In addition to soybean aphid, include two spotted spider mite in soybean field scouting and treatment decisions. Due to lack of rain in many counties in Wisconsin, coupled with hot temperatures, fields are once again capable of hosting economic spider mite populations. Recall that in 2005 in east central WI, and in 2006 in northwestern Wisconsin spider mite populations started earlier in July and were at very high levels in some fields by early August. Currently, we have not yet seen widespread outbreaks but have had a few reports from drier areas. 

 

Below are recommendations to follow for the month of August. This is a compilation of MN and WI recommendations summarizing spider mite scouting and management recommendations.

From Dr. Ian MacRae, Dept. of Entomology, University of Minnesota Northwest Research & Outreach Center, Crookston, MN and Bruce Potter IPM Specialist, University of Minnesota Extension Service, Southwest Research & Outreach Center, Lamberton, MN  

        CONFIRM you have a mite problem prior to taking action.  Absolutely essential and very easy to do.  Scout the edges and corners of fields first, pull plants and shake some of the bottom leaves over a sheet of white paper or other relatively light surface and look for 'moving dirt'.  That would be mites.  If they're on the edge, then check the rest of the field.  By the time economically treatable populations are present, stippling (yellow spotting) and general yellowing of leaves will be visible.

From Dr. Ken Ostlie, Field Crop Extension Specialist, Dept. of Entomology, University of Minnesota, St. Paul, MN. I agree that the guidelines for scouting and thresholds provided in Ken Ostlie’s article at http://www.soybeans.umn.edu/crop/insects/spider_mites.htm are as good for our region as anything out there. 

 

 

 

 

 

 

 

No specific economic threshold has been developed for TSM in soybeans. 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.

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.

 

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 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.

 

Edge treatments are not generally effective since mites are usually spreading throughout the field before any visual symptoms are noted. In 1988 fields progressed in only 2 or 3 days from symptoms visible at the field edge to sever leaf loss throughout the field. Treatment is recommended only if damage and mites are detected throughout the field. Use the previous scale following guide, treat 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.

 

 

Soybean Damage Symptoms

The picture below shows a soybean field affected by two spotted spider mites (TSM). This picture is from a 2005 infestation in Sheboygan County. TSM insert mouthparts into leaf cells and 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. TSM 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, or attributing leaf yellowing solely to drought.

 

TSM adults are tiny (less than two-tenths of an 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. TSM 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, TSM outbreaks do not occur. In the absence of these checks, TSM 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, TSM generations are completed in 4 to 14 days, with the fastest developmental rates above 91°F.

 

A hand lens is necessary to clearly see TSM (once you’ve developed a search image, TSM can be seen with good eyesight, unaided). 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 TSM 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.

 

 

The challenge later in the season, is that treatment may be difficult given the 21 and 28 day pre-harvest intervals of the organophosphates Dimethoate (a.i., dimethoate) and Lorsban (a.i., chlorpyrifos), respectively. In addition, yield expectations in mite-treated fields may require adjustment because of drought conditions.

 

 

Following any August rainfall, relatively cooler temperatures and high humidity can foster the most effective natural enemy of TSM - 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.

 

 

 

 

 

 

http://www.soybeans.umn.edu/crop/insects/spider_mites.htm