November 20, 2003 at 2:13 p.m.
Despite its appearance, the spider mite is not feeding on the tree or causing any harm. The mites were clustering together as they prepare to over winter. They eventually will move into the cracks and crevices of the bark and go dormant until spring. They may accidentally be brought into homes on Christmas trees or holiday decorations made from evergreens.
Spider mites are among the most troublesome pests on houseplants. While they may not cause any harm to trees, they can severely damage and even kill houseplants. Spider mites are arachnids, having eight legs and are more closely related to spiders than insects. They are extremely small and can be hard to see without a hand lens. Many produce silken strands of webbing that cover affected plants. They move from plant to plant by crawling, being transferred by human, and by being blown in wind or air currents.
Spider mites can have many generations during the growing season. Mites develop from eggs into larvae, then nymphs, and finally the adult stage. The warmer and drier the conditions, the more generations will occur. Adult females lay eggs on host plants, and mites can develop from eggs into adults in less than two weeks. The first evidence you are likely to notice is pinprick-sized discoloration on the leaves. You may also see fine webbing when spider mites are numerous.
Isolate affected plants from healthy ones. Spider mites can easily move to nearby leaves and infect new plants. You can sometimes deal with lighter infestations by knocking the mites off with water from a kitchen sprayer or shower. Repeated washing over several weeks can effectively reduce their numbers.
There are also several insecticide options. Horticultural oil and bifenthrin are effective. Insecticidal soap can work but can be less effective. Spider mites are difficult to control, so check every 10 to 14 days and spray as needed. Be sure to get good coverage, paying special attention to the leaf undersides where most of the mites are found.
If you are still having problems with a spider mite infestation, prune off the worst parts of the plant. Some plants may be pruned back to the ground. The new growth may be mite-free, but don't hesitate to throw away plants that are heavily infested and damaged. It's better to get rid of them than risk infesting all the others.
The Master Gardeners have books for sale that would make great holiday gifts:
•"Landscaping for Wildlife," $10.95 and
•"Woodworking for Wildlife," $9.95, published by the DNR.
•"So Easy to Preserve," $18 from the U of Georgia Extension Service.
•The 2004 Minnesota Gardening Calendar are also available for $12.
Stop in and take a look.