Spider mites are the bane of bonsai growers and growers of ornamentals in gerneral. There are several approaches, incompassing prevention, treatment, and environmental changes. Whether you choose an organic approach or the chemical route, you should find this article helpful. Keep in mind the principles of Integrated Pest Management (IPM):
- Monitor your plants regularly.
- Practice prevention by creating a non conducive environment for pests.
- As soon as you find signs of an infestation, use the lowest toxic level of treatment.
- Move to more toxic chemical controls only if necessary and warranted.
Know The Enemy
First make sure that you have spider mites. Red spider mites are just visible to the human eye and should be easily visible with a five power hand lens. They look like little red spiders. If you look carefully you should also be able to see eggs that look like microscopic pearls. They also make a visible webbing that looks like debris on the underside of the leaf, not on top. If the mites are very small, translucent, and appear to have two dark spots on their backs, you probably have two spotted mites, or one of their relatives. Always look on the underside of the leaf, that is usually where you will find them. Badly infested plants will have yellow and red mottling of the lower leaves.
If you don’t have a lens (and everyone should), use a piece of bright white paper under direct sunlight, or a strong lamp, shake the suspected leaves (or needled foliage) over the paper and watch for very tiny specks scurrying to get to the other side. Mites are programed to always go to the underside of the leaf. Mite damage will appear on the lower, older leaves first. Unlike aphids, mites are not interested in succulent new growth. Mites by themselves are not aggressive movers. They pretty much stay on the same leaf or nearby leaves for their entire lives. They are moved (vectored) by bird feathers, dog and animal hair, and clothing. They are usually worst in dry dusty conditions, although two spotted mites have been known to thrive even under very wet conditions.
Mites have their favorite species, and by learning which plants your mites prefer, you can use these as indicator plants. You can also isolate the preferred plants for treatment if necessary. Plants with hairs on the underside of leaves seem to be a favorite, although not necessarily. Among the all time favorites I have found are: Roses (yum!), Daylilies, Flowering Quince (Chaenomeles), Apples, (Malus), Blackberry and other Rubus, Boxwood (Buxus), and Junipers, but there are many, many other species that are susceptible. Plants grown indoors are especially vulnerable. Mites usually start becoming a problem in late spring and reach a peak by late summer or just at the peak heat of the season. They are definitely hot weather critters. In cold weather they move and multiply much more slowly. In summer their life cycle is about seven to ten days, that is, hatchlings are laying eggs after a week or two. Any treatment must take this into account. Just killing the adults does little good. Repeat treatments are almost always necessary to kill the emerging mites. In winter they begin moving off trees and shrubs to winter over on grasses.
Prevention
Mites, like aphids are easily dislodged from the leaf surface, at least before they have a chance to begin building webbing. A weekly hard blast of water can stop an infestation from occuring or slow it down once it starts. However, you must be able to spray the undersides of the leaves. Just hitting the top surface will do little or nothing. Concentrate your attention on the lower parts of the plant. Mites won’t be found on the upper and succulent parts of the plant. Introduce predators. Predator mites can be introduced in the spring just about the time the mites begin appearing and building their population. It is much better to use mites this way and have them grow with the mite population rather than to try to treat an infestation with predators. Predator mites are very expensive, so this is also a cost effective procedure.
Treatment
If you find more than an occasional mite, and most of the lower leaves have two or more mites and perhaps webbing, you are probably in trouble. Begin lower levels of control. First try blasting them off with a spray of water. Do this about every two or three days. It may or may not work. If the population continues to build, use an insecticidal soap designed for mites (it should be on the label), or introduce predators. Safer brand miticide and other insecticidal soaps MUST be tested for phytotoxicity before you use them. Many plants are seriously damaged by these soaps, including most maples. Spray a leaf or two that you won’t mind losing and wait three or four days to see if there is any damage. Insecticidal soaps work by contact so there must be thorough coverage. Dunking the foliage may be the best way to treat bonsai and containerized plants. Soaps do not kill the eggs, so they must be repeated every five to seven days. At least three treatments will be necessary. Treatment with predator mites is discussed below. Once mite count reaches about 40 per leaf, the population will really explode and mites will begin moving to other leaves and plants. In the worst cases, they will even begin moving to other species that are usually resistant. Treatment with predator mites at this stage is possible, but difficult and expensive. At this stage you may want to take a more toxic route.
There are some really effective agricultural chemicals for mites that have a fairly low toxicity. Unfortunately, the over the counter chemicals are the absolute worst products. They are highly toxic and not very effective. I will give you some names below of products that do work, but you should not use them unless they are permitted for your area and situation. The best way to find out what to use in your area is to contact your local Agricultural Extension Agent. It’s a simple call, they are in the phone book under county government, usually under Agriculture. They are PAID to do this, don’t think that you are imposing on them. Red spider mites are pretty easily dealt with. You can usually knock them out with water sprays, Safer’s miticide (and other insecticidal soaps), Orthene, Kelthane or Isotox, in order of increasing toxicity. These are all nasty chemicals except for Safer’s and Isotox may not be available any more. I don’t recommend it. The big problem is that there are few effective ovicides for mites, so you must follow up in five to seven days with a repeat treatment to kill the hatchlings. Usually three treatments are necessary to end an infestation. There are one or two ovicides available now, but I believe they are all Ag products, I don’t think any are available over the counter. Read and follow all label instructions carefully. Do not use more or less than the recommended amount or concentration. Using less can result in breeding mites with resistance to that chemical.
If you have two spotted mites, your job is even harder.
These mites are very difficult to control and most are resistant to most miticides. Don’t bother using typical over the counter insecticides, they will do nothing. If they are approved for your area and application, you can use a rotation of Pentac, Mavrik, and Avid. These mites can build a resistance to pesticides very quickly, so repeated use of the same one just breeds new problems for the rest of us. These two spotted mites can still be killed with Safer’s miticide. It is highly unlikely that they will build a resistance to this, but you must have physical contact with the miticide, so be sure to dip your plants or thoroughly spray them. Follow up treatment twice, at five to seven days. Always use pesticides as recommended