A common and potentially significant problem on a variety of houseplants, including both indoor ornamental and vegetable plants, is the two-spotted spider mite. Spider mites get their name from the wispy webbing they produce on infested plants.  Like spiders, they have eight legs so are not insects.

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Spider mite infestation on indoor pea plant. Note ‘stippling’, the tiny leaf spots which are mite feeding sites and the webbing (full of mites!) in the upper part of the photo. UConn photo.

Adult two-spotted spider mites, a common species both indoors and out in the northeast, are quite tiny at only about 1/50th of an inch long.  Because of this and their habit of feeding and reproducing on the undersides of leaves, they are often overlooked until populations are high and plants have sustained significant (easily visible) damage.

Both adults and the smaller, but similar nymphs feed on plant tissues by piercing cells with needle-like mouthparts and sucking out the contents. This results in cell death and a tiny yellow spot on the leaf. Once there are many spots, the leaf begins to look ‘stippled’ or off color and may die and fall from the plant. Many spots are bound to happen if conditions are warm and dry and the mites are not noticed early because they have a high rate of reproduction under these conditions.

If you see what could be early symptoms of a spider mite infestation, ie, leaves with a light amount of spotting or discoloration or a bit of webbing on the plant, check for mites by tapping symptomatic leaves over a sturdy white paper surface (a paper plate works great).  Mites will fall onto the plate and will crawl slowly allowing you to see them.

Where do spider mites on houseplants come from?  When it’s warm outside, they can easily hitchhike in on you, your pets, or even a breeze.  They may come in on any plant material brought in from outside or a newly introduced houseplant.  Because of their small size and the webbing they produce, they are easily picked up and moved around by air currents.

What can you do about them?  If you have an infestation as bad as the one pictured here (on indoor pea plants), the best solution is to dispose of the plants to protect others.  For a lighter or moderate infestation options for indoor plants include a spray of water in the tub or shower (a hand held water bottle can be used if the shower spray is too strong for the plant), insecticidal soaps, neem products or botanical oils.  Some soaps and oils can cause injury to sensitive plants.  If circumstances allow time, do a spot treatment on a few leaves and observe for injury (dead, browning or curling leaves) for a few days before treating the entire plant.  It’s important to get thorough coverage on the lower leaf surfaces, as that’s where many of the mites are.  Soaps and oils must coat the pest to be effective.  Typically, additional treatments must be made at approximately weekly intervals to clean up an infestation.  To avoid spreading mites to uninfested plants, avoid handling them after infested plants without washing your hands, gloves, tools, watering cans, etc. Note: If a plant must be treated in place (ie too large or delicate to move to the shower), protect nearby furniture, drapes and carpets with a covering.

When using pesticides, even organic or relatively safe products, always read and follow label instructions carefully.  Do not use products intended for outdoor use indoors.

By J. Allen