Mites are not true insects. As adults the have 8 legs and are actually related to spiders and daddy longlegs. They may be one of the most damaging agricultural pests. One reason is that they're nearly impossible to see so you don’t know you have spider mites until it is a serious problem. In fact, they usually appear as no more than specks of dust the size of the period at the end of this sentence. Spider mites are not only hard to see, they're nearly impossible to get rid of. If you have spider mites you will notice yellow dots on leaves, curled or distorted plant leaves, which is caused by the mites' piercing mouth parts. Sadly, most growers notice the webbing between leaves and other plants. This signals you have a serious infestation.
Like white flies, spider mitess life cycle is temperature dependent. A female lays about 100 eggs, but under 60 F she will do it over two months. At 70F there will be over 10,000 mites after a month, and over 80 F there will be over 10,000,000 mites in your grow area.
How to deal with mites – First, If you lower the temperature even just during the night cycle it will help. You can submerge plants under water as I mentioned in a the general pests post or spray the plant with insecticidal soap. Misting your plants often with just water also helps, since mites thrive in a warm and dry atmosphere. The higher the humidity the slower their metabolism and life cycle. The good thing is mites can’t fly. Look and see if some plants are not infested, or if you can see some that are only lightly infested vs. heavy infestation. (webbing is a heavy infestation). Since mites walk everywhere, you can put double sided tape around the plants to prevent mite migration. You may need to trim your plants so that the mites can’t walk from leaf to leaf, or leaf to wall to leaf etc. Mites can blow in the wind so keep that in mind if you have air flow (Which you should for optimal plant growth).
As with white flies there are predators of spider mites, there are predator mites. Unlike predatory wasps of white flies predator mites are not so efficient, since being mites they can’t fly. It is a non-chemical option, and I know many of you have asked for suggestions for that type of control. The type of mite that infests greenhouses is called the red mite or the two spotted mite Tetranychus urticae . The predator mite Phytoseiulus persimilis feeds on all stages of the two-spotted mites and is the most commonly used predatory mite in greenhouses. It does well under humid conditions so if your plants can tolerate high humidity this may be the choice for you. There are other species of predator mites, each has its own environmental preferences. You want to try and match up your grow environment with the best predator mite
If you are using a chemical you should know many miticides work by coming in direct physical contact with the mite. This means you need thorough coverage to the underside of the leaves where some spider mites are feeding and most lay their eggs. Often eggs are more tolerant to many miticides so repeated applications are often needed.
Ultimately how you deal with mites depends on your growing situation. If you have prized plants like bonsai you may want to try chemicals several times. If you are growing plants to harvest you may want to keep the humidity as high as possible and the temperature as low as possible until your plants are done, then sterilize everything with alcohol, bleach or peroxide. If you've tried everything and you can’t get rid of the mites, you may have to kill most plants and clean out the grow area and start new. Odds are the older plants are a source of mites spreading to younger plants so you may have to get rid of the oldest plants. Two years ago when I had a spider mite infestation I finally got rid of them at the end of the semester when I killed most of the plants and only kept a couple plants which I cleaned thoroughly two times with a 10% isopropyl alcohol mixture. Each time I carefully cleaned the plants I moved them to a room that did not have plants in it previously. Two years mite free!
If you or someone you know has mites, keep in mind mites can be spread easily on your hands and clothing (they even blow around with puffs of air). When it comes to spider mites, prevention is far easier than the cure!
Good Growing,
Dr. E.R. Myers
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