One of the most destructive pests for indoor gardens is the spider mite. Although there are many different varieties of mites, the most common one that affects indoor gardens is 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. Spider mites inflict very serious damage to plants and can weaken the plants defenses and cause it to become more susceptible to disease and further damage. Mite damage is a cycle that starts when the mites are introduced into your environment.
Mites can come into a room many different ways, possibly on unfiltered air, or hitching a ride on your clothes or shoes. The first line of defense is just to try 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 can be hard to see, so sometimes 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 produce 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. Just keeping mites out and killing them when you see them is the easiest way to prevent a large infection, 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 as keeping your plants healthy.
Chemical deterrents like neem oil can help to make your plants less attractive to the mites and help to stop them from ever becoming established. Neem oil doesn’t really work for killing mites so it will only help if you spray it before you ever see mites. 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, because 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 so sometimes washing the webs off with water can help.
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; but 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 a small area. This is why some greenhouses employ sacrificial plants, like sweet peas. The sacrificed plants attract all of the pests, then they remove that plant from the room and replace it discarding all of the pests.
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 every living adult female, mites can reproduce and repopulate so fast that you will always have them unless you treat continuously. Some gardeners will break down their areas and scrub with strong disinfectants and remove any debris from the area. After all that work there is no guarantee that the mites won’t come back, because all it takes is one to survive for the cycle to start all over.