2 Spot Spider Mite

Two female two-spotted spider mites with egg (Giles San Martin) CC BY-SA 2.0 Two-spotted spider mites ( Tetranychus urticae ) can become a serious threat to your citrus and other fruit trees, most vegetables, and many ornamentals, such as marigolds, roses, and salvia. The two-spotted spider mite (Tetranychus urticae) is the most common of all plant pest mites and has been reported on more than 200 plant species. They are mites and not insects. Not only do insecticidal soaps work on pesky insects, they also kill spider mites. Photo Credit: ©2003 Buglady Consulting.

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Tetranychus urticae Koch

Appearance and Life History

The two-spotted spider mite (TSM) is more closely related to spiders than to insects. There are thousands of species of mites. TSM is one of many phytophagous, or plant feeding species.

The TSM has four life stages; egg, larva, nymph, and adult. Under optimum conditions of high temperatures and low humidity, this mite can complete its development in 5 to 7 days. Under more normal temperatures and humidity, the generation interval is approximately 19 days. The following provides an overview of TSM life stages:

Egg - The spherical, shiny straw-colored eggs are very small. Webbing produced by the TSM, which helps fasten the eggs to the leaf surface, makes the eggs difficult to see. After an incubation period of from 3 (75°F or 24°C) to 19 (50°F or 10°C) days the eggs hatch.

Larva - The six-legged, colorless, larva resembles the body form of the nymph and adult. It is slightly larger than a TSM egg. During the larval stage, little food is consumed.

Nymph - The eight-legged nymph looks like an adult, but is ;smaller and not sexually mature. It goes through 2 nymphal stages, proto-nymph and deuto- nymph, before becoming an adult.

Adult - The adult female is eight-legged and about 1/60 inch (0.4 mm) long. The eight-legged male is only about 1/80 inch (0.3 mm) long. Their color varies from pale yellow to green to orange to brown. When viewed from above, they appear to have 2 pigmented spots, which are actually contents of the gut showing through the body wall. A female TSM will lay from 50 to 100 eggs during her lifetime. Unfertilized eggs develop into males, fertilized eggs into females. The sex ratio can vary considerably, but is usually female-biased.

The adult TSM overwinters in non-crop and weedy areas such as grassy waterways, roadsides, weeds, set-aside acres, and pastures. As temperatures warm in the spring, TSM begins feeding on common hosts such as clover, chickweed, and various grasses. Mating and egg laying soon follow and continue throughout the mite's lifespan. The time required to go through a generation is shortened by hot temperatures. Populations may increase significantly if high temperatures are associated with dry conditions. Dry conditions can reduce naturally occurring pathogenic fungi (primarily Neozygites sp.) and predators that normally keep TSM populations at non-economic levels.

Damage

TSM feeds by piercing leaf cell walls with its mouthparts and sucking out the cell's contents. These cells become non-functional, thus the damage is considered irreversible (no longer contributing to plant growth and development). However, this does not mean that plants are unable to produce new leaf growth if conditions conducive to plant development improve.

In the early stages of TSM activity on corn, the feeding damage may not appear to be as pronounced as on soybean. The typical 'V' or 'U' pattern of infestation is either lacking or is not visible since initial feeding occurs on the lower leaves and is somewhat obscured by the larger upper leaves. As with soybean, the damage in corn will begin near TSM overwintering sites. Depending on the severity of the feeding, symptoms may include some leaf yellowing to brown leaves and a general decline in plant growth. The damage may be enhanced by poor growing conditions and/or soil compaction.

Upon closer inspection of TSM-infested plants, the leaves appear mottled or 'sand blasted.' Although TSM damage on corn can be very severe in the western Midwest, corn has not been severely impacted by the TSM in the eastern Midwest. Outbreaks of TSM have been noted where corn has been treated with some insecticides and fungicides. These pesticides, although not normally toxic to TSM, greatly reduced the numbers of predators and pathogens that normally keep TSM populations in check.

Sampling Method

If hot, dry conditions persist for several weeks, watch for leaf discoloration (yellowing) especially along the field borders or near grassy areas within fields. Carefully inspect these areas for the presence of TSM. Shake some discolored leaves over a white piece of paper. Watch for small dark specks moving about on the paper. Also look for minute webbing on the undersides of the discolored leaves. These are sure signs of TSM activity.

Before considering control, it is very important that TSM are identified as the source of the problem since other crop stresses can cause leaf yellowing, such as diseases, herbicides, nutrient deficiencies, excessive moisture, or insects such as thrips. Once TSM has been positively identified in the damaged areas of the field, it is essential that the whole field be scouted to determine the range of the infestation. Sample at least 5 different areas of the field and determine whether TSM are present by shaking leaves over a piece of paper. If plant discoloration is noted, estimate the average percentage leaf discoloration for the plants within the damaged area. Repeat this procedure in other areas showing damage. Usually the areas that show discoloration first are those areas that are already stressed from factors such as poorly productive soils (sand or clay knobs, etc.) and/or soil compaction.

Management Guidelines

Corn Insect Control Recommendations:E-series 219-W (PDF)

Control may be necessary when 15% to 20% of the leaf area is covered with TSM colonies, moderate damage is noted, and hot, dry conditions are expected to continue. The greatest benefit from control normally occurs when miticides are applied from the pre-tassel through the soft dough stages of plant development. It is unlikely that the application of a control will be cost effective once the dent stage of plant development is reached. Thorough coverage of corn leaves with a miticide is difficult, but necessary for TSM control.

If control is necessary, contact your state Cooperative Extension Service or click here for control materials and rates.

SPIDER MITES (Tetranychidae)

Spider mites are a cannabis grower’s main pest. Spider Mites pierce the leaf tissue to get at the chlorophyll. The first thing you need to know about them is that they like a dry environment. Which means that if you spot them, you can try increasing the humidity of your greenhouse with generous spurts from a misting bottle. Try this for a few days, and see if they are still a problem. Because mold thrives on humid conditions, you want to make sure you don’t have any harmful mold, like Botrytis (Gray Mold), growing before trying to increase the humidity.

NATURAL ENEMIES OF THE SPIDER MITE

Stratiolaelaps scimitus (Hypoaspis miles) goes after Spider Mites in the soil.

APPLY WHEN:

If you’ve previously had a problem with Spider Mites, then release Stratiolaelaps scimitus when preparing your grow area. Otherwise shortly after planting your seeds. And again if you notice that, despite your best efforts, Spider Mites have arrived anyway.

APPLY WHERE:

Before they have plants to feed on, Spider mites hide in protected areas. As a preventative, Stratiolaelaps scimitus should be applied on the floor, where table legs meet the floor. Also on the floor around the perimeter of the room. Spider mites will also hide out at electrical and plumbing outlets, as well as where support beams meets the wall. All these spots should be attended. If your initial preparation didn’t keep them away, then, in the early stages, you can apply again, but taking extra care to get Stratiolaelaps scimitusinto nooks, chips, cracks and crannies. If you have a problem already, then release Stratiolaelaps scimitus in the soil.

Amblyseius fallacis

Amblyseius fallacis is a good preventer of Spider Mites. Amblyseius fallacis is to be used only when you don’t have a serious problem with Spider Mites. It will hang around, spreading itself evenly throughout the greenhouse.

APPLY WHEN:

If you’ve had Spider Mites before, then establish Amblyseius fallacis at the beginning, before planting. 2 mites per square foot should do the trick.

Phytoseiulus persimilis

This Spider Mite predator can be used with Amblyseius fallacis. They won’t get in each other’s way. Phytoseiulus persimilis likes Spider Mite webs, while Amblyseius fallacis doesn’t. Because Phytoseiulus persimilis like webs, it’s an ideal enemy of adult Spider Mites.

APPLY WHEN:

If there’s an outbreak of Spider Mites and webbing occurs, then Phytoseiuluspersimilis is your friend.

Mesosiuluslongipes (pronounced lon-gipes)

Mesosiuluslongipes is another good Spider Mite predator, which may be better suited to your grow environment if it’s fairly dry to begin with. They handle lower humidity, which is what Spider Mites like. But Mesosiuluslongipes needs more humidity as the temperature rises. So keep your greenhouse warm, use artificial light and apply 3 Mesosiuluslongipes per square foot for a week and then again the next week. That should be enough.

Californicus (Neoseiuluscalifornicus)

Californicus is a useful predator of many mites. It’s particularly useful for the control of Spider Mites, which like dry environments. But so does Californicus, and it can cope with different temperatures. Although, if it has to choose, it, like most Californians, will choose warm or hot weather.

APPLY WHEN:

If you’ve seen a few Spider Mites and are expecting more, then get your Californicus out there right away. They like a generous population on which to feed, and, Spider Mites taken care of, are likely to hang around and eat pollen until more Spider Mites show up.

Amblyseiusandersoni

This helpful predator will deal with many mites. It’s got a specific taste for Broad Mites, Hemp Russet Mites and Spider Mites.

APPLY WHEN:

Release Amblyseiusandersoni early, when Spider Mites are just showing their presence; that’s when Amblyseiusandersoni is most helpful.

APPLY WHERE:

You’ll get your Amblyseiusandersoni in either a shaker bottle or sachet. The bottle makes it easy to carefully release Amblyseiusandersoni onto the plants, close to the flowers. You’ll want to let out 2 or 3 Amblyseiusandersoni for every 10 square feet of greenhouse.

Keep sachets out of direct sunlight and use 1 every 2 meters along a crop row. release on the plant, on whichever leaf or twig suggests itself. A sachet should work for up to 6 weeks, but can last a bit longer if Amblyseiusandersoni become properly established.

Galendromusoccidentalis

Galendromusoccidentalis, another native Californian mite, can be useful to the grower who’s noticed Spider Mites becoming a bigger problem. Galendromusoccidentalis doesn’t feed on eggs, but likes nymph and adult Spider Mites. It also likes other mites, including Hemp Russet Mites. It can handle environments of 40% humidity and below.

APPLY WHEN:

Release Galendromusoccidentalis when you see the Spider Mites. 2 or 3 per square foot should do it. Do this twice a week, and keep in mind that these Californians need 11 hours of sunlight a day.

Stethoruspunctillum

2 Spotted Spider Mite Life Cycle

A family member of the ladybug, Stethoruspunctillum is a very small black beetle. These little friends could be your most valuable ally against dreaded Spider Mites. They find populations by smell, and fly between plants. They handle dry conditions and are ideal if the Spider Mites are getting out of control and you’re having problems regulating greenhouse temperature. They also leave eggs at the scene which will hatch and keep working against the Spider Mites. They are susceptible to insecticides, so they’re best used when you’re not using chemicals.

APPLY WHEN:

If there are lots of Spider Mites, immediately release 1 Stethoruspunctillum for every 10 square feet of greenhouse. You can reduce the ratio by 10 if Spider Mites haven’t become a real problem yet.

2 Spot Spider Mites

Preferal™

Preferal can also be used against Spider Mites. It can also help with Broad Mites, Hemp Russet Mites, Eriophyid Mites, Whiteflies, Thrips and Aphids. It’s made from a naturally-occurring fungus, Isariafumosorosea, which is delivered via blastospores for fast germination. The spores stick to insects, then germinate and penetrate through their skin, continuing to grow inside. This kills the insect and more spores are released to do the same to other insects. Preferal won’t harm the good pests, or at least it’ll do very little damage.