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Mary Lou Marchand, Master Gardener: Controlling indoor plant pests

I’ve brought in the houseplants that were out for the summer, taken cuttings of some plants I’d like to keep over the winter and brought in a couple of larger pots of plants in hopes that they will continue to bloom for a while and maybe survive the winter indoors.

No doubt I’ve brought in some insect pests in spite of showering the plants before bringing them in. If problems do arise, non-chemical, nontoxic alternatives are my first choice for indoor plant care.

In our climate, the common culprits attacking indoor plants include aphids, spider mites, mealybugs, fungus gnats and scale. A first defense against indoor plant pests is a plant shower. Many pests such as aphids and spider mites can be removed or kept at bay with regular showers. In addition, the water removes dust that blocks out the weaker winter sun.

Fungus gnats are small, non-biting insects that are often confused with fruit flies. Fungus gnats live near the top of houseplant soil. The larva feed on small roots causing damage to the plant. Control of fungus gnats and larva can be achieved in two ways. Letting the top inch of soil dry out completely will, over time, starve the gnats. Careful bottom watering that does not allow moisture in the top inch of soil works well, too.

Spider mite control can be frustrating. Remove infested leaves and discard severely affected plants. Again, showering the plants is the first step in controlling spider mites. If plants are too large to take to the sink or shower, wipe them with a damp cloth. Treat all susceptible houseplants at the same time. Insecticidal soap and horticultural oils are the primary chemical controls. This may have to be done every couple of weeks all winter.

Mealybugs, white waxy insects, are a common houseplant pest that infest all parts of the plant including the roots. They live in small colonies all over the plant. Hand removal is the best control but dabbing individual bugs with a cotton swab dipped in alcohol works as well. Getting rid of mealybugs can be a time-consuming chore.

A variety of scale insects affect house plants and they are difficult to control. Most scales are tan or brown and range from 1/16 to 1/8 inch in diameter. They are found on the stem and/or leaves and suck sap from the plant. There is no easy cure for scale. One method is to pick off individual scales or gently rub the scales from the leaves or stems. This is done on small plants with a small infestation as is dabbing each scale with a cotton swab dipped in alcohol. Insecticidal soaps are another safer alternative to some of the more toxic chemicals. Unless the plant is especially valuable, you may find it best to throw away an infested plant rather than let the scale spread to other houseplants.

Of course, not all houseplant problems are caused by insects. Overwatering can cause root rot while underwatering causes wilting. Spots on the foliage may be sunburn, water spots, fungi or bacteria. Good air circulation helps prevent fungal or bacterial leaf diseases. Knowing your plant’s water, light and heat requirements will help keep it healthy. If you do find pests, try using the least toxic method to combat the problem.

Beltrami Master Gardener Writers’ Group will be taking a hiatus for the winter and will return to sharing horticultural information in the spring. In the meantime, seek information from the University of Minnesota Extension website at where there are many links to reliable information for those who live in our northern climate.