Garden Detective

Garden Detective: Spiders are garden-friendly; spider mites are not

Q: I am having a big problem with spiders. Is there a “season” for them so they would be gone soon? They are on all my plants, even lemon trees. I have had spider mites on some plants and have used spray on those and so far they look OK. Do the spiders actually kill the plants? Is there anything I can do to get rid of them?

Betty Johnson, Sacramento

A: According to UC master gardener Annie Kempees, garden spiders and spider mites are two very different creatures.

Garden spiders are primarily beneficial and their activities should be encouraged in the garden. Pesticide control is difficult and rarely necessary as they are not found in large groupings.

Spiders resemble insects and sometimes are confused with them, but they are arachnids, not insects. Spiders have eight legs and two body parts – a head region (cephalothorax) and an abdomen. They lack wings and antennae. Although spiders often are found on plants, they eat mainly insects, other spiders, and related arthropods, not plants.

Spider mites are common pests that feed on many fruit trees, vines, berries, vegetables and ornamental plants. To the naked eye, spider mites look like tiny, moving dots; however, you can see them easily with a 10X hand lens.

Spider mites are tiny and difficult to detect. In some parts of California, spider mites may feed and reproduce all year on plants that retain their green leaves throughout the winter. In colder areas and on deciduous trees that drop their leaves, web-spinning spider mites overwinter as red or orange mated females under rough bark scales and in ground litter (such as fallen leaves) and trash. They begin feeding and laying eggs when warm weather returns in spring.

You’ll usually notice plant damage such as stippled or yellow leaves before you spot the spider mites themselves. Check the undersides of leaves for mites, their eggs and webbing; you’ll need a hand lens to identify them.

To observe spider mites more closely, shake a few off the leaf surface onto a white sheet of paper. Once disturbed, they will move around rapidly. Sometimes the mites will be gone by the time you notice the damage; plants will often recover after mites have left.

Spider mites have many natural enemies, which limit their numbers, especially when undisturbed by pesticide sprays.

Adequate irrigation is important, because water-stressed plants are most likely to be damaged. Dusty conditions often lead to spider mite outbreaks. Broad-spectrum insecticide treatments for other pests frequently cause spider mite outbreaks, so avoid these pesticides when possible.

Sprays of water, insecticidal oils or soaps can be used for management. Always monitor infestation levels before treatment.

Additional information on spiders and spider mites can be found at http://ipm.ucdavis.edu.

Garden questions?

Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to h&g@sacbee.com. Please put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call:

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