Gardening: Houseplant pests common


— The most common insect questions the Routt County Cooperative Extension Service Office receives about houseplants are associated with whiteflies, spider mites and scale insects. All of these insects damage the plant by sucking sap out of its leaves. This impairs the plant's vascular system and weakens the plant, making it more susceptible to many other diseases.

Whiteflies are the most common, with the majority of damage occurring during the larval stage. The adults are white and fly or flutter when a plant is moved or disturbed.

Spider mites are extremely small insects and usually cannot be seen on the plant. Plants infected with spider mites can have a yellowing, stippled look, and webbing is occasionally present on the plant.

Scale insects are more difficult to identify. Scale insects move around the plant only during their larval stage. As larvae, they are only about 1/10 of an inch long. Once they become adults, they attach themselves to the leaf and remain inactive. Some scale insects are soft, while others form a hard shell. To positively identify these insects, you need a magnifying glass and should look for irregular bumps on the leaf surface. Plants will have a sticky substance on the leaves or surface below from the excretion of scale insects.

Whiteflies, spider mites and scale insects commonly affect a wide range of houseplants. Other insects like earwigs are more seasonal problems. Other insects that can be a problem on indoor plants include mealy bugs and aphids.

The good news is treatment for all of these insects is similar. Below are general recommendations for control of insects on houseplants from North Dakota State University Cooperative Extension.

1. Examine plants thoroughly for insects before purchasing. Isolate newly purchased plants for two to three weeks before placing them near plants known to be free of insects and disease.

2. Before bringing plants back into the home in the fall, examine each closely for insects.

3. If a plant is heavily infested, disposing of it may be the best solution.

4. Potting soil should be pasteurized to prevent disease and insect problems.

5. Some insects can be removed by using a small artist's brush or cotton swab dipped in alcohol, a toothpick or tweezers, or by thoroughly washing the leaves with a mild detergent solution (two teaspoons solution per gallon of water). If the last method is used, care should be taken to avoid wetting the soil. This method should be repeated several times.

6. Avoid misting plants as this can contribute to and spread infectious diseases.

Stick traps also are effective on whitefly adults because they jump around and become entangled on the sticky paper.

Other control methods include using pesticides. To insure the safest and most effective results from a pesticide, read the entire label on the container. Some plant species may be injured by certain chemicals (phytotoxic reaction), so read the precaution section on the pesticide label before applying.

Horticultural oils also can be effective, especially on scale insects. Insecticides labeled for indoor use containing the active ingredient Pyrethrins are worth a look, too. When using either of these products, be sure and treat both upper and lower surfaces of the leaf. Repeated application will be necessary.

CJ Mucklow is the Colorado State University Cooperative Extension Agent for Routt County. Questions? Call 879-0825 or e-mail


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