Yarb Tales Spider Mites
January 25, 2016

The Heritage Herb Garden at the Ozark Folk Center graces the park with visual colors and textures, sweet and pungent aromas, and helps us to interpret the history of the human use of plants.

The past couple of weeks Yarb Tales have been telling the stories of pests that feed on plants and are especially pesky for indoor specimens. Indoor temperatures are tropical. Without freezes to slow down or force sap-sucking insects and mites into dormancy, these organisms continue to feed and reproduce. Spider mites are among these enemies to the indoor gardener.

Spider mites, Tetranychus urticae are major pests to many crops throughout the world. They are commonly called glasshouse red spider mites or the two-spotted spider mites.

Spider mites can be observed as red or brown specks embedded in fine webbing in the growing tips of plants. Magnification is needed to observe the details of adults, larvae, nymphs and eggs.

The adult bodies are elongated ovals with four pairs of legs, distinguishing them from insects, which have six legs. A pair of antennae protrudes forward out of the head. The color of the adults varies, according to the plant host, from orange, light green, dark green, dark yellow, reddish-brown or black. Spider mite adults usually have two dark spots on either side of the body.

Spider mites develop through eggs, larvae, first nymphal and second nymphal periods and then sexually reproduce during the adult stages. Development time is dependent upon temperature, relative humidity, host plant, and the age of the leaf. Temperature is the most important factor. From egg to egg, the life cycle of spider mites at 59° F takes 36 days. This cycle takes only seven days at 86°F!

A female spider mite produces all male eggs if she is not fertilized. When mated, she lays male and female eggs.  One mating fertilizes more than 100 eggs. The warmer the environment, the more frequently eggs are deposited.

Outside, during autumn, the females may mate, turn an orange color within five days and hide. These females do not feed. They hibernate until conditions are favorable for eggs to develop after being deposited.

When the temperature is consistently warm, the eggs are deposited on the undersides of leaves. When new, the eggs are transparent. They turn opaque and are finally beige when the larvae emerge.

New larvae are transparent except for two red eyes. They have three pairs of legs. After they have eaten they turn green or yellow.

The larvae become nymphs that develop another pair of legs. Two spots on the sides become visible. Their color becomes light to deep green. The first nymphal stage takes nourishment. The second stage rests and develops into adulthood.

Spider mite adults, larvae and first nymphal stages feed on plant sap and tissue. The damage looks like yellow, speckled areas on the tops of leaves. The spider mite webs decrease the amount of sunlight reaching plant leaves. These two things combine to weaken and often kill plants.

Spider mites move around by walking, hitching rides on gardeners, or attaching to web filaments and riding airwaves. It takes determination, good nutrition and timely application of sprays to control them.

Plant nutrition should be balanced. An overabundance of nitrogen will cause tender green growth which spider mites love. Be especially attentive to supplying adequate phosphorus, calcium, iron and copper to the plants that are susceptible to spider mites.

Daily misting will keep spider mite populations low. Dry periods cause the spider mites to feed to keep from drying up—increased feeding results in higher egg production.

Summer oil or soap sprays are effective controls that coat and suffocate pests. They must be thoroughly and regularly applied. Spider mites are notorious for building immunity to chemical controls.

Spider mite eggs are very small, numerous and well protected in webbing, making them difficult to destroy. The other developmental stages are easier to target and spray. When the minimum temperature is 68° F egg development takes five to seven days. At 86° F, egg development takes place in 3 days.  Re-spray according to your average temperature.  Be sure to test plant foliage for sensitivity to sprays.

Wettable sulfur kills spider mites and small beneficial predators such as predatory wasps and the mites, Phytoseiulus persimilis that feed on spider mites. I have observed live, adult spider mites the day following a thorough spraying with sulfur. On a microscopic level, sulfur remains a crystal even though it is in solution. The crystals are spaced in solution far enough apart to miss some mites on plant surfaces. Though sulfur is a non-metallic element necessary to plant growth, precautions for use are numerous. Sulfur is moderately toxic to mammals, including humans. Protect skin, lungs, and eyes when using. To avoid foliage damage, do not use sulfur within one month of using oil sprays and do not spray sulfur when temperatures will exceed 85° F.

If you are fighting spider mites, I hope this information helps. If I don’t see you in the future—I’ll see you in the pasture.