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Extension > Agriculture > Crops > Soybean Production > Pest management > Managing two-spotted spider mites on soybeans

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Managing two-spotted spider mites on soybeans

Ken Ostlie, Extension Entomologist
Bruce Potter, IPM Specialist, University of Minnesota Extension

spider-mites

Prolonged drought raises the threat of two-spotted spider mite (Tetranychus urticae) outbreaks in soybeans and corn. As the 2012 drought intensifies in Minnesota, infestations are reaching treatable levels. Spider mite outbreaks are rare, but have occurred more frequently in recent years (1988, 2007, 2009, 2010). Farmers and their crop management advisors may need a refresher on two-spotted spider mites, population dynamics, scouting, treatment thresholds, and miticide options. To make matters worse, trying to manage both spider mites and soybean aphids creates decision headaches

What are two-spotted spider mites?

spider-mite-life-cycle

Fig. 1: Life cycle of the two-spotted spider mite

Two-spotted spider mites are minute (< 0.002 inch), greenish, yellowish to orange arachnids with two dark spots on their abdomen. Note their 8 legs, not 6 as in insects. Spider mite adults are ca. half the size, or less, of the smallest soybean aphid nymph. These mites attack a wide variety of plants, including several crops (soybeans, dry beans, alfalfa, and corn), vegetables, ornamentals, and trees. Mites overwinter as eggs and move to crops from permanent vegetation. Hatching mites colonize the undersides of leaves. Look closely, you might even see the webbing that earns them the name "spider" mites. They disperse by spinning a silk thread that's caught by the wind.

Spider mites have a straightforward life cycle, progressing through three stages between egg and adult (see Fig. 1). Depending on temperature, development takes 5 to 19 days. Hotter temperatures (>90°F) speed reproduction while cooler temps slow it down. Recent hot temps have accelerated reproduction. With females producing up 100 eggs each (see spheres on photo), it's easy to understand how populations can explode, increasing up to 70X in as little as 6-10 days.

Why are problems worse in drought?

Spider mite populations are held in balance by natural enemies, weather and host quality. Drought triggers spider mite outbreaks in soybean and corn by upsetting this balance in four ways.

minnesota-map

Fig. 2: Intensifying drought increases risk of two-spotted spider mite outbreaks in soybeans and corn.

How do spider mites injure plants?

leaf

Spider mites injure leaves by piercing cells and sucking out cell contents. This injury produces white or yellow spots or "stippling" that is heaviest on the underside of the leaves (see photo). Leaves lose photosynthetic surface as feeding continues. Water loss from damaged leaf surface becomes uncontrolled. Both photosynthetic rate, and leaf water status, decline with increasing levels of spider mite injury.

As colonies grow, injury intensifies. Entire leaves progress from grayish green to yellow, brown or coppery, and then drop off. Damage begins in the lower canopy and progresses upwards. Spider mite feeding reduces effective leaf area and accentuates drought stress. The result is reduced pod set, fewer seeds, and smaller seed size. For farmers and crop advisors not familiar with spider mites, symptoms are simply mistaken for drought.

How do I scout for spider mites?

Infestations typically are first observed near field edges or where soybeans are stressed. If lower leaf loss, yellowed or browning spots are noted at the field edge, it's time for some detective work.

spider-mites-and-leaves

Fig. 3: When scouting fields for spider mites, note leaf loss, stippling on leaves, and presence of mites on plants.

scouting

If mite presence is verified, it's time to move further 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 of 20 locations. Check fields every 4-5 days when drought persists. Under these conditions, infestations can develop quickly.

Two-spotted spider mite infestation scale 0-5

Potter and Ostlie 1988

0. No spider mites of injury observed

1. Minor stippling on lower leaves, no premature yellowing observed

2. Stippling common on lower leaves, small areas on scattered plants with yellowing

3. Heavy stippling 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 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 stipling 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

When should spider mites be sprayed?

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

spraying

Fig. 4: Thorough miticide coverage is critical for control of two-spotted spider mite outbreaks in soybeans and corn: preferably 5 gpa by air and 20 gpa by ground application.

Damage is not reversible so it's important to protect the middle and upper canopy leaves. At a rating of 3, there is heavy stippling on lower leaves with some progressing into the middle canopy and very little in the upper canopy. At this point, mites will be common in the lower canopy, present in the middle canopy, and with scattered colonies in the upper canopy. Some lower leaf yellowing will be common with accelerated leaf loss in small areas. Note: In cooler weather, these symptoms will be less pronounced.

What miticides work on spider mites?

While there are numerous insecticides labeled on soybean, only a few have adequate mite activity (see Table 1). Your choices are basically limited to chlorpyrifos, dimethoate, bifenthrin or mixtures containing these ingredients. Many of the insecticides labeled against soybean aphid are either ineffectual against spider mites, or may even aggravate the situation. Most newer pyrethroid insecticides labeled for soybean (Asana XL, Warrior T, Proaxis, Delta Gold, Mustang Max, Baythroid), older pyrethoids(Ambush, Pounce, Pydrin), and their generic counterparts are weakly effective, neutral or even "flare" spider mites. Flaring, the rise of sprayed populations above untreated levels, occurs because these insecticides remove predatory mites and insects, and may even stimulate more rapid reproduction.

The insecticides labeled for two-spotted spider mites in soybean appear in Table 1.

Important points and miticides:

Table 1: Miticides recommended for two-spotted spider mites in soybean (*Restricted use product)

Chemical name Trade name Application rate (lb ai/acre) Re-entry interval Pre-entry interval Use comments
Chlorpyrifos *Lorsban 4E,
*Lorsban advanced
*Chlorpyrifos 4E, *Govern 4E
*Hatchet 4E, *Nufos 4E
*Warhawk 4E, *Yuma 4E
0.25-0.5
(8-16 fl oz)
24 hrs 28 days Check 3-5 days after application. If new nymphs, switch product. Do not make a second application to spider mites. Do not make more than within 10 days. Do not feed or graze treated soybean plants.
Chlorpyrifos = Gamma-Cyhalothrin *Cobalt 2.55E
*Cobalt Advanced
0.26-0.52 (13-26 fl oz) 24 hrs 30 days Do not graze or feed to livestock
Dimethoate *Dimethoate 4E, 4EC, 400
*Dimate 4E, 4EC
0.05 (16 fl oz) 48 hrs 21 days Do not feed or graze within 5 days
Zeta Cypermethrin + Bifenthrin *Hero 1.24E 0.10 (10.3 fl oz) 12 hrs 21 days Do not graze or feed treated forage to livestock. DO not make applications less than 30 days apart
Bifenthrin *Bifenthrin 2E, *Brigade 2E
*Discipline 2E, *Fanfare 2E
*Sniper 2E, *Tundra 2E
0.08-0.10 (5.1-6.4 fl oz) 12 hrs 18 days Do not graze or feed treated forage to livestock. Do not make applications less than 30 days apart.
Please note: The information provided in this table is summarized from miticide labels to help make decisions regarding product selection. Labels change so read and follow label directions on the product you purchase. The label is the ultimate authority for its application to two-spotted spider mites and may differ from the information provided in this table. Read and follow label directions!

More on two-spotted spider mites and their management.


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