite fungal pathogens that normally suppress populations under good crop moisture. Hot temperatures speed up spider mite reproductive rate such that predatory mites and insects may not be able to suppress populations. (Ostlie and Potter 2009).
Regardless of temperature, check soybean fields that are on the dry side through R5 soybean growth stage. This can be done while scouting for soybean aphid. Now is a good time to make sure you are not overlooking spider mite colonies in the field. It is easy to attribute leaf yellowing to drought stress, soybean virus leaf symptoms, etc. and miss a twospotted spider mite population because the mites are so small. Careful field inspection is required.
Soybean Damage Symptoms
Spider mites insert mouthparts into leaf cells. Contents of the individual, living cells are extracted resulting in many small white or yellow spots, called “stippling” (speckled or sand-blasted appearance in early stages). From a distance, affected fields are apparent by leaf yellowing. Often infestations start at field edges, but can also be seen within fields on knolls (drier soil) and can be patchy in distribution within the field.
With increased mite injury leaves become yellow, bronzed, brown, and may eventually drop off the plant. Apparently healthy plants within the field can have live mites. Spider mites disperse within and between fields by climbing to the top of plants and spinning silk strands that, when caught on breezes, allow mites to drift to new host plants.
Figure 1. Chlorotic stippling on soybean leaf from twospotted spider mite. (Photo: Tom Klubertanz)
Confirm presence of live mites in the field
Two-spotted spider mite adults are tiny (<0.002 inch), yellow-green with eight legs and dark spots on either side of their oval bodies. Eggs are round and white to light yellow and laid on the underside of leaves. Two-spotted spider mites in northern states overwinter as adult females in sheltered field margin areas. In most years, with adequate rainfall and a fungal pathogen as a primary natural control, outbreaks do not occur. In the absence of these checks, spider mites reproduce quickly with several overlapping generations during an outbreak (eggs, nymphs and adults found together on infested plants).
Figure 2. Twospotted spider mite adults on underside of soybean leaf (Photo: P. Sonnentag, UW-Madison Entomology)
Eggs hatch in 2 to 4 days; nymphs develop in 2 to 4 days; and adults can live up to 21 days with better survival in hot, dry environments. Depending on temperatures, generations are completed in 4 to 14 days, with the fastest developmental rates above 91°F.
Figure 3. Twospotted spider mite eggs on underside of soybean leaf. (Photo: P. Sonnentag, UW-Madsion Entomology)
A hand lens is necessary to see two-spotted spider mites and eggs. Use a 10X magnification hand lens to confirm presence of live mites. Adults, with dark spots on the body, can also be detected by tapping soybean plants over a clipboard onto a white sheet of paper. Dislodged spider mites can be seen as tiny brown/black specks moving on the paper. Monitor fields along edges, and importantly, within fields. Examine upper, middle and lower canopy leaves for stippling. As spider mite populations increase you may also find webbing on the undersides of leaves in infested field areas.
Treatment decisions are based on amount of leaf discoloration due to spider mites and continued presence of live colonies in the field. Consider treatment when 20% to 25% discoloration is found before pod set, or 10% to 15% discoloration after pod set.
Spider mite populations often start along field edges, and spot or field edge treatments may be an option. However, before spot treatments are applied, thorough monitoring of the entire field is recommended. If mite injury is evident in the field interior, the potential for economic populations within 1-2 weeks should be recognized. A whole field treatment may be justified based on the guidelines below.
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. You can assess mite damage using the following scale:
0 – No spider mites or injury observed.
1 – Minor stippling on lower leaves, no premature yellowing observed
2 – Stippling common on lower leaves, small areas or scattered plants with yellowing
3 – Heavy stippling on lower leaves with some stippling 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. Stippling, webbing and mites common in middle canopy. Mites and minor stippling present in upper canopy. (Economic Loss)
5 – Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stippling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy.
Choose an insecticide labeled for twospotted spider mite control in soybean. Options are fairly limited to the organophosphate class of insecticide, chlorpyrifos or dimethoate.
Weather conditions and natural controls
Following rainfall, relatively cooler temperatures and high humidity can foster the most effective twospotted spider mite natural enemy – a fungal pathogen, Neozygites floridana, that attacks all stages of mites and is host-specific to spider mites. During early infection stages, mites have a discolored, waxy or cloudy appearance and mite death occurs within 1 to 3 days of infection.
Production of infective spores depends on environmental conditions which must be cooler than 85°F and with at least 90% relative humidity. At least 12-24 hours of such conditions are believed necessary for extensive spread of the disease, and TSM populations may decline rapidly in response to fungal disease activity.