Controlling Spider Mite Infestations in your Greenhouse

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One of the common problems for the commercial greenhouse grower during summer months is an infestation of spider mites. Spider mites tend to become more active in hot weather, and are not usually a problem in cooler weather. Spider mites feed off leaves on a variety of ornamentals as well as certain types of evergreens and fruit trees. If left unchecked, spider mites can destroy, or at a minimum do significant aesthetic damage.

Spider mites are very small. They have four pairs of legs, no antennae and one main body. They are smaller than 1/50 of an inch. They are difficult to see using the naked eye, requiring a magnifying device to observe. Spider mites are not insects, but are members of the arachnid family. There are over a dozen species of spider mites that are common to North America. The life cycle of mites varies from 5 to 40 days, depending on temperatures; 85 to 90 degrees is ideal, at which the life cycle is 8 to 12 days. The female mite can lay from 90 to 200 eggs in her 30 day life.

Symptoms of Spider Mite Infestation
Spider mites eat by rasping the leaf surface and sucking the juices. Infested plants will exhibit leaves that have tiny specks of brownish-yellow discoloration. Often times when you see the leaf symptoms you can find the critters by looking at the underside of the leaf. In situations of more advanced infestation one can find fine webs spun about the plant. When you find webbing it may be too late, as the degree of infestation may be too severe to save the plant.

There are several methods of available for the commercial grower to control or eliminate spider mites. It is, however, the opinion of the author that the most sure and economical way to deal with spider mites is chemical treatment with a miticide. This article will briefly review some of the methods.

Cultural Control
Keep plants well watered – moisture stressed plants tend to favor mite development. High nitrogen in plants also favors mite development. Keep weeds under control, as weeds are often alternate food hosts. Populations do not show up on monitoring cards, as there is no winged state. Mites are transported on web filaments through air flow patterns, especially by doorways. Because they are transported on worker’s clothing it is best to arrange to do greenhouse tasks in mite infested areas at the end of the day.

Biological Control
Introducing natural predators can help to control spider mite populations. Identification of the species of problem spider mite is necessary to determine the kind of predator you will need to purchase. The spider mite predator Phytoseiulus persimilis is a voracious feeder of the common two-spotted mite and its eggs. Under favorable temperatures and humidity a single predatory mite will consume up to 30 eggs or 24 young spider mites in a day. If given optimal conditions, Phytoseiulus reproduces twice as fast as its victim, so can gain additional advantage in this way. Although it is the most commonly released predator of mites, there are other species which will do well under a variety of environmental conditions.

Chemical Control
Chemical treatments can be very effective; however it is critical that the correct chemical is used. Many insecticides will not control the spider mite, and in fact may cause it to flourish because the insecticide may kill its natural enemies. One needs a chemical labeled to control spider mites (a miticide). There are many different species of spider mites; miticide labels will specifically list the species of mites that will be controlled. Spider mites usually feed on the underside of the leaf so that they are difficult to control when sprays only land on the top. Some of the newer miticides exhibit translaminar movement in the leaf, so that the chemical will move from the top of the leaf through the tissue to the underside.

Monitoring
Frequent monitoring of your plants is key to managing potential mite problems. Detecting leaf discoloration symptoms at an early state is the best way to ensure that your small problem doesn’t become a big problem.