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Many of us enjoy a family picnic in the garden on a hot summer day, but none more so than a highly fertile colony of the two-spotted spider mite. Spider mites, or in their Latin guise, Tetranychus urticae, are a broad-spectrum pest that attacks greenhouse crops, strawberries, garden vegetables, and tropical house and landscape plants.

Spider mite damage initially appears as tiny white speckles visible on the topside of leaves. A little spot here, a little spot there—the damage gets missed as the flecks appear so insignificant. Under the right conditions, however, this damage can explode into something unmanageable.

Spider mites are a member of the arachnid family and share the characteristic web-spinning abilities of traditional spiders. Unlike their cousins though, their webs are super fine and spread densely over leaves and stem without the classical geometric form. Upon close inspection of spider mite webs, you’ll discover not only adults moving back and forth, but multiple generations using the web’s protection to carry out their business. If your damage has already reached this state, you have a lot of catching up to do.

Identifying Spider Mites

Many species of mites can live on plants. Spider mites are small—so small you’ll need a 10x hand lens to identify them accurately. In fact, it’s more likely you’ll find the damage before the mite by randomly examining leaves. And this is wise—since if you detect them early, mites can be easily controlled. The mites themselves are oval in shape, slightly bristled, and pale green as juveniles with distinct dark green spots as they mature. These green dots are actually the contents of their gut and what gives the two-spotted spider mite its name.

Spider mites feed through a piercing, sucking action, which leaves a white pinprick mark. For individual plants, the first indication of infestation is a collection of these marks in an area the size of a dime. It may take some thorough hunting, but with practice, you can spot damage a good distance away. As the population increases, the feeding areas become larger, eventually yellowing the whole leaf and causing it to drop. Spider mites move from bottom to top, so it’s likely you’ll see the first of the damage on the plant’s lower growth, with leaves becoming more spotted as the population grows and the mites move up the plant. In severe situations, entire plants will become yellow and collapse.

Spider Mite Lifecycle

The lifecycle of the spider mite is dependent on their environment. Under ideal conditions of high heat (80 degrees F) and low humidity (less than 50%), the spider mite can complete development from egg to adult in just 5-7 days. These peak times generally occur between June-Sept. Under more average conditions of spring and fall, a lifecycle will take up to 19 days. If you can regulate temperature and humidity in your growing area (such as inside a greenhouse), you can minimize the mite’s population growth. As hours of sunlight shorten and food becomes scarce, female spider mites will enter into a stage known as diapause. This changes their coloration from green to orange. During diapause, the spider mite will not eat or lay eggs. They are also much less susceptible to pesticides.

Natural Predators

There are a few natural insect predators of the spider mite, but none is as effective as Feltiella acarisuga. This predatory gall midge is found worldwide except in the neotropics. It has proven itself so successful that biological control producers began rearing it in the late 90’s and have achieved great success in controlling the pest of greenhouse crops. Almost as small as their prey, an adult Feltiella measures 2mm and resembles a delicate tiny fly with folded wings and long legs. Though it’s possible to see them laying eggs in a colony, it’s likely easier to witness them in their larval or maggot stage. During this one week period, the bright orange Feltiella maggots will feed on 15 spider mites a day and up to 80 spider mite eggs. If spider mite populations are not yet at the webbing stage, an introduction of 1000 midge per hectare can rapidly reduce the infestation.

How to Identify Spider Mite Damage

The best way to control spider mites is to start with prevention. Upon receiving any new plant or vegetable starts to place in your landscape or garden, sample at least 25% by tapping the leaves over a piece of white paper. If you discover bugs dropping, use your 10x magnifying glass to determine if these are spider mites.

Because spider mite populations can increase so rapidly, and because many crops are at risk, it’s essential that you scout for possible threats if you’re growing a vulnerable crop or landscape plant during the hot weather. For large crop situations, walk back and forth every couple of rows to examine plants for any discoloration or speckling. If you know that spider mites are already present, look for the fine webbing indicating the population is growing. The web generally starts small between two easy points, like between the leaves on the tip of a branch. The webbing, when dense, has a shimmery appearance, and is very sticky. Ideally, have a cloth and a bucket with soapy water with you to wipe off the webbing as you find it.

For greenhouse situations, follow the same method as for large field crops but particularly inspect the tops of plants closest to the glass. Spider mites will congregate quickly here. Some hothouse growers will monitor their staff as they emerge from a day’s work in the crop noticing anyone who comes out itchy. Spider mites exude a skin irritant that can cause sensitive people to scratch. Many hotspots can be discovered this way.

Popular Plant Hosts

Spider mites are particularly attracted to the following plants:

  • Annuals
  • Apples
  • Apricots
  • Beans/peas (pods)
  • Cherries
  • Cucumbers
  • Ficus or Benjamin fig
  • Hemp
  • Hops
  • Peaches
  • Plums
  • Roses
  • Strawberries
  • Tomatoes

Alternate weed hosts

  • Chickweed
  • Clover
  • Various grasses

Natural Prevention and Management

There are two essential keys to successful spider mite control:

  1. Habitat management: Keep the humidity level high, above 65%, and temperatures lower than the ideal 80 degrees F.
  2. Early detection: If you can catch the spider mite prior to webbing or distribution throughout the crop, you have a very good chance at eliminating it before you start to lose plants. Note which plants they prefer and monitor accordingly.

If you discover spider mites, you have a few options:

  • Washing: For individual plants like landscape trees or perennials, wash down the entire plant, focusing on the undersides of the leaves. Repeat every two days for 10 days until spider mites are absent from new growth 10 days after the last wash.
  • Beneficial insects: Purchase and apply beneficial predators. Distribute evenly throughout plants or crop. Monitor your crop for beneficial predators throughout summer season.

By informing yourself and taking these simple steps, you can control spider mite populations before they wreak havoc in your garden.

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