Spider Mites

Class: Arachnida
Order: Acari
Family: Tetranychidae

Identification: Ok Master Gardeners, get your hand lenses out because we’re looking at some small critters. Spider mites are coming on strong this time of year; in fact many of you may have already armed up for battle. Spider mites feed on the underside of leaves and are most notable when they spin webbing. This webbing protects them for rain runoff (and sometimes your pesticide sprays), predators and provides structure to lay eggs on. Spider mites are not insects; they are more closely related to spiders. Thus, spider mites have eight legs (usually) and two body segments (the ceplothorax and the abdomen). If you get an up-close personal look at spider mites, you will see that they have a very boxy shape (different from the tear-drop shape of predatory mites), two stylets (or whip-like mouthparts) and have some sparsely placed course hair on the abdomen. Spider mites are pale yellow to green (or red depending on the species) in color for most of the feeding season while overwintering mites turn orange (Sound familiar? It amazes me of how similar many spider mites and herbivorous insects are to plants). There are many kinds of spider mites in our area. The most common pest that I’ve seen is the twospotted spider mite (easily recognized by its two spots, of course). Other common species include the Mc Daniel spider mite and citrus red mite. We also have a few common species of predatory mites. Identification of predatory mites is very important. Most of our endemic predatory mites are milky white in color, very fast and have a teardrop shape. Predatory mite eggs are much larger than spider mite eggs and have a football shape.

Life History: Spider mites undergo a similar metamorphosis as some insects, called gradual metamorphosis. Adult spider mites look similar to immature spider mites. Females lay spherical eggs. The eggs hatch into six-legged, first instar larvae. The next two stages are called the protonymphal and deutonymphal stages, which have the more characteristic eight legs. The final developmental stage produces the adult spider mite. Spider mites are reproductive machines. The twospotted spider mite can produce up to 100 in her lifetime. Total generation time can range between 20 days when it is cool to 5 days when temperatures get warm. You do the math. In optimal conditions, a population can explode. When populations do explode and your plant dies, spider mites will web up to the highest part of the plant and try to catch a breeze. This is how spider mites disperse and infest other plants; they balloon from plant to plant by sending a long piece of silk into the wind.

Damage: Spider mites have unusual feeding habits. The two stylets, that I previously mentioned, are inserted into individual plant cells. The mite basically sucks up the cells contents. This causes a very distinctive type of damage called ‘stippling’ or ‘bronzing.’ High populations can easily kill plants. Spider mites can have a huge host range. For example, the twospotted spider mite occurs on 180 different species of plants.

Control: Many plants can tolerate spider mite infestations. Have you ever seen a green Skimmia shrub? In fact, I thought Skimmia was supposed to be stippled white in the leaves (I kind of like it). Most Skimmia in the Northwest have citrus red mites feeding on them; however mortality due to spider mites is not very common. Plant health is key for suppressing spider mite populations. Spider mites respond very well to high nitrogen content. Over fertilization or water stressed plants actually increase the spider mites ability to lay eggs. Healthy plants also have natural defenses against spider mites. When a spider mite feeds on a plant leaf, the plant responds by releasing volatile chemicals that actually make spider mites sick. To add even more hardship to the spider mites, these volatile chemicals are also used to call for help. Predatory mites use the specific chemicals released by spider mite infested plants as a cue to come and get dinner! Scientists are just revealing this chemical interaction between the plant, spider mite and predatory mite. Recently, scientists have found that plants down-wind from infested plants release these chemicals too (even though the spider mites haven’t gotten there yet). Are these plants talking?

This easily leads into the most important control for spider mites. Traditionally in agriculture, spider mites are considered secondary pests (in fact some entomologists claim that spider mites were never agricultural pests until the use of DDT). When some pesticides are sprayed, we can usually predict a subsequent spider mite outbreak. This is because we’ve just killed or sterilized the predatory mites (or in some instances, we’ve actually made female spider mites more fecund with our pesticides). There has been a large effort to determine pesticide compatibilities with the use of predatory mites. So if you need to decide whether to spray or pray, collect leaf samples and get out your hand lens to monitor your spider mite population. Generally, spider mites are under biological control when you have a ratio of one predatory mite for every ten spider mites, or less. Other mite predators include green lacewing larvae, small lady beetles, big-eyed bugs, minute pirate bugs cecid flies and the spider mite destroyer lady beetle (sounds like a tough crowd, eh?). If you’re able to recruit the spider mite destroying lady beetle (looks like a very small, black lady bug), you are one lucky person. These beetles are insatiable mite feeders!