Healthy plants can fend off pests better than struggling plants can. To keep your plants robust, you need to mimic the conditions it favors. Tropical plants require humidity. Placing your plants on pebble trays with water, making sure the base of the pot is above the water level, is an easy way to add humidity. Don’t forget to fertilize your houseplants as needed in winter at partial strength.

Inspect your plants routinely, checking the underside of the leaves where pest hide. Clean off the leaves of large-leaved plants weekly. Identifying a problem early makes the treatment easier.

Common Pests

Aphids

Aphids are usually found feeding on new growth or the undersides of leaves. They suck plant sap, resulting in yellowing and misshapen leaves. In addition, growth may be stunted, and new buds deformed. As aphids feed, they excrete a sugary material, called honeydew, which makes leaves shiny and sticky. Sooty mold fungi may grow on the honeydew, producing unsightly dark splotches on the plant’s surfaces.

Control: With minor infestations, handpicking, spraying with water, or wiping the insects with a cotton swab dipped in rubbing alcohol may be effective. For larger infestations, spray with insecticidal soap or neem oil extract.

Fungus Gnats
Adult fungus gnats can be seen running across or flying near the soil surface under a houseplant. The adults do not feed on houseplants but can be a nuisance. Fungus gnats are especially a problem when overwatering occurs.

Control: For plants that can tolerate it, allow the soil to dry between watering. Dry conditions will kill the larvae. Do not allow water to stand in the saucer beneath houseplant containers. Products that contain strains of the biological control agent Bacillus thuringiensis can be applied to the soil of houseplants and watered into the soil for control.

Mealybugs
Mealybugs are small, pale insects related to scales. They can have a cottony appearance. Mealybugs are most commonly found on the lower surfaces of leaves and where the leaf attaches to the stem. They suck plant sap, causing stunted and distorted growth and sometimes plant death. Like aphids, mealybugs excrete honeydew, providing the opportunity for the growth of sooty mold fungi.

Control: Light infestations can be controlled by removing individual mealybugs by hand or by wiping each insect with a cotton swab dipped in rubbing alcohol. An insecticidal soap spray may also be used or spray with neem oil extract.

Spider Mites
Mites are not insects but are more closely related to spiders. Since they are extremely small, plant damage is typically the first sign of their presence. A silky web is often seen with heavier infestations.

Spider mites damage plants by sucking plant sap. Damage includes light-colored speckling on the upper surface of leaves and results in a plant with an overall faded look. If the mites are left unchecked, leaves become bronzed or yellowed, and the plant dies. Spider mites tend to thrive environments with low humidity.

Control: Spray sturdy plants forcefully with water, including the undersides of leaves, to dislodge mites and break up their webs. Plants also can be sprayed with insecticidal soap or neem oil extract. It is often necessary to spray once a week for several weeks to control mites.

Scale
Scale insects are covered by a waxy covering. The covering can be scraped off to locate the insect living beneath it. Scales vary in appearance depending on age, sex, and species. Some are flat and appear like fish scales stuck to a plant. Others look like waxy, colored masses. They are usually found on stems and the undersides of leaves but may be found on upper surfaces as well. Scales feed by sucking plant sap.

Control: Carefully scrape the scale off the plant. Plants can be sprayed with insecticidal soap or neem oil extract.

More information on identifying and controlling houseplant pests can be found on this University of Maine Cooperative Extension Bulletin.