Archive for Thursday, February 21, 2008

The green thumb

February 21, 2008

Houseplants are particularly vulnerable to insect pests this time of year. Look closely at your plant each time you water - catching an insect infestation early can make a crucial difference.

A few weeks ago, the leaves on my bonsai began turning yellow. I thought the poor little tree was just lacking water, but when I looked closer, I noticed aphids feeding on the leaves and stems. A few treatments have returned the plant back to normal, but I am watching the tree and my other houseplants more closely now.

Identification is the first step to controlling insects on houseplants. Many of the creatures are visible with the naked eye - but so small that their features are difficult to distinguish. If you need help with identification, collect insects in a jar or plastic bag and bring them to the Douglas County Extension Office, 2110 Harper St., for free identification. You can also bring in the plant, but be sure to protect it from the cold temperatures.

The most common pests associated with houseplants are aphids, spider mites, scale, thrips, whiteflies, mealybugs and fungus gnats. All of these insects may reappear after the first attempt at control, but 2-3 treatments or applications usually take care of the pest.

Aphids are soft-bodied insects that suck sap from bud, leaves and twigs. Different species of aphids range in color from green to gray, yellow and red. They are best identified by their pear-shaped bodies and the tubelike structures called cornicles that project from their rear ends. Aphids are best controlled by washing the plant with a strong stream of water or with an insecticidal soap. There are a few conventional insecticides labeled for use on houseplants.

Spider mites are harder to see without a magnifying lens. They also suck sap from the plant. If you suspect spider mites, hold a white piece of paper under the leaves and shake the plant lightly. Mites that fall to the paper will look like small, rapidly moving spots on the paper. Spider mites can be controlled with the same methods as aphids.

Scale insects are named because of the shell-like coverings that protect their bodies. They are most commonly found on the stems and can be flaked off with a fingernail or treated with insecticidal soap.

Thrips and whiteflies are less common, but also suck sap from leaves and stems. Thrips typically leave silvery patches on the leaves and black specks (fecal matter). Whiteflies simply cause the leaves to pale and drop off. Treat both with insecticidal soap or insecticides labeled for control of the insect on the plant.

Mealybugs are slightly larger than the other insects and covered with white, fluffy filaments that resemble strands of cotton. They tend to group at the base of leaves. Treat mealybugs with the same methods as aphids, listed above.

Fungus gnat adults are harmless, but their larvae sometimes feed on plant roots in moist soil. Allow the soil to dry out between waterings to help control this insect. If they continue to be a nuisance, treat with a product that contains Bacillus thuringiensis v. israelensis - a common brand name is Gnatrol. Fungus gnats can also be controlled with pyrethrins, such as bifenthrin.

Jennifer Smith is the Douglas County Extension AgentHorticulture for K-State Research & Extension. She can be reached at 843-7058 or smithjen@ksu.edu.

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