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If you’ve ever had to deal with an infestation of red spider mites on either house or garden plants, you’ll know that they are a force to be reckoned with. It’s difficult to get rid of them. The first sign may be some pale yellow speckling of the leaves – and then you turn the leaf upside down and (if you have good eyesight) you see the little white eggs. Most people are blissfully unaware of their existence… until the moment when, in response to some secret cue, they launch their coup…. swarming up to the top, where they will hold what appears to be a gigantic mite festival (or is it a political gathering where they plan their takeover of the world, who knows?), webbing over the plant and seriously damaging it.
These little monsters are renowned for their ability to spread like crazy, and it only takes a few to survive for the population explosion to start all over again. Also, they develop resistance to chemical insecticides if such are overused. Nonetheless, numerous sources will still recommend chemical pesticides – which we of course frown upon, not only because of their toxicity but because they aren’t perfectly effective against mites over time!
Here’s the good news: Help is at hand in the form of essential oils. Several essential oils are effective against spider mites – and what’s interesting is that the mites appear not to be able to develop tolerance to them. This may theoretically be because essential oils have numerous molecular components (often over 100 molecules identified) and a “broad spectrum” action.
Neem essential oil is considered by many to be the spider mite “weapon of choice”. It is also effective against several other insect species. You can pick it up at many garden supply stores. Neem oil solidifies at room temperature, so you will need to leave the bottle of neem in a pan of warm water to liquify. Then add 2oz neem to 2oz plain dish soap, and mix thoroughly. Then add this to 1 gallon of water – and spray. You will need to be careful to spray every single square inch of the infected plants, and the area around the plants also, in order to be sure to wipe the mites out. Application once every two weeks is often suggested – and if the plants are fruit / flower crops, it is advised to cease spraying before the flowering season commences, so that the crop is not tainted with neem! If you actually manage to get rid of the mites completely, consider yourself a pro.
Several other essential oils have been tested by scientists for their action against spider mites. A 2010 study found various essential oils effective against red spider mites that had become resistant to the chemical pesticides chlorfenapyr, fenpropathrin, pyridaben and abamectin.
Lemon Eucalyptus was found the most effective essential oil, followed by peppermint, citronella Java, red thyme, caraway seed, clove leaf, and pennyroyal essential oils.
In these tests, the LC50 was measured. The LC50 is a technical term for the concentration required to kill 50% of the population of the mites – and it is measured in microg/cm3. The most toxic essential oil (Lemon Eucalyptus) required a concentration of 19.3 microg/cm3. The least toxic had an LC50 of 23.7 microg/cm3.
Putting this into perspective, there are 1000 cubic centimeters in a liter, and 1000 micrograms in a milligram. Given that 1ml of water is approximately 20 drops
A further 2010 study indicated essential oils of Citrus peels – particularly of Citrus aurantium (bitter orange) and C. senensis (sweet orange) – highly effective against red spider mite.
Further studies have found sandalwood essential oil effective against two-spotted spider mite.
Tea Tree has been found highly effective against scabies mites, it is likely that it will be effective against spider mites also.
And there are many others.
I am looking at the studies to attempt to find out the concentrations of oils required. Measuring the LC50 will not give us a figure that we need. The sandalwood study discusses 0.1% solution – that’s 1ml oil to 1 litre of water – as being effective on mites on roses without harming the plants. However I have not been able to find recommended application amounts for the other oils. Maybe someone can help with this part?
So I cannot make recommendations here, apart from the neem, which is in common use. If you are willing to experiment, you might start on a plant (or even just a part of a plant) that is not of immense concern. Mix 1ml of essential oil with 1ml of plain, unscented dish soap. Then add to a liter of water. Spray the solution on the plant / area of the plant, and note whether it harms the plant / is effective against the bugs. If you are doing significant volume spraying, of course wear the appropriate protective equipment, you can breathe a lot of essential oils if you are spraying them, maybe more than you want to! Again, I would not advise to use essential oils on vegetables, or crop plants that are flowering / fruiting, as you may taint the crop.