Spider Mites

General Overview 

Spider mites are very common, and serious, pests of outdoor plants, and occasionally indoor plants. They are spiders, not insects; they have 8 legs and insects have only 6. Spider mites are very small, only about 1/50 of an inch long, and are difficult to see. They have piercing-sucking mouth parts for feeding on plant sap and can cause plants to lose vigor so that they may be unable to overcome a severe infestation, resulting in the plant's death. They can multiply rapidly and in large numbers, cause leaves to take on a dusty, dull appearance. Leaves then yellow and drop or turn brown or tan. 

Symptoms and Diagnosis 

Since the mites are so small, the first sign is generally a plant that looks dull or in poor health. Leaves may appear stippled and curled. Fine webbing may also be evident under the leaves or between the leaf and the stem. When a leaf or branch is tapped over a white sheet of paper, small specks that appear as dust or pepper may be seen to move. Spider mites are among the most ubiquitous of pests, attacking a wide variety of field, garden, greenhouse, nursery, and ornamental plants, as well as several weed species. 

Life Cycle 

Outdoors, adult female spider mites overwinter under loose bark, in cracks in the soil, in leaf litter, and in other protected places. In the spring, adults emerge and begin laying eggs. Each female typically deposits 70 eggs and lives for only 30 days. Eggs hatch in 5–7 days, and as many as 10 generations may occur each year. Spider mites can go from egg to mature adult in less than two weeks. The young look similar to the adults, but newly hatched young have only six legs and do not possess the characteristic dark spots on the back. Reproduction of the two-spotted spider mite is favored by hot, dry conditions, so serious damage is likely to occur in mid-July to September.  

Integrated Pest Management Strategies 

1. Knock mites off plants with water. Spraying with a strong stream of water (particularly the undersides of leaves) will provide some control. Spray plants frequently to control future buildups. For severe infestations, affected plants or plant parts can be removed and destroyed. There are several natural predators that feed on spider mites. The use of chemical insecticides to control other garden pests can result in the death of these beneficial insects and a subsequent increase in the population of spider mites. 

2. Use insecticidal soap. Many insecticidal soaps are also effective in controlling mites so check the label. Other soap products may actually be labeled as miticidal sprays. These soaps are specially formulated to kill mites and not damage plants. They can be very effective if used frequently until the problem is under control. 

3. Use superior horticultural oil sprays. Highly refined oils sold as superior or horticultural oils are also very effective in controlling mites. The oil suffocates the mites. Unlike dormant oils, these oils are highly refined and under proper conditions, can be applied to plants in foliage without damage. Follow label directions to avoid damage to some plants that may be sensitive. Superior oils are also considered nontoxic and are less harmful to beneficial insects. When spraying indoors, protect surfaces that may be damaged by an oil residue. 

4. Use chemical insecticides or miticides. A very safe product made from the seeds of a tropical tree is called Neem. Other chemical controls include malathion, bifenthrin, and cyfluthrin. Be sure to follow all label directions when using pesticides. Many pesticides are very harmful to bees and fish when used improperly. Be sure to check the label to make sure the product is effective against spider mites.  

5. To limit future problems, inspect plants regularly. With regular inspection, pest problems can be caught when just beginning and control is easier. It is also recommended that newly acquired indoor plants be isolated for 2–3 weeks to limit the introduction of pests indoors. Bringing plants indoors in the fall is another way of introducing mites indoors. 

Organic Strategies 

Strategies 1 and 5 are strictly organic approaches. For organic approaches to Strategies 2 and 4, consult the Organic Materials Review Institute (OMRI™) for appropriate insecticidal soap and miticide products.