Monitor houseplants for insect problems all through the winter.
Healthy, well maintained plants are the best defense, since pest problems are more likely to occur when plants are stressed. Insects also can arrive with new houseplants or gift plants, so isolate these plants for several days when you first bring them home to avoid spreading any pests they may carry to other plants.
Here are some of the pests to watch out for:
• Spider mites: Tiny mites often are difficult to see without a magnifying glass, but look for white webbing near where leaves join the stem, stippled foliage and weak, off-color leaves.
Wash the plants off with water in the shower to knock off the mites. Or wash or wipe them off with a solution of half rubbing alcohol and half water (this on a few leaves first and wait two to three days to see if there is any damage from the alcohol before treating the entire plant).
Using a soft clean rag or a cotton ball, gently wipe the leaves and stems. Pay careful attention to the undersides.
You also can try treating with an insecticidal soap that is sold and labeled for treating houseplants.
• Scale: A characteristic sticky, clear honeydew is a sign of the small, immobile, rounded scale insects, usually found on stems and veins of leaves. Scale can be difficult to get rid of, but you can try washing or wiping the insects off with the alcohol solution or treating them with an insecticidal soap.
• Mealybugs: Easy to spot, these insects resemble crowds of tiny cotton puffs. Wash or wipe them off with the alcohol solution or try treating them with an insecticidal soap.
• Whiteflies: Whiteflies are a major problem in many greenhouses because they can quickly move to neighboring plants. Look for tiny, white, mothlike insects that often are found on the undersides of leaves. They will fly up in a cloud when plants are disturbed. Carefully vacuum them up, install sticky tape to trap them or use an insecticidal soap.
• Fungus gnats: The adult black gnats that fly around the plant are harmless, but their larvae in the moist soil feed on plant roots. Fungus gnat larvae thrive in moist soil and often are a problem in overwatered plants or overly moist flats of seedlings.
Allow the potting medium to dry out as much as your plants will tolerate between waterings. To kill larvae, you also can treat the growing medium with Bacillus thuringiensis (Bt/H-14), which is labeled for fungus gnats. Use yellow sticky cards placed horizontally on the edge of the pot to trap adults and monitor their population.
A 1-inch layer of sand on top of the medium surface will discourage fungus gnats from breeding in the pot, but also will make it harder for you to judge when to water.
• Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.