Spider mites are members of the arachnid class along with spiders and ticks. The term "spider mite" comes from their behavior of spinning fine silk webs on infested leaves and new growth. The two spotted spider mite, Tetranychus urticae is the major spider mite pest of ornamental, floral and vegetable crops grown in greenhouses. Adults have 8 legs and an oval body, with 2 red eyespots near the head end of the body. Females usually have a large dark blotch on each side of the body and numerous bristles covering the legs and body. They look sort of like bristly black dots, and at 1/60th of an inch they are practically microscopic. Their eggs are visible too - they are very tiny, white and are laid grouped closely together. Infestations decrease plant production and the physical damage mites cause makes plants unattractive and weak.

Many houseplants indoors are hospitable to spider mite infestations, and the same goes for plants grown in an indoor growroom or greenhouse. Two spotted mites damage plants by piercing single leaf cells and sucking out the contents, causing the cells to collapse and die. This is the cause of the speckled browning found on leaves with spider mite infestations. The undersides of leaves may appear yellowish and crusty. In heavy infestations the mites remove nearly all the chlorophyll, and leaves crumple, die and fall off. When large populations have been present for a few weeks, fine silk webbing may cover large areas of the plant. The mites disperse from a plant of declining food quality on threads of webbing and drift or are blown on to other plants. If you look closely at these webs you can see tiny little spiders running back and forth.

Life Cycle and Environment
In a given colony of two-spotted spider mites, both adult males and females are present, however females usually out number males three to one. This factor accounts for their high reproduction rate as a single female can lay on average over 200 eggs in her life time. (Bessin.) Females normally lay eggs on the underside of leaves.

The rate at which a two spotted spider mite develops from an egg to an adult is greatly dependent on environment. Their life cycle is accelerated at higher temperatures. This is one of the reasons that these mites are such large greenhouse pests. In the artificial environment of a warm lighted greenhouse, the mites are able to reproduce quickly and to be active throughout most or all of the year.

A Healthy System Approach
In an ideal situation, the plants themselves would be healthy enough not to succumb to light mite populations (meaning, a few mites may be present but they are causing no significant damage,) predator populations would be at a sufficient number to keep damage below the critical level, and environmental conditions would be such that they would favor the good guys and not the bad guys. It is possible to reach these goals in any grow area.

The first step in the program is to keep plants healthy. Mites are more serious on stressed plants, and are often unable to gain a foot hold on healthy, vigorous plants. Different kinds of nutrient additives are avaiable whick strenghten plants from the inside out, helping to make them more pest resistant.

The second step it to regularly monitor for mites before they cause damage. Tetranychus urticae live in colonies on the underside of leaves and are very hard to see with the naked eye. Their tiny webs on the bottom of leaves are sometimes more easily seen then the little spiders are themselves. The spiders look like tiny black dots. It is possible to see them just by looking, but they are much easier viewed with a magnifying glass. They can be seen on both tops and bottoms of leaves, but they seem to prefer staying on the underneath part.

The first sign of an infestation is a sprinkling of tiny yellow dots on the upper side of plant leaves. The dots are the dead plant cells caused by the mites' feeding behavior. By the time this damage is spotted it is often too late to stop some damage from occurring. The ideal is to spot them before they start causing noticeable problems. If you look regularly you will definately notice a couple mites on leaves weeks before serious damage has been caused. If you take action at this point, you will be able to prevent a much larger problem.

Physical Control
It is possible to physically control mites before their populations get too high. They are easily washed off with a spray of water. They can also be wiped away with a wet cloth. This is easier to do on one or two plants than on many, so it may not be as applicable in larger growing operations.

What IS practical however, is the washing off of young plants before they are transplanted into their final growing places. If you have had problems with mites or if you are receiving new plants from another grower, you would be wise to rinse the leaves of your young clones or seedlings before introducing them to your main growing room. Be sure to rinse the bottom part of the leaves.

It is also very important to keep the growing area clear of dead plant material. Dead leaves should be removed from growing areas as soon as possible, as they often contain mite colonies and clusters of eggs. Dead plant material is also a breeding ground for fungus gnats.

If you choose to use pesticides, we highly recommend using botanical ones. Spider mites are usually not killed by regular insecticides, so be sure to check the pesticide label to see if "miticide" is present. Pesticides claiming "for mite suppression" are usually weak miticides and will not perform well. There are several natural sprays we carry including GC Mite, Pyrethrins and Neem oil. All of these will break down quickly after only a few days. Insecticidal soaps are also effective agains spider mites.Some natural sprays can be used right up until harvest, but it will be up to you to decide if that is a good thing to do or not. Be careful when using sprays on flowers and buds as you might damage them or leave them open to mold.

There are three main kinds of predator mites, each suited to a particular environment. If you are not sure which kind will work best you can usually get a standard mix of all three.

• Phytoseiulus persimilis: One of the most effective predators of pest mites on ornamental and fod plants. Well adapted to humid greenhouse conditions. When temperatures are around 70°-85° F. and 70%Rh., this predatory mites species can reproduce about twice as fast as the two-spotted spider mite. Well adapted to humid greenhouse conditions. Not appropriate in hot, dry environments.


• Mesoseiulus longipes Does best in warm to hot greenhouses. Tolerant of lower humidities (40% RH at 70° F). Frequently used to control spider mites in hot greenhouses on tall plants. It tolerates lower humidities than P. persimilis.


• Neoseiulus californicusDoes best in warm humid conditions, but will also tolerate low humidity (40% - 80%rh at 50° - 105°F). Occurs naturally along coast and inland valleys of California.
  excellent general predator for control on roses and vegetable crops in greenhouses. It tolerates higher temperatures and lower humidities than P. persimilis.

You should do a series of two or three releases over a two week period. This will ensure that your predators eat all stages of spider mite populations (eggs, nymphs, adults.) Start with 1 predator for every 20-25 spider-mites (count spider-mites on a few leaves, & average them out for a rough estimate). This gives control in about 4-6 weeks. Using more predators speeds up the process. For example, using 1 predator for every 5 spider-mites brings control in about 2 weeks.

What to do if you already have a Large Problem
In the case of a high pest population, you will have better luck supressing it if you eliminate part of it before introducing the predators. This can be done by pruning off highly damaged areas (which probably have large populations on them) and by using a botanical pesticide. Insecticidal soaps and horticultural oils can be used up to two days before release of predators (be careful, residual pesticdes will last a month and will kill your new predators.)

You can also release twice as many predators on the first release as is normally called for. That will make a quick dent in your pest populations.

Bessin, Ric Two-Spotted Spider Mite. Cooperative Extension Service Bulleting University of Kentucky College of Agriculture. http://www.uky.edu/Agriculture/PAT/recs/crop/pdf/entfa310.pdf

Daar, Sheila, Olkowski, William, Olkowski, Helga . 1991.Chapter 6: Pests of Indoor Plants in Common-Sense Pest Control. The Taunton Press.Newtown, CT