Most houseplant problems are due to poor environmental conditions or poor, inconsistent care. However, indoor plants are also prone to insect pests. Scale and mealybugs are two common insect pests on houseplants. Horticulture specialists with ISU Extension and Outreach share information on these pests and how to control their spread. To have additional questions answered, contact Hortline at 515-294-3108 or hortline @iastate.edu.
Houseplants with small “bumps” on the stems or covered with a sticky sap, are likely 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, 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 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 locations, the nymphs insert their slender stylets (mouthparts) into the plant and begin sucking sap. The covering or shell develops soon after feeding begins. Scale insects remain at these feeding sites for 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.
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 may reproduce at any time of year, scale-infested plants should be sprayed with insecticidal soap or other houseplant insecticide every 7-10 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 is often best to discard houseplants that are heavily infested with scale, as control is nearly impossible and the insects could spread to other houseplants.
Small, white cottony masses at the base of leaf stalks on houseplants are probably an indication of mealybugs. Mealybugs are piercing-sucking insects. They insert their slender beaks into plant tissue and extract the plant’s sap. Light infestations cause little harm to plants. However, heavily infested plants may decline and eventually die.
Mealybugs on houseplants are difficult to control. Unless the plant is particularly valuable, it’s often best to throw away an infested plant before the insects spread to other houseplants. Standard control remedies for houseplant pests can be successful if done with diligence and persistence. On lightly infested plants, pick off individual mealybugs and egg sacs or dab each one with an alcohol-soaked cotton swab. It is also helpful to syringe plants with a forceful spray of lukewarm water to dislodge the mealybugs from the plants.
Mealybugs can also be controlled with insecticide sprays. Use aerosol or hand pump spray products made specifically for houseplants. These products may contain insecticidal soap, pyrethrin, neem, permethrin or other ingredients. Granular systematic insecticides applied to the soil surface of infested houseplants may also be effective. When using insecticides, carefully read and follow label directions.
Photo: Scale on houseplant.
Caption: A small scale infestation causes little harm to healthy houseplants. However, a heavy scale infestation may result in poor, stunted growth.
Photo: Mealybug presence.
Caption: Small, white cottony masses are a sign of mealybugs. An infestation could lead to a plant’s decline and eventually death.