Some Background

Spider mites, not insects, are members of the order Acari. The main distinguishing characteristic that differentiates this order from those of insects is the number of legs these creatures have. Insects have three pairs of legs; adult spider mites have four pairs.
Mites are common pests of indoor plants worldwide; there are, perhaps, thousands of mites belonging to several families known to be phytophagous or plant-eating. There are many names for these pests, but the most familiar are the words “spiders,” “mites,” or, in the proper combined form, “spider mites.”
We say proper because spiders are members of a different order, most commonly, Aranea. Signs of spider mites, such as webbing and leaf stippling, often go unnoticed until infestations occur.
Spider mites tend to be survivors, often — depending upon species — doing their best in hot, dry conditions unfavorable to many pests.
In the United States, the number-one spider mite species is what is known as the two-spotted mite (whose adults have a dark spot on each side of their body and are sometimes connected over the back to present a saddle-bag-like appearance). They are also known as red mites due to their overwintering coloration.
Scientifically, they are known as Tetranychus urticae and will be the major topic of this chapter. However, other spider mite types, such as the carmine, European, southern, and citrus red mites, can also cause spider mite damage.
Acarid family mites (including in-ground bulb mites), Eriophyid family mites, false spider mites, and Tarosonemid family mites, among many others, will be the main concern on this page. The ubiquitous two-spotted mite, which is reportedly the most common mite pest, will also be of main concern.
Troubles with spider mites can start anytime, but two times of year are especially problematic: in the winter, when humidity levels tend to drop from the greenhouse heater pumping out warm air coupled with the usual lack of plant material and less rigorous irrigation schedule, and in the summertime, when greenhouse temperatures are on the borderline oven-bake setting.
In both cases, using predatory mites can be an effective measure to keep populations under control.
Interiorscapes usually occur in the winter, mostly because the building’s heating system constantly cranks out dry air.
Due to lower light levels in interiorscapes, plants tend to be more stressed and thus more vulnerable in the wintertime. But mites can infest plants at any time. These eight-legged demons are very opportunistic.
How They Become a Problem
In interiorscapes, the usual occurrences are in winter, mostly due to lower light levels and dry air caused by building heating systems. These environmental factors create perfect conditions where spider mites live.
Indoors, the spider mite infestation process typically begins as people come into contact with the plants. Spider mites are known hitchhikers, and if, let’s say, interiorscaper technicians or greenhouse scouts proceed in their duties from one account to the next or from one greenhouse customer to the next, the mites can be carried with them and deposited in a new location.
Outdoors, though mites are not flyers and have no wings, they can soar like birds on the slightest gusts of wind. Because mites are small, the two-spotted mites range from 0.3 to 0.5 millimeters in size, and they are not typically noticed while roaming.
Two-spotted mites, many of which go through a red-coloration phase in the winter (depending on location), live through those bleak months in plant and weed debris, cracks in floors, and any out-of-the-way places they can lie undisturbed.
For this reason, if your facility ever becomes infested with these pests, it is advisable to do an extremely thorough cleaning before planting new crops in anticipation of another growing season.
If you don’t, you’ll run a very high risk of having spider mites visit your crops again. And this is true of any of the mites listed herein.
A Serious Pest
Fortunately, most are readily visible to an observant scout with keen eyes (though a hand lens helps a lot), a focus on what to look for, and a meticulous approach with clarity in their duty.
Some mites, however, are so small, like the Acarid family mites, Eriophyid family mites and Tarosonemid family mites, that they may go unchecked for a considerable time before their presence is noted. With many of these small mites, their damage is first detected.
Often, there is a huge time before the greenhouse or interiorscape managers can figure out and isolate the causation. Unless those tasked to identify these problems have prior experience with these little mites, one must outsource the identification task to experts.
Even many cooperative extension officers lack the experience to identify these pests, so waiting for the final identifier to come to your aid may take some time. However, since many scouts have difficulty seeing even the comparatively large two-spotted spider mites, they are often discovered only by the damage they cause or the webbing they produce.
Some Advice

This forms a tent-like haven where the females can lay their tiny pearl-like eggs and raise their young. The best way to view these meager beginnings of a colony-in-the-making is to eyeball along the midrib against a good light source.
If the spider mite infestation isn’t noticed early on, the webbing will become apparent. It has engulfed whole leaves, flowers, and even entire plants. This webbing is also a good place to wage war against these pests.
As written on the “The I in IPM” page, wiping leaves [read: wiping webs] with a soapy sponge is a practice that can help in the battle against two-spotted and other web-producing spider mites.
Please note, however, that if you see tiny, individual silken strands randomly strung from one plant to another at any distance greater than one inch, you’re probably not seeing spider mite webbing.
Instead, especially if seen in a structure where chemicals are not used, you’ll have spiders working in your facility. You, therefore, will have no reason for alarm. Spiders, though creepy, are good guys to have around as they eat insects.
Do keep an eye on the situation, though. And remember, too many spider webs can impact your population of other good critters, like parasitic wasps since spiders are not very selective in their predatory ways.
Signs of Spider Mites
A spider mite feeds by inserting its piercing mouthpart into a plant’s phloem or inner cells. The mite collapses the cells by drawing the plant’s juices or sap. This feeding activity will normally show up as clusters of individual cells which are yellowish and sunken. The plant will take on a stippled appearance.
Mites are very capable of transmitting plant diseases, but are seldom responsible because they stay put unless hitchhiking or getting blown around. Diseases will be the least of your worries. Between the chlorotic stippling and the webbing, infested plants can look pretty unhealthy — especially as their dying leaves begin to dry up, shrivel and fall off.
Outdoors, two-spotted mites and many other species fare less well, especially when facing natural conditions where spider mites thrive—hot and dry environments. Rainstorms and high winds — and even natural enemies — have a considerable impact on many of these mites.
If you could just move the plants outside for even a little while. There are many ways to put mites to the test. What with web-wiping, increasing the humidity by misting and spraying your plants with plain water, and the vast array of predatory insects and mites available to eat spider mites, spider mites are fairly simple pests to deal with.
Unless, of course, you get hit with one of those less common spider mite pests mentioned at the beginning of this page. Those, though little, take a big bite and can be a much greater challenge.
Some Solutions to Treat Spider Mites
Other predatory mites, such as Phytoseiulus persimilis, are very good at really reducing a given population and are an excellent option when trying to control spider mites effectively. Sulfur sprays can help and are reasonably safe to use. They’re especially useful for targeting the less-common families of pest mites.
Chemical controls can also be effective, but great care must be taken as spider mites can easily develop resistance to many pesticides. Strategically using such methods can help eradicate spider mites without harming beneficial organisms.