How to detect houseplant and greenhouse pests

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Last Updated: 
February 19, 2003

CORVALLIS - Many houseplant and greenhouse pests are tiny and often go unnoticed to the untrained eye. Soft-bodied mites, thrips, mealybugs, scales, whiteflies and aphids are all commonly seen on indoor plants.

Winter is a good time to check for these tiny pests on your houseplants and in backyard greenhouses and sunrooms, explained Amy Dreves, entomologist and assistant with the Oregon State University Master Gardener Program.

If you know what pests you have, control is simpler and often can be done in a less-toxic manner than just applying a broad spectrum pesticide, hoping to kill whatever may or may not be there.

These tiny critters are termed "soft-bodied," as they don't have as hard an exoskeleton as beetles and grasshoppers do. Soft-bodied pests tend to have piercing, sucking mouthparts that they use to extract plant sap and chlorophyll. This sucking causes "stippling," or tiny discolored spots, streaking or discolored scarring, yellow bronzing or curling of leaves. The damage may cause leaves to drop off and ultimately may kill a plant.

Soft-bodied pests can move from plant to plant, spreading plant diseases such as viruses, bacteria and fungi while they feed.

How do you find tiny houseplant pests? And if you find them, how do you know what they are? Dreves offers some strategies for figuring out if you have a soft-bodied pest problem on your indoor plants:

  • Examine your houseplants and greenhouse plants regularly for pests, such as each time you water and feed your plants.
  • Breathe lightly on your plant and watch closely for movement of the pest reacting to your carbon dioxide.
  • Tap or brush branches or leaves of your plants over a white piece of paper so you can easily see what drops onto the paper. Use a 10- to 16-power hand lens or a magnifying glass to help identify critters.
    There are some of the major identifying characteristics of the major groups of soft-bodied pests you might encounter on houseplants or in the greenhouse.

Mites are tiny "cousins" to spiders, with eight legs. Almost invisibly small, mites look like moving little dots of red, yellow or green. In the colder months mites often turn orange. Look for spider-like webbing and plant parts that have bronzing color, yellow stippling of leaves or a dirty appearance.

Thrips are also tiny, "sliver-shaped" insects, less than 1/20th inch long. Adults have two pairs of feather-like wings that fold flat over their backs when they rest. The larvae, or young are typically yellow-orange. The adults are tan to brown-black. The damage they do to plants looks similar to that of mites, without the "tell-tale" webbing. Thrips leave black specks of their feces, scattered like a sprinkling of pepper on leaf surfaces, and are attracted to bright blue color.

Mealybugs look like little fluffs of greasy or waxy white cotton that are oval, somewhat raised and distinctly segmented with a serrated body margin. They are about 1/4 to 3/16th inch long. They snuggle near crevices between stems and leaves and on the underside of leaves. Unlike scales, they move slowly when disturbed. Mealybugs secrete sticky honeydew that makes the leaves look shiny. Leaf distortion, spotting and growth deformities are all symptoms.

Scales are hardly recognizable as insects at all. Resembling raised scabs or bumps 1/16th to 1/8th inch in diameter on leaves and stems, scales can be white, black, brown, gray or tan. Some resemble tiny oyster or turtle shells. They come off easily by scraping them with your fingernail. Scales can also leave a honeydew residue.

Aphids are most common on tips, buds and under leaves of plants. They can be winged or unwinged. They can look colored, powdery or wooly and are usually 1/16th to 1/8th inch long. They secrete honeydew. Plants infested with aphids will have curled and distorted leaves.

Whiteflies resemble flying dandruff or tiny light colored moths (1/12 inch long) as they move about and around a plant. A whitefly-infested plant will look wilted and faded. You may also see a honeydew secretion on the leaves. Look on the underside of leaves for oval-shaped, minute, somewhat transparent and flat, legless immature whiteflies. They are similar to scale insects but they do not move.

Author: Carol Savonen
Source: Amy Dreves