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Free Your Garden From Spider Mites and Fungus Gnats

By Susan Eitel
Published: December 1, 2016 | Last updated: April 23, 2021
Key Takeaways

When it comes to dealing with common garden pests, you cannot let them get the upper hand! Here are a few ways to deal with spider mites and fungus gnats.

Spider mites and fungus gnats are common pests that multiply fast and can quickly get out of hand. Once you see signs of these pests, flying adult fungus gnats and webbing from the spider mites, it is a clear indication they are well-established and you have an infestation. Therefore, monitoring and prevention is the key to keeping them at bay.

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Spider mites

Spider mites are incredibly tiny arachnids with eight legs and an oval-shaped body. Their mouths pierce and suck, causing damage by puncturing the plant cells. Fluid loss eventually fades and then kills the plant. Spider mites look much like a grain of sand to the naked eye and hang out mostly on the underside of leaves.

There are more than 1,000 species of spider mites but the one you’ll see most often is the two-spotted spider mite, which has a dark spot on either side of its body. Under ideal conditions, the two-spotted spider mite hatches in three days and can become sexually mature within five days. An adult female can lay as many as 20 eggs per day. Females live for two to four weeks. That amounts to hundreds of eggs laid and can add up to a million mites in a month. Eggs are round and clear and later become a creamy color just before hatching.

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Fungus gnats

Adult fungus gnats are small flies from the Sciaridae family. They are black with thread-like antennae, long slender legs and clear wings. The larvae feed on plant roots, causing weak, slow growing plants. Larvae can be clear or cream colored with shiny black heads. They look like small black specks to the naked eye. They grow to be about a quarter-inch long. The flying adults do not cause damage to the plant, but they lay eggs in grow media. The larvae feed on the roots, making the plant weak and slow growing, potentially leading to plant death.

Controlling Spider Mites and Fungus Gnats

Spider mites breed profusely, which allows them to adapt quickly and resist pesticides, rendering chemical controls over a long period of time futile. Monitoring becomes a must. Monitoring spider mites is as simple as keeping a 10x hand lens or microscope in the growroom and checking the underside of leaves. Sometimes this is the only way to detect them before you see signs of webbing. Yellow sticky cards placed horizontally on top of the media and hung horizontally above plants will not only alert you to the presence of adult fungus gnats, but also help control them.

Pest prevention includes keeping floors, growing surfaces, substrates, tools and clothing clean and disinfected. A separate set of tools for the indoor growroom is essential. Disinfecting tools with isopropyl alcohol and handwashing before moving from one plant to another will stop the pests from being transported via you and your tools. These pests can also be transported indoors on your clothes and on your pets. Changing into clean clothes and footwear before entering the growroom and keeping pets away will make a big difference.

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Atmospheric control can also help, so keep air well-circulated and the humidity around 50% with day-time temperatures around 75°F and night-time temperatures around 60 to 65°F. You can also spray the plants with water, ensuring you get the undersides of leaves, and vacuum the plant with a Shop-Vac (this can be done during both vegetative and flowering stages). If these precautions are met and you end up with pests, there are options available for control.

Beneficial insects are a great option for any garden. Ladybugs are a general predator and will eat almost anything. They are relatively inexpensive and you can keep them in the growroom by spraying a special food on the leaves that gets them breeding. I consider mine pets.

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Phytoseiulus persimilis are bright reddish orange mites that feed on two-spotted spider mites. These predators need a relative humidity of 60% that can be achieved by misting plants and floors. Persimilis will reproduce faster than spider mites at warmer temperatures, so keep the room around 75°F.

Hypoaspis miles are tiny beige mites that live on the top layer of grow media. They feed on fungus gnats as well as root mealy bugs, sciarid flies, springtails and thrip larva as an added extra bonus. I suggest you visit your closest indoor garden specialist for more information and release rates.

There are many chemical and organic sprays available on the market as well. There are also choices for insecticidal sprays, which are best used for non-edibles. Insecticidal soaps, neem oil, habanera pepper spray and pyrethrum are some examples of organic controls that can be homemade, or purchased.

For safety’s sake (yours and the plant’s), always be mindful of proper application rates and methods. Keep in mind that these sprays kill both pests and beneficials and cannot be used together.

If you stay vigilant and use this advice, spider mites and fungus gnats won’t be able to get the better of you.

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Written by Susan Eitel

Profile Picture of Susan Eitel
Susan Eitel has a degree in landscape horticulture from Humber College in Toronto, Ontario. She has worked in the hydroponics industry for more than 25 years. Susan has always been interested in beneficial insects and integrated pest management. She lives in the Niagara area with her beloved husband and dog.

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