By: Teo Spengler
If your plants are attacked by two-spotted mites, you are going to want to take some action to protect them. What are two-spotted spider mites? They are mites with the scientific name of Tetranychus urticae that infest hundreds of different plant species. For more information about two-spotted mite damage and control of two-spotted mites, read on.
You may have heard of spider mites, but perhaps not this particular kind. So exactly what are they? These garden pests are as tiny as mites can be. In fact, one alone is barely visible to the naked eye, so you aren’t going to be able to inspect it and count its spots.
But finding one mite alone is not very likely. By the time you see two-spotted mite damage and think about two-spotted spider mite control, you are likely to have a large mite population. These mites live on the underside of plant leaves.
As you prepare to fight two-spotted spider mite damage, it helps to understand the pest’s life cycle. Here is a summary of what happens.
The mature female two-spotted spider mites overwinter on host plants. They pass the winter either under the host plant’s bark or else on the base of neighbor plants. In spring, the females mate. They lay 2 to 6 eggs a day on the bottom side of the host plants’ leaves, laying perhaps 100 in their short lifetime. In less than a week, the eggs hatch. The new mites lose their exoskeletons three times in their first few weeks. They then become mature adult mites, mate and lay eggs.
If you see two-spotted spider mite damage on your plants, they probably have mites in all stages of development. Generations tend to overlap. In hot dry weather, the infestations are particularly severe and control of two-spotted mites becomes important.
You might find two-spotted spider mite damage on either deciduous or evergreen trees or garden ornamentals. Even garden veggies can be at risk. Two-spotted mites suck essential plant fluids from the leaves. With a serious infestation, the foliage yellows or appears mottled. You will likely see fine, silken threads over the leaf surface.
Even with heavy infestations, you may not be able to spot the actual mites on your plants. To confirm your suspicions, hold a piece of white paper under a stippled leave and tap it. Tiny moving spots on the paper means you need to think about treating for two-spotted mites.
The best way to begin treating for two-spotted mites is to apply a pesticide specific to mites called a miticide. Ideally, you should start treating for two-spotted mites before your plants are seriously damaged.
Apply the miticide for control of two-spotted mites every 7 days or so. Since mites can develop resistance to chemicals, switch to another type of miticide after three applications.
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Red spider mites. Photo by: Catherine Eckert / Shutterstock
Dealing with predatory insects and other pests is one of the least favorite tasks for most gardeners. It’s even more challenging when they are difficult to spot.
Prolonged drought raises the threat of two-spotted spider mite (Tetranychus urticae) outbreaks in soybeans and corn. When the drought intensifies in Minnesota, infestations can reach economic threshold levels.
Here, you’ll find strategies for managing spider mite outbreaks, including how to scout for them, when to treat and which miticides may be effective.
Spider mite outbreaks are rare, but have occurred more frequently in recent years—including in 1988, 2007, 2009 and 2010. To make matters worse, you’ll need to make tough decisions if you’re trying to manage both spider mites and soybean aphids.
Overview: Two-spotted spider mites
Two-spotted spider mites are minute (less than 0.002 inch), greenish, yellowish to orange arachnids with two dark spots on their abdomen. Note they have eight legs, not six like insects. Spider mite adults are about 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, and 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 (Figure 1). Depending on temperature, development takes five to 19 days. Hotter temperatures (more than 90 degrees Fahrenheit) accelerate reproduction, while cooler temperatures slow it down.
With females producing up to 100 eggs each (clear, round and shiny spheres on Figure 1), populations can explode. In fact, there can be as many as 70 times more mites in as little as six to 10 days.
Why problems worsen 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.
It accelerates spider mite movement from surrounding permanent vegetation and alfalfa as it dries down or is cut for hay. Under drought conditions, cutting sparks a mass movement into adjacent soybean and corn.
It improves the food quality of soybean.
It diminishes or stops fungal disease activity that attacks mites, such as Neozygites. Disease outbreaks are fostered by cool, highly humid conditions that favor spore formation and mite infection. Hot, dry weather stops these diseases.
It speeds spider mite reproduction, so predatory insects and mites can't keep up.
How spider mites injure plants
Spider mites injure leaves by piercing cells and sucking out cell contents. This injury produces white or yellow spots or stippling that’s heaviest on the underside of leaves (Figure 3).
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.
This leads to reduced pod set, fewer seeds and smaller seed size. Farmers and crop advisors who aren’t familiar with spider mites may mistake symptoms for drought.
Infestations are typically first observed near field edges or where soybeans are stressed. If you notice lower leaf loss or yellowed or browning spots at the field edge, it’s time for some detective work (Figure 4).
Examine plants at the field edge first, especially adjacent to roadsides, drainage ditches or alfalfa.
Pull plants and examine the leaves from the bottom upwards.
Look at the underside of leaves. Note stippling or webbing. Examine for mites with a hand lens or by tapping infested leaves over a white sheet of paper.
Determine how far mites and symptoms have progressed up the plant.
When scouting fields for spider mites, note field edge symptoms, leaf loss and stippling on leaves and presence of mites on plants.
If you verify mite presence, 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 two plants in each of 20 locations.
Check fields every four to five days when drought persists. Under these conditions, infestations can quickly develop.
When to spray spider mites
When a soybean field reaches a three on the following scale, spray the middle and upper canopy leaves to protect them.
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 with yellowing on scattered plants.
3 Spray threshold: Heavy stippling on lower leaves with some stippling progressing into the middle canopy. Mites present in the middle canopy, with scattered colonies in the upper canopy. Lower leaf yellowing is common, and there’s some lower leaf loss.
4 Economic loss: Lower leaf yellowing is readily apparent. Leaf drop is common. In the middle canopy, stippling, webbing and mites are common. Mites and minor stippling present in the upper canopy.
5: Lower leaf loss is common, with yellowing or browning moving up the plant into the middle canopy. Stippling and distortion of the upper leaves are common. Mites are present in high levels in the middle and lower canopy.
The full pod (R4) and beginning seed (R5) stages are critical in determining soybean yield. Spider mite feeding reduces photosynthetic area and accentuates drought stress. This reduces pod set, seed number and seed size.
If leaves drop or plants are killed, pod fill stops in its tracks. Pods on mite-stressed plants are more likely to shatter, which compounds yield loss. Only a 10 to 15 percent reduction in effective leaf area will justify an insecticide/miticide application to avoid yield losses. Unfortunately it's not easy to estimate a 15 percent reduction in effective leaf area.
Damage isn’t reversible, so it's important to protect the middle and upper canopy leaves. At a rating of three, there’s heavy stippling on the 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. In cooler weather, these symptoms will be less pronounced.
Miticides that work on spider mites
While there are many insecticides labeled for soybean, only a few have adequate mite activity (Table 1). Your choices are basically limited to chlorpyrifos, dimethoate, bifenthrin or mixtures containing these ingredients.
Some insecticides may aggravate the situation by causing populations to increase. Flaring—when sprayed populations rise above the untreated level— can occur because these insecticides remove predatory mites and insects, and may even stimulate more rapid reproduction.
Most pyrethroid insecticides, except bifenthrin, aren’t terribly effective against two-spotted spider mites in Minnesota. By eliminating natural enemies, these products may actually flare spider mites. This is why you should check for mite problems within seven to 10 days after spraying for soybean aphids.
Don’t count on chlorpyrifos and dimethoate to control heavy infestations, even though they’ve performed well against two-spotted spider mite in Minnesota in previous outbreaks. Neither product kills mite eggs, so hatching spider mites begin rebuilding within a few days. Numerous reports of these insecticides failing to control heavy mite populations were reported in 2010. Bifenthrin has a longer residual and may control hatching eggs for a few days. High temperatures shorten the residual of all products.
Don't bother with edge treatments make a decision for the whole field.
Miticides primarily rely on direct contact to kill mites. Because mites usually occupy the underside of leaves, thorough coverage is critical. Don’t skimp on water. Use 20 gallons per acre for ground application or 5 gallons per acre for aerial application (unless the canopy is open). At lower water volumes, performance may suffer.
Re-scout treated fields five days after application to determine if egg-hatch and re-building populations might require re-treatment. Continue scouting on a regular schedule until soybeans reach stage R6.5, or environmental conditions become unfavorable for mites.
Don’t re-spray with the same product. Switch products (and modes of action) between applications to ensure miticide resistance won’t develop.
Table 1 shows application rate in pounds (lbs) and fluid ounces (fl oz) of active ingredient per acre, entry intervals and use comments for several miticides. The information is summarized from miticide labels to help producers select products. Labels change, so read and follow label directions. The label is the ultimate authority for its application to two-spotted spider mites and may differ from the information provided in this table.
|Chemical name||Trade name||Application rate (active ingredient per acre)||Re-entry interval||Pre-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 lbs (8-16 fl oz)||24 hours||28 days||Check three to five days after application. If there are new nymphs, switch products. Don’t make a second application to spider mites. Don’t make more than one application within 10 days. Don’t feed or graze treated soybean plants.|
|Chlorpyrifos + -lambda-cyhalothrin||*Cobalt 2.55E, *Cobalt Advanced||0.26-0.52 lbs (13-26 fl oz)||24 hours||30 days||Don’t graze or feed to livestock.|
|Dimethoate||*Dimethoate 4E, *Dimethoate 4EC, *Dimethoate 400, *Dimate 4E, *Dimate 4EC||0.05 lbs (16 fl oz)||48 hours||21 days||Don’t feed or graze within five days.|
|Zeta-cypermethrin + Bifenthrin||*Hero 1.24E||0.10 lbs (10.3 fl oz)||12 hours||21 days||Don’t graze or feed treated forage to livestock. Don’t make applications less than 30 days apart.|
|Bifenthrin||*Bifenthrin 2E, *Brigade 2E, *Discipline 2E, *Fanfare 2E, *Sniper 2E, *Tundra 2E||0.08-0.10 lbs (5.1-6.4 fl oz)||12 hours||18 days||Don’t graze or feed treated forage to livestock. Don’t make applications less than 30 days apart.|
* Indicates restricted use product.
A: Honestly, mites are so tiny that you might be carrying them into your garden or indoor grow room yourself. They can ride in on your skin, your shoes, your clothes. Your dog or cat might have mites on them. But the most common source for spider mites is actually live plants. It’s a good idea to quarantine plants, even ones you purchase at garden centers or are given by friends, for a couple weeks before putting them where you want them to end up. This way, you catch the pests before they can make it onto your established plants.
While demolishing spider mites may take a little bit of time, it can be done — and your plants will be happier for it. Do you have any other techniques you’ve used in the war against mites? Let me know!
There are few pesticides available for use in home gardens and landscapes that are effective against twospotted spider mites.
Insecticidal soaps are made from potassium salts of fatty acids (don’t make a homemade soap solution as this can burn and damage plants). Horticultural oils are made from either petroleum oils, vegetable oils (like cottonseed oil), or neem seed oil.
Soaps and horticultural oils are reasonably effective against mites and have little impact on people, animals and nontarget insects.
These products will only kill mites that the pesticide directly contacts. They do not have any residual activity.
Target the underside of leaves as well as the top.
Repeat applications may be needed.
Long-lasting insecticides, such as bifenthrin and permethrin can be used on twospotted spider infestations. However, these insecticides also kill natural enemies and could possibly make infestations worse in the long run.
Twospotted spider mite infestations occur when it is hot and dry.
Water plants thoroughly before spraying pesticides for spider mites.
Spray in the early morning or early evening.
These steps will reduce the risk of further stressing plants and causing injury.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
If twospotted spider mites continue to be a problem after control efforts have been attempted, and the plants are valued, consider hiring a landscape professional to treat them. Landscape professionals have the training, experience, and wider array of pesticide products to effectively deal with spider mite infestations.
Jeffrey Hahn, Extension entomologist and Suzanne Wold-Burkness, College of Food, Agricultural & Natural Resource Sciences