For self reliant gardeners an indoor garden is often the choice to ensure food for the winter months. To maximize the yield, all precautions are taken and yet you still find your plants suffer and/or die due to pest damage. I hope you find these organic methods to control spider mites, the most frequent and damaging pest to an indoor garden.
What Are Spider Mites?
Spider mites are members of the family Tetranychidae, it includes about 1,200 species and are known to feed on hundreds of different plants. Spider mites are less than 0.04 in (1 mm) in size and quite varied in color. They lay small, spherical, initially translucent eggs and many species spin silk webbing to help protect the colony from predators; they get the “spider” part of their common name from this webbing. Because they have 8 legs, they are not considered an actual insect which typically have 6 or less.
Because the mite is so small, they can be difficult to spot until the plant has been completely infested. A female spider mite (fertilized once for life) can lay up to 20 eggs a DAY, each of these can start producing in as little as 5 days. Dry and hot are prime conditions for a population of spider mites to invade your plants. Optimal conditions are 80° and above. One female can lay up to 20 eggs per day and can live for 2 to 4 weeks, laying hundreds of eggs. Because of their quick reproduction spider mite populations adapt quickly to resist pesticide control, so chemicals can lose their effectiveness when used over a prolonged period.
So, while you could use an insecticide to kill the mites, their reproduction rapidity and quick reworking of their resistance creates resilience to insecticides quickly. They can catch you off guard and stomping out an infestation could get harder than you think. With each spider mite duplicating itself 10 times a day it doesn’t take long until you’ve got literally thousands on your plants. The spider mites will then migrate from plant to plant riding on currents of air and infest the entire indoor garden.
Areas of a Plant Effected by Spider Mites
The tiny size of spider mites makes them very difficult to see on your plants. It is then, critical that when growing in warm temps indoors that you keep a look out for signs of infection. Growing much in my greenhouse, I was never sure why spider mites did so very much damage to my plants before I could eliminate them until one day a friend came to visit. During her stay she discovered an infestation I had not noticed yet. Why? Because I thought that spider mites always created webs. (and from my experience in the most new and tender growth of a plant) Evidently, the webs are simply a defense mechanism and not all spider mites are capable of it or employ it.
Spider mites both live and feed on the plants you find them on. They have tiny piercing mouth parts with which they bite into the plant cells of leaves to suck out the chlorophyll. As spider mites feed on the chlorophyll plant cells become damaged. The plant leaves will display a rough yellowing as they lose chlorophyll (because chlorophyll is what makes the plant green) and cell walls are being punctured. All of this causes incredibly debilitating damage to your plants and will eventually kill them.
In the early stages of spider mite infestation the only signs of this pest are plant stress, slowed growth, and damaged leaves. But the very first sign most gardeners see, called stippling, appears on the tops of leaves as tiny white to yellow specks. By the time most gardeners see stippling on leaves, an infestation is well underway.
I was only paying attention for the appearance of webs. So learn from my experience, looking for signs that your plant may be infected is crucial.
Things to Observe if you Suspect You Have Spider Mites:
- examine the tops of leaves for tiny white to yellow specks
- browning of leaf edges
- plants have a dull color, it’s vibrancy has faded
- the underside of the plants leaves for any black, red, or brown spots
- very fine webbing (especially in new growth) this is almost a certain sign of spider mite infestation
- notice the connection of the leaf to the stem … when their numbers are still small, spider mites will hang out there and under the leaves
Again, check for webbing and spots on the underside of the plant. Yet, if you do not find evidence by observation alone, you can take a white sheet of paper and gently tap the leaves of your dull plant. If your plant has spider mites, then you will see them on the white paper. They will generally appear as pepper and (depending upon the level of the infestation), appear to move.
Organic Methods to Control Spider Mites
Even with the most preventive measures, it is still quite possible for a plant to get spider mites. Once you have identified the problem, then you need to get rid of them quickly because they multiply quickly. There are a number of ways you can do this. The following spray recipes are contact sprays meaning the spray has to make contact with the mite to be effective. You will need a good sprayer. An adjustable nozzle is helpful for getting under the leaves of the plants.
Spraying your plant with the following spray recipes will require repeated application as the eggs of the spider mite hatch in 3 to 4 days. You will want to stay ahead of them.
Before you spray:
Remove all heavily infested or dead leaves from the plant and discard them in a plastic bag. Do not forget to look for any leaves which have fallen off of the plant. Even though they are not attached, they will still be a hiding place and breeding ground.
Citric Acid for Organic Spider Mite Control
Citric Acid is a colorless, crystalline carboxylic acid found in almost all citrus fruits and plants. This acid was first extracted in 1784 by a Swedish chemist, but not until recently has it been used for the control of many insects. Citric acid is a contact killer and needs to be applied directly to the insect to be effective. Since it is an acid it can be harmful to plant material, so use it in a weak mix. Citric acid is often mixed with Isopropyl alcohol to make a spray that dehydrates and kills on contact. Orange Guard makes a water based spray or you can most certainly make your own. Simply mix around 3 teaspoons of powdered citric acid per quart of water and spray the plants, particularly the undersides of the leaves where they tend to congregate.
To use lemon juice instead of citric acid proper, mix 4 tsp. lemon juice, one cup of water and one cup Isopropyl alcohol in a trigger spray bottle. Spray the mixture on plant leaves being attacked by aphids.
No citric acid to be found? A simple spray of Isopropyl alcohol can work! Use 1 part alcohol to 2 parts water and spray away. This is probably one of the most affordable organic methods to control spider mites and kills them on contact.
Note: Citric acid is a severe eye irritant and moderate skin irritant so be sure to exercise caution in handling it. 
Organic Spider Mite Control Using Rosemary Oil
Rosemary is a natural insecticide and using a mix of rosemary infused oil and water will take care of your spider mite infestation. A study conducted by the University of British Columbia found that the use of pure rosemary oil and rosemary oil based pesticides had “caused complete mortality of spider mites at concentrations that are not phytotoxic to the host plant.”  They found that rosemary oil, not only repelled spider mites but could effect oviposition (egg laying) behavior. With a 2:1 ratio of water (2 parts) to rosemary infused oil (1 part), spray the oil mixture on the undersides of the leaves of the plants to serve as a natural spider mite killer.
Required items to make your own rosemary infused oil (NOT essential oil)
- 3 ounces rosemary leaves (fresh or dried)
- olive oil, grape seed oil, or sunflower oil (enough to cover the rosemary)
- crock pot or double boiler
- small hand held fine steel screen colander strainer
- nozzle sprayer
- Bronner’s Castile Liquid Soap (Unscented, Eucalyptus, or Peppermint are all fine)
To Make the Oil:
- Place rosemary and oil in crock pot or double broiler
- Heat on low for 5-6 hours (if you are in a hurry shorten the length of time to 1-2 hours on high
Note: A lower temp and longer interval will yield a more potent oil. If using the high setting be careful to stir frequently and don’t let it scorch! I really recommend the low setting.)
- Let cool a bit then strain the leaves out of the oil using a hand held steel fine screen colander
If you’d like more detailed instructions see how to make an infused oil. (at this link, I use dandelion to infuse, but the recipe is good for most any herb)
Turn off your fan if you use one. If spraying outside, be sure it is on a day with little wind.
Add a tablespoon of the liquid castile soap (Again, Unscented, Eucalyptus, or Peppermint are all fine) to emulsify the mix and help it stick to your plants better. Be sure to shake the sprayer after closing and shake frequently during the application as well. DO NOT APPLY THIS MIXTURE UNDER LIGHT, the oil will magnify your lights and quickly burn your leaves. Apply under light and turn them off. Using a good nozzle sprayer adjust the spray to go up to get under the leaves for best coverage and effect.
NOTE: If you are dealing with and outdoor infestation, spray after the sun goes down or early in the morning. Don’t spray in full sunlight.
If you have an infestation, spray once a day for 4 days, then every other day for a week. Then twice a week for 2 weeks, and finally once a week for maintaining once all signs of live mites and their damage have stopped. Spider mites hibernate so you will see them again most likely if you don’t do a weekly maintaining spray. Store your rosemary oil in a cool area.
It is fairly simple to keep spider mites from infesting your plants. If you clean your tools regularly, spray your plant with rosemary oil to keep the spiders from breeding, then the odds are ever in your favor of having a great garden. Your food and your survival is ultimately your responsibility but because you have a system which requires little maintenance does not mean that there is no maintenance. A careful eye is the first line of defense in organic methods to control Spider Mites!
- How to Control Spider Mites on Marijuana Plants
- Insecticidal Soaps for Garden Pest Control
- US EPA – Pesticides – Fact Sheet for Citric acid and salts
- Miresmailli S, Isman MB. Efficacy and persistence of rosemary oil as an acaricide against twospotted spider mite (Acari: Tetranychidae) on greenhouse tomato. J Econ Entomol. 2006 Dec;99(6):2015-23. doi: 10.1603/0022-0493-99.6.2015. PMID: 17195668.