Dry Weather Reminder for Soybeans: Two-spotted Spider Mites

Eileen Cullen, Extension Entomologist

Due to lack of substantial rain in many counties throughout Wisconsin, soybean fields are once again hosting, or capable of hosting, economic populations of two-spotted spider mites.

There is a strong connection between drought stressed soybeans and spider mite activity. The following four points are taken from Ostlie and Potter (2009): 1) Drought stress accelerates spider mite movement to soybeans from surrounding permanent vegetation and alfalfa as it dries down or is cut. 2) Drought stress improves the food quality of soybean for spider mites. 3) Drought also diminishes or stops activity of spider mite fungal pathogens that normally suppress spider mite populations. 4) Hot temperatures speed up spider mite reproductive rate so that predatory mites and insects cannot keep up. In 2009, we have had a ‘cool’ drought, which may be dampening spider mite reproductive rate so far. Drought coupled with hot temperatures in the mid-90s would increase spider mite problems further.

Don Genrich, UW Extension Agriculture Agent, Adams County, reported that several soybean fields in the Arkdale area of Adams County were treated for spider mites August 3rd. Bryce Larson, UW Extension Agriculture Agent, Calumet County, reported a conversation with a Sheboygan County farmer during the recently concluded Farm Technology Days of economic spider mite populations on his farm. August 4^th, Tom Novak (CCA, Total Crop Management) and Gregg Kerr, (CCA, Kerr Agronomics) reported spider mites in soybeans from Jefferson County and western Wisconsin, respectively.

Because spider mites are so tiny, scouting and treatment thresholds are not based on number of mites per plant. Instead, confirm presence of live mites in the field and judge the degree of leaf stippling (symptoms from spider mite feeding), as well as field pattern of feeding damage in the field (field edges vs. throughout field), to support your treat or no treat decision. Information on weather and natural controls (fungal agents) is also provided below.

Figure 1. Example of advanced economic twospotted spider mite damage, Fond du Lac County 2007.  Note soybean leaf stippling and leaf bronzing in this field from 2007. Photo: Mike Rankin, UW Extension Fond du Lac County, Agriculture Agent

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”. Another good word for the leaf symptoms would be “speckling”. 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.

Scout fields now to make sure you are not overlooking spider mite colonies in the field. It is easy to attribute leaf yellowing to drought and miss a two-spotted spider mite population because the mites are so small. Careful field inspection is required, but fairly easy to detect once you look at plants closely.

Figure 2. Chlorotic stippling of soybean from twspotted spider mite injury (Photo: T. 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).

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.

A hand lens is necessary to clearly 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.

Figure 3. Two-spotted spider mite adults on underside of a soybean leaf

Treatment Decision Guidelines

No numerical economic threshold has been developed for two-spotted spider mites in soybeans, largely because it is not practical to count individual spider mites at the commercial field level.

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.

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.  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. The treatment guideline scale above is used to help guide treatment decisions.

Edge treatments are not generally effective since mites are usually spreading throughout the field before any visual symptoms are noted. Treatment is recommended only if damage and mites are detected throughout the field. Use the previous scale as a guide; treat when injury progresses to a rating of 3. Check fields every 4-5 days if drought persists since damaging infestations can develop quickly.

Weather conditions and natural controls

Following additional August 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. The picture below shows a healthy TSM on the right vs. a TSM with fungal infection on the left. 

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.

Do not assume rain will eliminate mite populations. Heavy rains eliminate stress on the plant, but you need to make sure spider mite problems do not continue to persist and impact yield.

Scout now and through R5 for spider mites, their signs, and damage. If soybean aphids reach economic threshold density (250 aphids/plant) in a soybean field with spider mites also present, choose an insecticide also labeled as a miticide for twospotted spider mite control. The organophosphate insecticide active ingredients chlorpyrifos and dimethoate are two such products.  

References and Related Links

Ostlie, K. and B. Potter. 2009. Managing Two-Spotted Spider Mites on Soybeans and Corn in Minnesota. Just for Growers, MN Soybean Production. http://www.soybeans.umn.edu/crop/insects/spider_mites.htm

Cullen, E. 2006. Spider mites: A to Z. pp. 130-133. In Proceedings 2006 Wisconsin Fertilizer, Aglime, and Pest Management Conference, University of Wisconsin, Madison, WI. http://www.soils.wisc.edu/extension/wfapmc/2006/index.php

DiFonzo, C. 2005. You might have mites. Field Crop Advisory Team Alert, 20(14): July 28, 2005. Michigan State University, WWW document.

http://www.ipm.msu.edu/CAT05_fld/FC07-28-05.htm#4

Hammond, R., and B. Eisley. 2005. Twospotted spider mites. Crop Observation and Recommendation Network (C.O.R.N.) Newsletter. July 18, 2005 - July 26.

http://corn.osu.edu/index.php?setissueID=93

 Klubertanz, T.H. 1994. Twospotted spider mite. p. 92-95. In L.G. Higley and D.J. Boethel (ed.)

Handbook of soybean insect pests. Entomol. Soc. Am., Lanham, MD.