Banishing Insects that Prey on Container Plants

By Bill Marken, Suzanne DeJohn, and The Editors of the National Gardening Association from Container Gardening For Dummies, 2nd Edition

Of the thousands of insect species that call your plants home just a handful pose problems for gardeners. By far, most insects you see on your plants are benign, or even beneficial, so carefully identify an insect before deeming it a pest to be controlled. Here are the most common insect pests that you’re likely to find infesting your container plants and the best ways to control them:

Aphids: Aphids are tiny, pear-shaped pests that come in many colors, including black, green, and red. They congregate on new growth and flower buds, sucking plant sap with their needlelike noses. Aphids leave behind a sticky “honeydew” substance that may in turn be colonized by black, sooty mold. The honeydew often attracts ants, too, and while the ants themselves aren’t plant pests, they can signal you to look for aphids and other honeydew-producing pests.

Aphids are easy to control. You can knock them off sturdy plants with strong jets of water from a hose, or use insecticidal soap, spinosad, or pyrethrins..
Borers: Borers tunnel into the wood of fruit trees, white birches, dogwoods, shade trees, and rhododendrons, among others. Look for borer-resistant plants. Keep susceptible plants growing vigorously and watch for signs of borer damage — dead bark, sawdust piles, partial wilting, and poor performance. When you find borers, cut off and destroy severely infested stems and limbs. Inject Bt or parasitic nematodes into remaining borer holes, depending upon the plant and type of borer.
Geranium budworms: Geranium budworms are very frustrating pests of geraniums, nicotiana, ageratum, and petunias. The small caterpillars bore into flower buds and eat the flowers before they open or feed on open blooms. To confirm the presence of these heartless monsters, look for small holes in blossoms or the tiny black droppings the caterpillars leave behind. To control, pick off infested flower buds and spray with Bt, spinosad, or pyrethrins.
Mealybugs: These small, sucking insects, most common on houseplants, cover their bodies with a white cottony substance that makes them easy to identify. They usually feed in groups, forming a cottony mass on branches and stems. Wash off small numbers with a cotton ball dipped in rubbing alcohol; for larger infestations, spray with insecticidal soap or neem.
Scale: These tiny insects look like small bumps on plant stems and leaves. They hide under a turtlelike shell that serves as a shield. These pests suck plant sap and can kill plants if present in large numbers. The first sign of scale is often the sticky honeydew they secrete. Remove and destroy badly infested stems. Clean off light infestations with a cotton ball soaked in rubbing alcohol. Spray with dormant oil in winter or summer oil during the growing season.
Spider mites: Spider mites are tiny, eight-legged spider relatives that you can barely see without a magnifying glass. If the population gets big enough, you can see telltale fine webbing beneath the leaves. And as the mites suck plant juices, the leaves become yellowish with a silvery or bronze stippling. The plant may even start dropping leaves. Mites are most common in hot, dry summer climates and on dusty plants. Houseplants, tomatoes, and roses are commonly infested.

A daily wash with a strong jet of water from a hose can help keep infestations down. You can also control spider mites with insecticidal soap and summer oil sprays.
Thrips: Thrips are another almost-invisible troublemaker. They feed on flower petals, causing them to be discolored and the buds to be deformed as they open. They also feed on leaves, causing them to become deformed and giving them a stippled look. Impatiens, roses, and gladioluses are commonly infested.

Many beneficial insects feed on thrips, especially lacewings. Insecticidal soaps and hot pepper wax are also effective against thrips.
Whiteflies: Whiteflies look like small, white gnats, but they suck plant juices and can proliferate in warm climates and greenhouses. They tend to congregate on the undersides of leaves. You can trap whiteflies with yellow sticky traps sold in nurseries. Insecticidal soaps, summer oil, hot pepper wax, and pyrethrins are effective sprays.