Those houseplants you kept outside all summer may be harboring unwanted guests.

Even though your plants have been inside for the winter for a couple of months, it's still important to check them for insects and other pests. Even one or two insects can be the start of an infestation.

Pests to look for are aphids, mealybugs, spider mites, whiteflies and scale.

Aphids are small, black or green, crawling insects that live in thick colonies along the soft parts of stems, sometimes covering them entirely. They excrete a sweet liquid called honeydew, which often attracts ants. Aphids are quite destructive. They cause distorted and curled leaves, malformed flowers and hardened buds.

Mealybugs are about the same size as aphids, but are white and fuzzy. Masses of them, resembling pieces of cotton, nestle and lay eggs where leaves and branches attach to stalks. Mealybugs can stunt and kill plants by depleting their fluids.

Whiteflies also are small and white, but unlike mealybugs, will take flight if you shake the plant. You'll never find just one whitefly on a plant. They come in quantities and can spread to all the plants in your house.

Spider mites are miniscule, but very destructive. An infested plant will have yellow-speckled leaves and may be covered with tent-like webs. The mites look like grains of cayenne pepper. They're easy to spot if you hold a piece of white paper under the plant and shake the leaves so the mites drop onto the paper.

Scale appears as blister-like areas on stems and the undersides of leaves. These areas can be circular, oyster shell-shaped or rectangular and are colored white, brown, gray or black. Scale also produces honeydew, which may drip from the leaves. Scale seriously stunts growth. If not treated, it can kill the plant and spread to other plants.

With a small infestation, you may be able to wipe the pests off with a cotton swab dipped in alcohol. Rinsing the plant under a gentle stream of water for a few minutes can wash away insects and eggs. Keep checking the plant for a recurring infestation during the next couple of weeks.

It's also important to gently dig around the roots and look for pests in the soil, such as sow bugs, millipedes and slugs. You can remove most of these by hand, or you may want to rinse the soil off with a garden hose and repot the plant in new soil.

Serious, persistent infestations can be treated with an insecticidal soap spray or a multipurpose insecticide containing a miticide. Each is available from gardening centers. Be careful to follow the directions on the label exactly. Excess spray can be toxic to the plant. If you spray too closely to the leaves, the aerosol propellant can freeze them.

After spraying, keep the plant in a large plastic bag for a few hours to contain and concentrate the spray, then isolate it for about two weeks or until you're sure it's pest-free. It also is a good idea to follow all these precautions if you've bought a plant or received one as a gift. Even well-cared-for nursery plants can harbor pests.

Is setting your plants outside worth all the hassle? The advantages are great. Sunlight, rain and warm weather produce a flush of healthy growth, and the pests usually are easy to get rid of. Overall, it's worth it.

"Growing Green" is contributed by Lehigh County Extension Office Staff and Master Gardeners. Lehigh County Extension Office, 610-391-9840; Northampton County Extension Office, 610-746-1970.