Yard and Garden: Houseplant Pests

Just as there are different types of indoor plants – foliage, flowering, succulents and florist’s plants – there are different pests that can invade houseplants. ISU Extension horticulturists explain how to recognize and manage pests on houseplants. To have additional plant and garden questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.

How can I determine if my houseplant has spider mites? 

Spider mites are tiny (about 1/50 inch long when mature). As a result, they are difficult to see with the naked eye. A convenient detection technique is to hold a sheet of white paper under a stem and then shake or tap the stem against the paper. The mites, if present, will show up as tiny, slow-moving specks on the paper. Examination of an infested plant may reveal very fine webbing produced by the mites. Also, check the plant for symptoms of mite feeding. In light infestations, plant foliage will appear to be speckled with tiny tan spots. Heavy mite infestations can turn the foliage to greenish yellow and eventually tan or brown. Heavily infested plants often drop their leaves. 

How do I control spider mites on my houseplant?  

If the mite infestation is light, spraying infested plants with water should help reduce the mite population. Move infested plants into the bathtub or shower and forcefully spray the plants with water to dislodge the mites. Several miticides (pesticides that control mites) are available for houseplants. Check for products at your local garden center. Sprays available to home gardeners usually contain insecticidal soap, horticulture oil or pyrethrin. When using miticides, carefully read and follow label instructions. Thorough spraying, especially to the undersides of the leaves, is important for control. These sprays have no residual activity and only control mites that are contacted directly. Repeated applications will be necessary. Also, keep plants watered and fertilized to promote plant vigor and reduce the impact of mite feeding.  

It’s often best to discard houseplants that are heavily infested with mites. Control efforts will likely be ineffective and the mites could spread to other indoor plants.  

I have small, white, cotton-like clumps on my houseplant. What are they?  

The small, white, cotton-like clumps on your houseplant are probably mealybugs. The body of each mealybug is oval and about one-fourth inch long. However, the soft, segmented body is concealed by filaments of white wax that cover the insect. The filaments extend out from the periphery of the body and may be up to one-half inch long.  

Mealybugs have needle-like sucking mouthparts. They insert their needle-like beaks into plants and suck sap from the plant tissue. As they feed, mealybugs excrete a sweet, sticky liquid called honeydew. The honeydew accumulates on the lower leaves of the plant, table or floor.  

Mealybugs lay eggs in a compact, white, waxy sac, usually in the area between the leaf stalk (petiole) and stem. Three hundred or more yellowish or orange eggs may be deposited by a single female. The eggs hatch into tiny, immature mealybugs called nymphs that move about on the plant searching for a place to settle and eventually insert their beaks into the plant and begin sucking sap.  

As mealybugs feed, numerous waxy filaments start forming as white, thread-like projections located along the edge of the body. The filaments grow, curl and tangle until the entire body is covered. Mealybugs usually remain in one place for their entire life span of four to 10 weeks.  

How do I control mealybugs on a houseplant?  

Mealybugs on houseplants are difficult to control. Unless the plant is particularly valuable, it may be best to throw away the infested plant before the insects spread to other houseplants. The standard remedies for houseplant pests can be successful if applied with diligence and persistence. Picking off individual mealybugs and egg sacs or dabbing each one with an alcohol-soaked cotton swab may be satisfactory for lightly-infested plants. Similarly, syringing the plants with a forceful spray of lukewarm water may dislodge a light mealybug infestation.  

Mealybugs also can be controlled with insecticide sprays. Use aerosol or hand pump spray products made just for houseplants. These products may contain any of several different ingredients, including insecticidal soaps, pyrethrin, neem or a synthetic pyrethroid, such as permethrin, bifenthrin or resmethrin. Granular insecticides that you apply to the soil of infested houseplants also may be effective. Use with caution and read and follow all label directions.  

In most cases, houseplants that are heavily infested with mealybugs should be discarded. Control will be extremely difficult (if not impossible). 

The leaves on my houseplant are covered with a sticky sap. There are also small “bumps” on the stems. What is the problem? 

The houseplant may be infested with scale insects. These small, inconspicuous insects are covered with shell-like coverings. They attach themselves to stems or leaves, and suck sap from the plants. As they feed, the scale insects excrete a sweet, sticky substance called honeydew. The honeydew accumulates on the plant’s lower foliage, furniture, carpeting or other objects beneath the infested plant.  

The life cycle of scale insects consists of the egg, nymph, and adult stages. Eggs are laid below the scale coverings of the adult females. When the eggs hatch, the nymphs crawl from underneath their mother’s scale and move a short distance to their own feeding site. The newly emerged nymphs are also called crawlers. At their new location, the nymphs insert their slender stylets (mouthparts) into the plant and begin sucking sap. The covering or shell develops soon after feeding begins. The scale insects remain at these feeding sites the rest of their lives.  

A small scale infestation causes little harm to healthy houseplants. However, a heavy scale infestation may result in poor, stunted growth. In severe cases, death of infested plants is possible. 

How do I control scale on a houseplant?  

Scale insects are difficult to control. Systemic insecticides are generally ineffective. The shell-like covering protects the scale from contact insecticides. The only time scale insects are vulnerable to contact insecticides is during the crawler stage. Since scale insects on houseplants don’t reproduce at a specific time, scale-infested plants need to be sprayed with insecticidal soap or other houseplant insecticide every seven to 10 days until the infestation is eliminated. Small infestations can be controlled by individually scraping off the scales or by dabbing each scale with an alcohol-soaked cotton swab. It’s often best to discard houseplants that are heavily infested with scale as control is nearly impossible and the insects could spread to other houseplants.