Controlling Spider Mites

Spider mites, also known as two-spotted mites, become a particular problem for the gardener through the winter. Normally, they hibernate in ground litter or under the bark of trees or shrubs. However, if they stow away onto a plant being brought indoors, the artificial lights, and warm, dry, conditions of most heated homes will allow them to keep infecting plants.

The spider mite, Tetranychus urticae, is a tiny, 8-legged pest related to the spider & tick. Adults are normally green or yellow but turn red when the day lengths shorten in the autumn. They attack plants by stabbing the underside of the leaves and sucking out the sap. This damage causes a distinctive stippling effect due to the loss of chlorophyll. As their numbers increase, the number of white speckles on the leaf increases and the leaf eventually dies. Once the spider mites begin reproduction, a distinctive “webbing” forms, usually under the leaf and then at the growing tip of the plant.

What makes this pest truly difficult to control is its rate of reproduction. Each female will lay up to 12 eggs per day. Mating is not required for egg production. At 21ºC, these eggs will hatch in as few as 3 days and will develop into adults in only 14 days. If left unchecked, 10 spider mites in May will become 100,000 by July!

Spider mites have been found in greenhouses across North America and Europe and are known to attack over 200 species of plants including azalea, camellia, citrus, evergreens, hollies, ligustrum, pittosporum, pyracantha, rose, and viburnum; fruit crops such as blackberries, blueberries and strawberries; vegetables including tomatoes, squash, eggplant, cucumber; and trees such as maple, elm, ash, black locust, and poplar.

The Predatory Mite

Unlike the spider mite, the predatory mite, Phytoseiulus persimilis, is a welcome insect in the garden & greenhouse. It is a fast-moving insect with an orange teardrop-shaped body. The species is a specialized predator the two-spotted spider mite and feeds on all stages of its prey, from egg to adult.

The adult P. persimilis is a voracious eater, eating between 5-20 prey per day. It uses its sense of smell to find plants infected by spider mites. As soon as it comes into contact with spider mite webbing, it will intensify its search.

P. persimilis can be purchased from many biological control companies. Often it is shipped in a glass vial or on trays of bean leaves. The easiest method of application is to sprinkle about 20 adults on each plant. These predatory mites prefer to work their way up a plant, searching for food so try to introduce them as low down as you can. In addition, if many of your plants are infected, keep them close together with their leaves touching so that these predators will be able to easily move from one plant to another.

Unlike the spider mite, P. persimilis prefers humid conditions and Misting will not only help it multiply but will keep down the numbers of the spider mite. A relative humidity of 70% is ideal for P. persimilis. Once its food supply is exhausted, the numbers of P. persimilis will decline.

Other cultural controls

As previously mentioned, misting plants at least twice a day will keep spider mite numbers down. Mite populations can also be reduced by spraying the underside of the leaves with a jet of water to break up the webs and wash the mites off.

Soap sprays are also very effective at controlling spider mites. The Active ingredient, potassium or sodium salts of fatty acids, is not toxic and can safely be used indoors. A homemade spray can also be prepared using ordinary dishwashing detergent. Mix 5 tablespoons of detergent in 1 gallon of water and spray the plants, especially the underside of the leaves.

Resources

Arzeena Hamir is an agronomist and President of Terra Viva Organics.