Regional Gardening Reports :: National Gardening Association

In the Garden:
Pacific Northwest
December, 2008
Regional Report

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This bougainvillea blooms for 2 to 3 weeks on every 6 weeks, bringing springtime cheer to my indoor garden.

Indoor Plant Pests

For those of us who garden indoors as well as out, bugs aren't a pesky problem confined just to the garden. They can be as much of a nuisance on the windowsill as in the perennial bed. Luckily, there are really only five common indoor pests so identification and treatment are relatively easy.

I've found that control of these five pests can be accomplished with three simple steps. I've had great success with them, and with a little diligence, you can, too.

First, keep your plants happy. If you provide optimal growing conditions for your plants, you'll minimize infestations. Bugs on houseplants serve a purpose, undesirable though they may be from our point of view. They clean up the environment by breaking down vulnerable or unhealthy plant tissues into their original organic elements. Bugs are often indicators that houseplants are stressed and there may be something wrong with their growing conditions or your cultivation techniques.

Inspecting your plants on a regular basis can help you identify a pest problem early on and gives you an opportunity to treat it while the population is still small. A shower every three to four weeks keeps dust off the leaves and will usually rinse off any pests that have moved in and made a home in the potting soil or on the foliage.

If you discover a pest population during one of your routine inspections and you can't rinse them off in the shower or sink, there are some organic controls at your disposal. The one you use will depend upon the specific pest and on the plant. Since some plants are more sensitive than others, it's always best to test on a leaf or two and wait a few days for a reaction. If everything looks fine, you can safely treat the entire plant. Here are my solutions for the most commonly encountered houseplant pests:

These small, soft-bodied insects may be pale green, pink, black, or yellow, depending on the species. They like to cluster on tips of new growth and leaf undersides, sucking out plant juices and causing leaves to become distorted and yellow. Aphids secrete a sugary fluid called honeydew that may spur the growth of a sooty, black fungus on leaves and cause stickiness on leaves, floors, and rugs. Aphids like fast-growing plants, such as hibiscus.

To control, place the plant in the shower and knock aphids off the leaves with water, or wash the leaves in a sink full of soapy water. Cut back on high-nitrogen fertilizers, which stimulate new growth. For widespread infestations, spray the foliage with insecticidal soap, neem oil, or horticultural oil.

Fungus Gnats
These small, dark-colored flies jump and fly across the soil surface and around the house. Although they generally don't harm plants, they're a nuisance. The adults lay eggs in potting soil. If numerous enough, the larvae can damage plant roots.

Fungus gnats feed on organic matter and fungi in the soil. They especially like moist, rich soil. To control, let the top layer of the soil dry out between waterings to discourage their egg laying. Avoid using fish emulsion fertilizers since they foster the fungus that gnats like to eat. Or, you can drench the soil with Bacillus thuringiensis 'israelensis' (B.t.i.) or neem oil to control the larvae.

These soft-bodied insects group in white, cottony masses and suck sap from plants. Like aphids, they excrete large amounts of sticky honeydew. While adults tend not to move once settled on a leaf, flower, or stem, the young (crawlers) can move around the plant. Mealybugs favor cacti and jade plants.

For small infestations, you can wash the leaves in a shower to dislodge the insects, or dab individual mealybugs with a cotton swab doused in rubbing alcohol. The alcohol will desiccate the insects, killing them. For larger infestations, spray with insecticidal soap, neem oil, or horticultural oil.

These insects may be flattened and brown, or thick, white and covered with a waxy or woolly substance. They appear like small bumps on leaves and stems. Leaves of infested plants turn yellow, and the overall vigor of the plant declines. Like mealybugs, the young are mobile, while the adults tend to settle in one area to feed.

Rubbing the scales off the stems by hand or dabbing them with rubbing alcohol often controls small infestations. For larger populations, spray horticultural oil or neem oil to cover the shells and suffocate the scale. Scale insects may hang on the stems and leaves even after they are killed.

Spider Mites
These small insects aren't usually evident until their population soars and they form their characteristic webbing. Their feeding can cause stippling of the leaves and leaf drop. They thrive in hot, dry conditions and are particularly fond of cyclamen, Norfolk Island pine, and schefflera.

High humidity discourages mites from establishing, so mist your plants on a regular basis and wash off infested plants in a shower or sink full of soapy water. Spray insecticidal soap or horticultural oil on severely infested plants. If you have many plants indoors, consider releasing predatory mites, such as the two-spotted mite (Phytoseiulus persimilis). These mites feed on spider mites and reduce the population without harming the plants, pets, or people. It will take a few weeks for them to control an infestation.

These tiny, white insects feed in large numbers on leaf undersides, sucking out plant juices. They secrete honeydew that may cause the growth of a sooty, black fungus on leaves. Their feeding can cause leaves to turn yellow and drop. They are easily disturbed and fly around when you brush against an infested plant. They are often found on hibiscus and ivy.

Whiteflies are attracted to the color yellow, so you can trap them by hanging yellow cards coated with a sticky substance, such as Vaseline, around your plants. Or suck them up with a vacuum cleaner as you shake your plants. In serious cases, spray plants with insecticidal soap, neem oil, or horticultural oil. In houses with many plants, consider releasing the whitefly parasite Encarcia formosa. This miniature wasp kills whitefly larvae but doesn't harm plants, pets or people.

With proper care and feeding, and careful observation coupled with quick action, your indoor plants can remain healthy and pest free, giving you as much pleasure as your outdoor garden does.

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