Spider Mites by Stephanie Suesan Smith, Ph.D.

Spider mites are common pest problems, especially for indoor plants.  They suck out the sap from leaves and leave behind bruises and other problems.  This may make the leaves become discolored or even drop prematurely.  If enough leaves drop, the plant dies.

Regular insecticides will not kill spider mites.  They are arachnids, not insects.  You must use a special chemical called a miticide to kill them.  First, though, you need to understand their life history to know what you are up against.

Spider mites are small and can be difficult to see with the naked eye.  Their colors range from red and brown to yellow and green, depending on the species of mite and the time of year.  Many spider mites produce webbing, or webs, when they are present in great quantities.  It is usually this webbing that catches the eye of plant owners.

The most important mite is the twospotted spider mite.  This is the mite that attacks indoor plants, vegetables, fruits, and outdoor flowers.  It produces a lot of webbing.

Spider mites develop from eggs, which are usually laid near the veins of leaves.  Most spider mite eggs are round and extremely large in proportion to their mother.  After the eggs hatch, they remain stuck to the leaves and can be useful in diagnosing the presence of spider mites.

Most spider mite activity peaks in the summer.  Spider mites can become full grown in as little as a week after being born.   After mating, females may produce a dozen eggs daily for a couple of weeks.  You can see how this rate of reproduction can rapidly overtake a plant.

Dry conditions favor spider mites.  They feed more.  At the same time, most of their natural enemies have trouble living under very dry conditions, so the spider mites reproduce unmolested.

Spider mites are hard to control.  There are natural enemies such as a small, dark-colored lady beetle known as a “spider mite destroyers”, pirate bugs, big-eyed bugs, and predatory thrips.  Other mites also eat spider mites.  One of the reasons spider mites may get out of control is that common pesticides kill the predators of mites but not the mites themselves.

One thing that helps control spider mites is adequate watering during dry periods.  This helps the plants resist spider mites, and the water itself can blow spider mites off the plant.  Most pesticides are ineffective on spider mites.  The most effective pesticide is the use of horticultural oils at the summer rate.  Great care must be taken to get the oil on all leaf and stem surfaces to kill the spider mites.  Since horticultural oils do not kill eggs, the treatment should be repeated in seven days to kill the newly hatched mites.