Dreaded Houseplant Pests

by Connie Holland
Adams Co. Master Gardener
Gettysburg Garden Club

Little did I know that for this article I would be writing from recent experience. My tender succulent plant collection lives under artificial grow lights during the winter and goes outside during nice months. Recently, I noticed aphids on one. I was shocked because these plants have never had any sign of aphids until now. Aphids, often called plant lice, are tiny soft-bodied insects that damage plants by sucking out the sap causing curling, crinkly or mottled leaves and ruining flower buds. They especially like stems, new growth, and flower buds. Aphids also carry viruses from plant to plant and secrete sticky “honeydew” that acts as a breeding ground for ugly black “sooty mold”. On my plants they seemed to appear from nowhere. How did that happen?

Actually, there is an explanation based on the aphid’s life cycle. Aphids overwinter as fertilized eggs in the soil. In spring, or in my case, after weeks in my warm basement, eggs hatched into wingless females. Successive generations are produced until a colony becomes overcrowded when generations of winged females, capable of flying, are produced. Winged females reproduce until the summer’s end when both winged males and females are produced. Those females mate and lay eggs that overwinter in the soil starting the life cycle again. This is what happened to my succulent. Outdoors all summer near aphid-infested milkweed plants (milkweed is considered an aphid magnet), winged females laid eggs in the succulent’s soil.

Next came a challenge to get rid of aphids. It is not advisable to spray insecticides indoors. A more environmentally and gardener friendly way to get rid of aphids is to blast them off with water. I covered the soil at the plant’s base with plastic wrap to avoid overwatering and to prevent soil washout. A vigorous water spray washed the aphids off. Checking often, I keep an eye out for any more egg hatch, especially on adjacent plants since aphids reproduce so quickly, there may be none one day, and a week later there are hundreds.

For outdoor aphid control, beneficial insects play an important role as an alternative to the appropriate insecticide. Lady beetles (both adult and larvae), lacewings, some flower flies (larvae), and tiny parasitic wasps all consume aphids. Remember certain insecticides destroy beneficial insects as well as pests.

Another very annoying household pest is the fungus gnat, a dark tiny fly-like pest that flies around plants and is annoying. Fungus gnats develop in moist potting soil, feeding on root hairs, and emerge as adults every 30 days. The best way to get rid of fungus gnats is to allow soil to thoroughly dry. This can eliminate the eggs. Sometimes covering a plant’s surface soil with sand discourages fungus gnats. Commercially available yellow sticky cards work extremely well to trap gnats flying around. It is amazing how many insects get stuck on them! Those are my preferred method of dealing with fungus gnats. Be sure to quarantine any new houseplant to ensure you are not bringing in any gnats. As a last resort, a soil drench with an insecticide that targets soil gnats can be very effective. Commercially available granules impregnated with such a targeted insecticide work well when watered in on top of the soil.

Mealy bugs are another of the most annoying and destructive houseplant pests. They suck juices out of new growth causing ugly spotting, yellowing and ultimately leaf drop. They look like small bits of white cotton. Young mealy bugs can crawl so this is the best time to treat an infestation. A female can produce over 500 eggs within a 1-2 week period. Mealy bugs also secrete sweet “honeydew” leading to black “sooty mold” showing up on foliage and stems. Eliminate infestations using a Q-tip or cotton ball dipped in rubbing alcohol. Be sure to check frequently. Commercially available yellow sticky cards may also work to trap mealy bugs.

When it comes to the most dreaded insect scale, I speak from experience. Months ago I purchased a sale orchid discounted because it had no blooms. Little did I know it was harboring scale, small bumpy brown oval-like insects resembling tiny helmets. Scale might as well be wearing helmets because their tough exterior shell is somewhat impervious to insecticides. The orchid was finally freed of scale after weeks of scraping them off with alcohol soaked Q-tips. So far it is scale free. However, I still examine it closely. Other plants of mine infected have not faired so well. A lovely fern, entered in a show, developed scale probably caught from an adjacent plant at the show. It had to be discarded because scale on a fern or plant with a lot of foliage is almost impossible to rid of scale without using systemic insecticides that can harm beneficial insects.

Another common houseplant pest is the whitefly, a tiny white fly very hard to control. When an infested plant is disturbed, they can be seen flying all around. Damage appears as pale yellow leaves that eventually drop off. Untreated, whiteflies can kill a plant and are best avoided by not taking possession of an infected plant. Check any plant before purchase by gently moving the foliage and watching for the white flies. This is especially important because they are so tough to control. If a plant is infested, or to monitor routinely for whiteflies, keep yellow sticky cards present at all times.

The best advice I can provide to prevent houseplant pest problems is to quarantine any newly acquired plant for at least 2-3 weeks away from other plants in order to give any pests presentenough time to show themselves. This applies to any new plant, inside or out.