spider mite (67967 bytes)

Spider Mites

By Judy Sedbrook, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver County

The most common spider mite, the two-spotted spider mite (Tetranychus urticae), is commonly encountered on houseplants but will also attack shade trees, shrubs, other herbaceous plants and agricultural crops. Each colony may contain hundreds of individual mites.

spider mite colony (15034 bytes)

Spider mite colony

Mites are members of the arachnid class along with spiders and ticks. Hard to visualize well with the naked eye (about 1/60th of an inch in length when full-grown), a hand lens is necessary to positively identify their presence. Mites can be observed by shaking infested leaves over a white piece of paper. If there are mites present, you will see little dots about the size of a period on this page. If it looks like dust but moves, it's a mite. These mites are greenish yellow to orange and have two distinct black spots on each side of the body, which give rise to their name.

The two-spotted mite thrives whenever conditions are favorable for plant growth, but are most likely encountered under dry and hot summer conditions when their populations tend to explode. This happens because high temperatures (up to 100oF) decrease the lifecycle from three weeks to a mere 5 days and low humidity allows the mites to more easily remove waste products from their bodies via evaporation, thus enhancing feeding and reproduction. Spider mites that may infest honeylocust, linden, elm, willow, and oak are most destructive in the summer.

Spider mites on conifers and broad-leaved evergreens are cool weather pests. They feed heavily and reproduce quickly in spring and fall. Activity is low during the hot part of summer, although damage is often at a maximum and becomes easier to see when other plants are green and growing normally.

Spider mites also attack some garden crops including sweet corn and beans.

All stages of the spider mite live on the undersides of leaves where very fine silk webbing can often be found when mites are very numerous. This webbing can also be found toward the tops of plants where the humidity is lower and can extend from leaf to leaf to cover the entire plant.

The mites suck out plant juices causing a loss of chlorophyll. The damage first appears as yellow or whitish speckled areas on the upper surfaces of leaves. As damage progresses, the speckling moves toward the leaf tip and leaves take on a bronzed color on the upper surface. Infested broad leaves may be cupped downward or become distorted. The leaves eventually turn brown and prematurely fall off. This may eventually result in the death of the plant.

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Spider mite damage

Spider mites are also a known source of allergens causing asthma, hay fever, and contact urticaria.

CONTROLS:

  • In a normal year, their natural enemies control mites including a fungus disease that is present when temperatures are below 85F and relative humidity is high.
  • There is some evidence suggesting that the phytoseiulus persimilis mite, lady beetles and certain thrips can be used to control the two-spotted mite.
  • Washing infested plants periodically with a strong stream of water will provide some control.
  • Insecticidal soap is an alternative for home landscapers who prefer not to use pesticides.
  • Spider mites can be exterminated with a variety of miticides. Horticultural oils, used at the "summer oil" rate of 2%, are possibly the most effective miticide for home use. Most other insecticides are not effective on mites and some, especially carbaryl (Sevin), result in increased mite damage by killing their natural enemies. Always read the label before applying any pesticide.

For more information on spider mites, see CSU Fact Sheet 5.507.

Photos: Judy Sedbrook

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Date last revised: 01/05/2010