It’s that time of year when cool nights get us thinking that it’s time to bring in the houseplants that have enjoyed a summer outside. If you don’t carefully check the plants before moving them inside, you could be bringing in pests as well.

Houseplant Hitchhikers Fall into Three Groups

(1) Plant-sucking pests – These tiny insects and mites live on the plant and suck juices from leaves and stems, often leaving a sticky residue behind. This group includes aphids, spider mites, whiteflies, scales, and mealybugs. Scales and mealybugs are not really mobile and remain in place on the plant, hidden under leaves or in stem crotches. Whiteflies, aphids, and spider mites are usually found beneath leaves, too, but they can move about on the plant and may even leave the plant if indoor conditions don’t suit.

(2) Soil-dwelling pests – Most of the pests in this group are simply using the moist soil in your plant’s pot as a comfy home. Earthworms, slugs, sowbugs, millipedes, earwigs, and ants might live in or under the pot and may feed on organic material in the soil but don’t really do any damage to the plant. Fungus gnat larvae can also live in the perpetually wet soil of overwatered houseplants (see Fungus Gnats Are Common Where There Are Potted Plants). These fly larvae can feed on and damage the roots of houseplants. The tiny adult flies that develop can become a nuisance pest in your home.

(3) Accidental pests – Some larger leaf-feeding insects such as grasshoppers and caterpillars feed on various outdoor plants and could accidentally be carried inside. Spiders and crickets and miscellaneous other creatures may be hiding on the plants. These accidental pests will soon leave the plant and die once indoors, they don’t reproduce and develop on the plant as do the true plant pests in item (1) above.

An Ounce of Prevention…

• Before moving houseplants back inside, remove the saucers and give them a good shower with the hose to dislodge any casual visitors. Water the pots thoroughly or soak the pot for 10 minutes to flush out ants and some other soil-dwelling pests. Some infested plants may need to be repotted in fresh, sterile potting soil to eliminate soil pests and their eggs.

• Unfortunately, warmer, drier indoor air seems to encourage some plant-sucking pests such as spider mites (see Spider Mites Can Infest Your Houseplants) making them especially difficult to control. An application of insecticidal soap to the foliage may knock back the infestation but if the plant is heavily infested and you don’t particularly value it, consider discarding it.

• Once inside, give your transported houseplants a bit of separation, especially from any plants that didn’t summer outside. When leaves from different plants are touching, pests can move from infested plants onto uninfested plants. It’s a good idea to quarantine returning pots for a while if you suspect hitchhiking pests. See Pests on Your House Plants for tips on how to tell if your plants are infested.

• Don’t overwater your houseplants. That encourages fungus gnats especially. Letting the soil dry out completely between waterings is the best way to eliminate fungus gnats.