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Published 09/27/2019

Spider mites are one of the most destructive pests for indoor gardens. Although there are many different varieties of mites, the most common is the two-spotted spider mite. Spider mites feed on the contents of plant cells and use a sharp  proboscis to pierce the plants and drink the cellular fluid. Mite damage is a cycle that starts when the mites are introduced into your environment. Spider mites inflict very serious damage to the plant and can weaken the plant’s defenses, causing it to become more susceptible to disease and further damage. 

Mites can come into a room in many different ways, possibly on unfiltered air or hitching a ride on your clothes or shoes. The easiest, and first line of defense is to keep the mites out of your room in the first place. Use clean practices when moving things into and out of the grow area and always monitor your plants for signs of pests and diseases. Spider mites are usually first visible as black dots on the underside of leaves along the veins. They are sometimes hard to see, but using a jewelers loupe can help. The sign of a major infestation is when your plants start showing signs of webbing. The webs are a defensive structure that the mites utilize to reproduce and take shelter. Once the webbing starts, it becomes harder to completely eliminate the mites and the damage may already be severe to your plants.     

Mites can be controlled several ways, either through chemical means or physical. Keeping mites out and killing them when you see them is the easiest way to prevent a large infestation, but that won't work when you have a lot of large plants. This is why it is important to maintain proper environmental conditions. If you maintain proper airflow and humidity, mites can be kept to a minimum. Washing your plants with water or neem oil can help to keep mites from getting established as well, helping to keep your plants healthy.

Chemical deterrents like neem oil can help to make your plants less attractive to the mites, and they can also help to stop them from ever becoming established. Neem oil doesn’t work for killing mites so it will only help if you spray it before you ever see the pests. Once the mites are in the area, it is important to switch to something that can actively kill them. Pesticides like pyrethrum and azadirachtin are both effective at killing mites and have a relatively short half life. Pesticides are only as effective as their application, so it's important to pick the right one for your situation. Pyrethrum is available in several different forms either as an aerosol, a ready to use formula, or a concentrate. Azadirachtin is typically available as a concentrate. When you are applying pesticides through water, it is important to use a good sprayer that can create a very fine mist and coat the plants evenly. When you have a heavy outbreak it is important to spray consistently since the pesticides do not kill the eggs. This means you need to spray every four to five days for the first few weeks, then once a week as a maintenance spray. The webs that mites produce can protect them from the spray. Washing the webs off with water can help increase the effectiveness of pesticides.

Mite damage to your crop can be dramatic. Mites will rob your plants of oil content and will cause an overall lower quality product. When a crop is damaged early by mites, the plant may be susceptible to later diseases. If the damage is limited to later periods of development, it may be just fine. Some growers will notice that mites seem to be attracted to one plant or confined to a small area. This is why some greenhouses employ sacrificial plants, like sweet peas. The sacrificed plants attract all of the pests and can more easily be removed or replaced, helping to further control infestation.

Once a garden has mites, it can be very difficult to completely rid the area of them. Mites undergo what is known as  diapause, which is a physically dormant period that mites can enter when conditions are unfavorable. The mites in diapause are able to resist environmental stresses and can survive for months without food. Unless you can kill or remove every living adult female, mites can reproduce and repopulate quickly, creating a continuous and frustrating problem.