Regional Gardening Reports :: National Gardening Association

In the Garden:
Upper South
February, 2011
Regional Report

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Keep a watchful eye on your houseplants for insect pests in order to control them before they cause major damage.

Houseplants Under Attack

You know the routine for houseplants. Provide moderate 60- to 70-degree temperatures, keep humidity levels up by grouping plants on pebble-filled trays or with a humidifier, provide plenty of light, avoid cold drafts, regularly water and fertilize, and check plants in the fall for pests, then treat accordingly. Even with all that, somehow pests can still manage to find houseplants. We'll leave the mystery of how they appear to greater minds, but, instead, look at ways to deal with them once they make their appearance.

Here are the most common houseplant pests, and some options for safe ways to control them.

Considering that there are about 4,000 different species of these soft-bodied insects, I guess it should come as no surprise that a few are going to show up on houseplants. These are sap-sucking insects that tend to cluster on new growth and under leaves. They are usually about one-eighth-inch long and maybe green, yellow, brown, red, or black. A tell-tale sign of aphids is the sticky fluid they secrete, known as "honeydew."

The simplest control is to prune off the most heavily infested houseplant parts, then wash the plant. For small plants, cover the soil with plastic and hold the plant under a faucet. For larger plants, again cover the soil with plastic, and put them in the shower for their bath. Organic controls for aphids include insecticidal soap and neem spray.

Fungus Gnats
Gray-black, eight-inch-long flies that resemble mosquitoes, fungus gnats are usually most obvious when watering, as they'll fly up from the soil. The flying stage doesn't cause much damage, but the quarter-inch long maggot stage, with its black head and white body, feeds on the root hairs of houseplants.

Because they like moist soil, one remedy is to let the soil surface dry out between waterings. Also avoid using fish emulsion fertilizers. Or catch the egg-laying adults with yellow sticky traps placed horizontally at the soil surface. Another option is to drench the soil with a neem solution or with Bacillus thuringiensis var israelensis (Bti), a highly selective, biological control.

Those little white cottony masses adorning so many of my houseplants this year are an especially common problem. These soft-bodied, wingless insects to one-quarter-inch long suck the sap out of plants, especially on new growth. Other signs of mealybugs are leaves that yellow, curl, and eventually drop. Mealybugs also secrete honeydew, like aphids.

For light infestations, dab mealy buds with a Q-tip dipped in rubbing alcohol. Washing plants to remove these pests is another simple option. Repeated insecticidal soap and neem sprays are necessary when an infestation gets out of hand. For plants that can withstand cold temperatures, such as gardenias and citrus, two days of temperatures at 36 degrees will also reduce a mealybug infestation.

For an insect that is immobile under either a hard or soft shell as an adult, the teeny babies (the nymph stage) can definitely travel swift and sure. How they know that a new bay plant has been brought into my house is amazing; even more so is how fast they get to it. I've also had them on orchids, cycads, citrus, and other houseplants. These shelled, eighth-inch bumps are most often found along the stems and leaf veins, where they suck the sap of plants.

Because scales are so prolific and very hard to eradicate, it is very, very important to monitor plants as often as you can. At the absolute first sign of them, rub or pick them off by hand or swab with a Q-tip dipped into rubbing alcohol. If the infestation gets worse, cut off and dispose of infested branches, twigs, and leaves. Beneficial insects, such as ladybugs and lacewings, are natural predators, but not many of us want these indoors. Light horticultural oils or neem oil may also be used to control infestations.

Spider Mites
Not a true insect but actually a type of arachnid, spider mites are teeny-teeny-tiny, or about one-fifieth an inch long. Which means you probably won't notice them until they have multiplied into a very large population. At that point, you will notice light dots on leaves where they have been sucking, along with yellowing, dropping leaves, or fine webbing, especially on the undersides of leaves.

To control spider mites, first remove heavily infested parts, then give plants a good washing. If possible, increase the humidity around your houseplants. There is also a predatory mite, Phytoseiulus persimillis, that can be used for control if you don't mind additional "bugs"in your house. Insecticidal soap or light horticultural oils are other possible controls.

If a cloud of white, flying insects burst forth when you jostle your plants, then whiteflies are your culprit. About one-sixteenth-inch long, these moth-like insects attach themselves to the undersides of leaves and suck the juices from new growth, causing stunting and yellowing of leaves.

Since whiteflies are attracted to the color yellow, yellow sticky traps are the first line of defense. These can be purchased, or you can make your own by spreading yellow note cards with Vaseline and setting them among plants. The whitefly parasite, Encarcia formosa, is a miniature wasp that feeds on whiteflies, but doesn't harm plants, pets, or people. Insecticidal soap or neem sprays will also control whiteflies.

Bottom Line
Though houseplant pests may show up even with good care, it still pays to grow the healthiest plants possible, providing the best conditions for them. It also pays off to check plants at least once a week for pests and initiate controls immediately. If you decide to use an organic control, look for one that combines insecticidal soap and neem. Remember that there are multiple generations of insects in a short period of time, so it makes sense to re-apply controls once a week for at least several weeks until the pests are under control. With this effort, you'll be surrounded by beautiful, healthy houseplants.

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