Houseplant Pests

A perfect holiday gift, a house plant, may bring unwanted guests with it - insect pests. Worse yet, the indoor environment offers perfect conditions for insect pest populations to explode.

Many insects that thrive on indoor plants are not easily noticed when populations are low. The descriptions below will help you to identify the common culprits on your plant and select an appropriate control strategy.

Since plants may be in close proximity to pets and humans, the least-toxic control options available are advisable for indoor plants.

Spider mites

Signs and Symptoms: Spider mites, or red-spiders, appear as tiny dots moving over the plant surface. They have very fine piercing/sucking mouthparts and feed by sucking out plant fluids. Their feeding activity on plant tissue results in a browning of the foliage ("bronzing"), due to the loss of chlorophyll. Webbing over parts of the plant is often seen when populations are high.

Control Options: Chemical - Chemical control options include insecticidal soap or a 2% solution of horticultural oil. Biological - Various predatory mite species (e.g. Phytoseiulus persimilis, Amblyseius californicus) are available commercially for release on mite-infested plants in the home. Mechanical - Wash plants with soapy water (2 tsp. mild detergent/gal.water) and a soft brush, making sure to wash on the undersides of leaves. Alternatively, hosing tough plants with a strong jet of water knocks off mites and destroys their webs. Cultural - Low temperatures, high humidity, and excess moisture are unfavorable for spider mite development. Place potted plants on gravel in water-filled trays or mist plant foliage periodically to increase humidity around the plant. Avoid high temperatures and dry soil. If plant species are particularly susceptible to spider infestations, consider discarding them.

Scale

Signs and Symptoms: Heavily infested plants are often covered with small, disk-like or waxy coverings, and underneath each covering is the scale organism feeding on the juices of the plant. Two types of scale can attack indoor plants: soft scales and armored scales. Soft scales produce honeydew while armored scales do not. Feeding activity results in the distorting, stunting, and yellowing of foliage.

Control Options: Chemical - Chemical control options include insecticidal soap or a 2% solution of horticultural oil. Mechanical - When populations are low, scales can be rubbed off plants by hand, using garden gloves, a toothbrush, or a dry or alcohol-dipped cotton swab. Other strategies include washing plants with soapy water (2 tsp. mild detergent/gal. water) with a soft brush, making sure to wash on the undersides of leaves. Heavily infested plant parts may require pruning. Cultural - Mist frequently to dislodge crawlers. Use slow-release fertilizers with moderate proportions of nitrogen. Scale thrive on plants with overly succulent growth.

Mealybugs

Signs and Symptoms: Mealybugs are slow-moving insects that are covered with a white, waxy substance. Intense feeding activity results in loss of color, wilting, and death of the affected plant part. Mealybugs also produce large amounts of honeydew that coats the foliage and serves as a substrate for black sooty molds.

Control Options: Chemical - When populations are low, mealybugs found on a plant can be killed by dabbing the visible insects with alcohol on a cotton swab (alcohol dissolves the wax around the insect and its egg masses and kills them). Other options include insecticidal soap, a 2% solution of horticultural oil, pyrethrins or rotenone. Mechanical - Wash plants with soapy water (2 tsp. mild detergent/gal.of water) and a soft cloth. When plants are lightly infested, mealybugs can be killed by rubbing them off with your fingers. Where the plants are few and the mealybug infestation is light, prune out the infestation. Dislodge mealybugs by hosing down plants frequently. Cultural - As in the case of scale, use slow-release fertilizers with moderate proportions of nitrogen.

Bruce Barrett, Associate Professor, Department of Entomology, UMC (573)882-3446