# Managing Spider Mites in Corn and Soybean: Thresholds & Treatments

## July 25, 2013

Banks grass mite (top) and twospotted spider mite (below). Note difference in dark coloration.(Photos by Jim Kalisch)

For information on scouting, differentiating species, and the important role that beneficial predators play in spider mite management, see Part 1.

## Treatment Thresholds

Researchers in Texas have developed economic injury levels for spider mites in field corn. Although these were originally developed based on data from TSM, additional research has shown that the BGM has the same damage potential as the TSM, so this information can be used for either species in corn. To use this procedure, the per acre control costs (miticide + application costs) and the expected value of the crop (yield [bu/acre] x corn grain value [\$/bu]) must be estimated. A two-step sampling procedure is used.

For other market values, use the following formulas to determine an economic injury level.

1.  For percent infested leaves the formula is (cost of control x 600) ÷ (price per bushel x bushel yield)

2.  For percent of leaf area damaged the formula is (cost of control x 312) ÷ (price per bushel x bushel yield)

First, check the field for the presence or absence of spider mites on individual green leaves on a corn plant. Record the number of infested green leaves (containing one or more live spider mites) and the total number of green leaves on each plant. Repeat this procedure on at least 10 plants from different portions of the field.

Compare the percentage of infested green leaves to the first value in Table I associated with the appropriate control costs and crop value. If your sample equals or exceeds the value in the table, estimate the percentage of leaf area damaged by spider mites and compare that value with the second value in the table. Spider mite damage can be described as chlorotic spotting of the leaf surface caused by mites sucking out plant juices at a feeding site. At either step, if the sample value is less than the value in the table, control of spider mites is unlikely to be profitable.

 Table 1. Economic injury level for the Banks grass mite or twospotted spider mite on corn based on percentage of infested leaves per plant and percentage of total leaf area damaged. Control cost per acre Market value per acre (\$) 400 600 800 1000 1200 1400 1600 1800 2000 % infested leaves per plant / % of total leaf area damaged \$5 7/4 5/3 4/2 3/2 2/1 2/1 2/1 2/1 2/1 \$10 15/8 10/5 8/4 6/3 5/3 4/2 4/2 3/2 3/2 \$15 22/12 15/8 11/6 9/4 8/4 6/3 6/3 5/3 5/2 \$20 29/16 20/10 15/8 12/6 10/5 9/4 8/4 7/4 6/3 \$25 37/20 25/13 19/10 15/8 12/6 11/6 9/5 8/4 8/4

For example, if your estimated control costs are \$15 per acre and the crop value is \$1600/acre (200 bu/acre x \$8.00/bu), if 6% or more of the green leaves are infested, then you need to estimate the percentage of leaf area on the corn plant that is damaged by mites. If 3% or more of the total leaf area is damaged by mite feeding, it will likely pay to control spider mites in this example.

Research also has shown that corn is unlikely to benefit from treatment for either BGM or TSM after the full dent stage.

No research has been conducted that would allow calculation of an economic injury level for twospotted spider mites on soybeans. On soybeans, the following scale is suggested to assess damage by spider mites (Ostlie and Potter 2012, http://www.soybeans.umn.edu/crop/insects/spider_mites.htm):

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 and some 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.

Damage from mites may be confused with that caused by drought and several foliar diseases, so be sure to base treatment decisions on the presence of mites, rather than just apparent injury symptoms. Fields may be spot treated if the infestation is localized, but check other areas for mites (especially downwind of infestation) and extend treatments into these areas if large numbers of mites are found. Although late-season infestations may accelerate soybean senescence and increase pod shattering, caution should be used in deciding to treat with pesticides because many of the pesticides used for mite control have 21-28 day preharvest intervals.

## Control

##### Spider mite control can be tricky.
Early season insecticide applications, particularly pyrethroids and organophosphates, can knock out many natural predators of spider mites, opening the door to unchallenged population growth. Pursue insecticide applications with this in mind.

For effective control, spider mites must come into contact with the miticide. Since mites are found primarily on the underside of the leaves, they are difficult to reach with low volume applications. Using three or more gallons of water per acre by air to carry miticides may increase effectiveness. Aerial applications are generally more effective if applied very early in the morning or in the late evening. Applications made at these times avoid the upward movement of sprays, away from the plants, on hot rising air.

Eggs are difficult to kill with pyrethroid or organophosphate miticides, so reinfestation is likely to occur seven to 10 days after treatment as a result of egg hatching. The reinfestation is frequently heavy because natural enemies have been reduced or eliminated. A second application may be necessary to kill newly hatched mites before they mature and deposit more eggs.

In many cases, especially with TSM, slowing the rate of population increase is all that can be accomplished with a miticide application.

## Control Products

Mode of action class 1B; organophosphate

• Dimethoate: labeled for use in soybean and corn. Multiple products; Dimethoate 4E, 4EC, 400, Dimate 4E, 4EC
• Chlorpyrifos: labeled for spider mite control in soybeans. Multiple products; Lorsban 4E, Lorsban Advanced, Chlorpyrifos 4E, Govern 4E, Hatchet 4E, NuFos 4E, Warhawk 4E, Yuma 4E

Mode of action class 3A; pyrethroid

• Bifenthrin; labeled for use in soybean and corn. Multiple products; Bifenture 2E, Brigade 2E,Discipline 2E, Fanfare 2E, Sniper 2E, Tundra 2E

Mode of action class 10B

• Zeal (etoxazole); labeled for spider mite control in corn; active against eggs and larvae

Mode of action class 12C

• Comite (propargite); labeled for spider mite control in corn

Mode of action class 23; tetronic and tetramic acid derivatives

• Oberon (spiromesifen); labeled for spider mite control in corn; most effective against egg and immature stages.
• Onager (hexythiazox); labeled for spider mite control in corn west of Highway 281 in Nebraska. Does not control adult mites.

Combination Products

• Hero (zeta-cypermethrin and bifenthrin); labeled for spider mite control on corn and soybeans
• Cobalt (chlorpyrifos and gamma-cyhalothrin); labeled for spider mite control on soybeans
• Swagger (bifenthrin and imidacloprid); labeled for spider mite control on soybeans
• Tundra Supreme (chlorpyrifos and bifenthrin); labeled for spider mite control on corn and soybeans

Robert Wright
Extension Entomologist, Lincoln