Mites are tiny spiders that suck the living juices from nearly a thousand different garden plants. Being closely related to ticks, some varieties of mites also suck blood from mammals, like us! The study of mites and ticks is called acarology.
Mites prefer soil that is high in organic content with plenty of moisture. Unfortunately, that describes nearly all of our container plants and many of the microclimates found in Bay Area gardens. A highly adaptable critter, there are over 42,000 species of mites worldwide. Aside from nest mites, dust mites, varroa mites, and many others that attack birds, animals, and bees, respectively, common garden mites include gall mites, spider mites, and thread-footed mites.
Spider mites get their name because they build protective webs around eggs and feeding areas. The most common spider mites in the Bay Area are the two-spotted spider mite, the strawberry spider mite and the Pacific spider mite. They are often found on the underside of leaves, where they pierce plant cells to feed. These mites are very small, usually less than 1 mm (0.04”) long, so they are all too easy to overlook until the damage becomes significant. It doesn’t take long for a population to develop, either. A single female can lay 20 eggs a day and live for up to 4 weeks. Since each offspring hatches within 3 days and becomes sexually active in only 5 days, a single, fertilized female and her offspring can produce millions of spider mites in a single season!
An interesting note: female spider mites have two sets of chromosomes, like we do, but males only have one. If an egg is fertilized, it will hatch female. If it is not fertilized, it will hatch as a male. Also, female spider mites are able to “decide” whether to lay male or female eggs, depending upon environmental conditions. To control spider mites, insecticidal soap is your best bet. Neem oil, diluted and combined with a surfactant (dish soap) can help. If chemical pesticides are used, repeated applications will be necessary (and progressively ineffective, as mites can develop resistance).
Most thread-footed mites, also known as white mites, feed on fungi and algae, a few varieties have evolved to attack leaves. Specifically, the cyclamen mite and the broad mite are able to inject toxins that thin the cell walls of mature leaves. Damaged leaves display puckering, twisting and stunting.
The only known effective chemical pesticides against mite infestations are endosulfan, dicol, and ethyl bromide fumigation. Endosulfan was globally banned due to its toxicity to humans and its ability to accumulate in an environment, dicol is considered a “moderately hazardous” pesticide, closely related to DDT, and the state of California classified ethyl bromide as carcinogenic and a reproductive toxin - not anything you want to be spraying on food plants. Broad spectrum pesticides do more harm than good because they also kill beneficial insects that feed on mites. You can buy predatory mites that help control mite infestations. If an infestation is discovered, sprays of water, insecticidal oils (neem, cottonseed or canola) soaps can be used to displace mites and make life harder for them. Garlic extract and oil of clove, rosemary, cinnamon, mint and others can also be effective. These natural treatments can be dangerous to plants, however, so use them carefully. The same goes for sulfur, especially on cucurbits. Observation and prevention are far easier than eradication.
One of the easiest ways to avoid mite infestations is to create a quarantine area for new plants. This protects established plants from new infestations and gives you the time needed to see if a new plant is carrying any pests or diseases. Also, proper irrigation reduces water stress in established plants, making them better able to protect themselves. Mites prefer dusty conditions, so keeping garden paths, trees, shrubs and other areas clean can significantly discourage mites. Encouraging beneficial insects, such as lady beetles, lacewing, and pirate bugs, by providing water and habitat, can significantly reduce mite populations without the use of pesticides or sprays.
I hope this information inspires you to grow more of your own food. You can ask your garden questions on my Home page.