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Mertensia virginica en masse.
Mertensia virginica en masse. Photo © Mary Free

Managing Houseplant Pests

By Linda Cornish Blank, Extension Master Gardener

An unhealthy houseplant is most often the result of improper care. Too much or too little water, light, or air circulation cause many plant problems. If a plant is struggling or stressed due to improper care, the likelihood of pest infestation increases. Providing a houseplant with the growing conditions it needs is the best way to keep it healthy and minimize pest infestation.

Pest Detection

Be on the lookout for pests. Early detection is key to managing pests and often preventing infestation.

Honeydew, excreted by brown soft scale, which has collected on a nearby leaf.
Honeydew, excreted by brown soft scale, which has collected on a nearby leaf. Photo Colorado State University Extension
  1. Regularly inspect plant leaves, stems, and soil surface for pests. Inspection needs to be done year-round. You will contend with pests on your houseplants when they are outdoors in the summer. Indoors, pests on houseplants often increase during winter months.
  2. Examine leaves that are discolored or that contain holes. These may be evidence of pests.
  3. Watch for honeydew that pests excrete as waste. This waste gives the leaves a shiny appearance and is sticky to the touch. It can be found on leaves as well as on items around and underneath the plant.
  4. Check plant containers along edges, drainage holes, and saucers.
  5. Check for pests when you water. Water causes some pests to move, making them easier to detect.

Common Pests

Houseplants are susceptible to a variety of pests. Scale, mealybugs, aphids, and spider mites are four of the most common.

Scale

Soft scale adults and immature crawlers
Whitney Cranshaw, Colorado State University, Bugwood.org_

In my decades of keeping houseplants, scale is by far the most common and persistent pest that I’ve encountered. The brown soft scale, Coccus hesperidum Linnaeus, is one of the most common. It is round or oval, measuring 1/16-1/18 inch in diameter.

Scale infests both plant leaves and stems. Eggs hatch underneath the cover of the mother scale continuously over several weeks. The newly hatched “crawlers” have limited mobility and usually move short distances in search of feeding sites. After the crawlers settle to feed, they begin to produce a brown protective body covering. They excrete honeydew, which consists mainly of excess sap they ingest and then excrete as waste.

Scale feeds into the plant tissue sucking out its sap. Feeding injury to the plant may result in yellowing leaves, poor growth, and stunted plants. Sustained infestations can cause die back.                                  

Mealybugs

Several species of mealybugs are common pests of houseplants. Mealybugs are soft-bodied insects, whitish in color, with filaments extending from its sides and tail end. They are about 1/8-1/4 inch long.

Mealybug Whitney Cranshaw, Colorado State University, Bugwood.org
Mealybug
Whitney Cranshaw, Colorado State University, Bugwood.org

Mealybugs may infest different parts of the plant. Insects on leaves and stems are most visible but may also be found below the soil on the main stem and on roots. Some species tend to move to roots when growing conditions are less favorable, returning to stems and leaves when plants are actively growing. When full grown, most female mealybugs produce cottony material in which [BC2] to lay eggs; the eggs hatch within a few days. The newly emerged crawlers move about the plant. Often, infestation occurs during this movement. Like scale, mealybugs excrete large amounts of honeydew as waste on the foliage.

Mealybugs suck sap from plants causing leaves to become discolored and plants to be stunted or wilt.

Aphids

Aphids on plant stem
Aphids on plant stem – Image by M W from Pixabay 

Aphids are small, soft-bodied, pear-shaped insects about 1/16-1/8 inch long. They are usually green but may be pink, brown, black, or yellow. Some aphids have a woolly or powdery appearance due to a waxy coat. Adults may or may not have wings.

Aphids are usually found feeding on new growth or the undersides of leaves. Some feed on roots. Eggs generally hatch in the spring. Populations increase rapidly as generations can be completed in two to three weeks. Like scale and mealybugs, aphids excrete honeydew as they feed. Sooty mold fungi may grow on the honeydew, producing dark splotches on the plant’s surfaces.

Aphids suck plant sap causing leaves to yellow and the plant to wilt. Growth may be stunted and new buds deformed. 

Spider Mites

The twospotted spider mite, Tetranychus urticae, is one of the most common houseplant pests. It is oval shaped, yellow or green in color with two dark spots, and measures only 1/50 inch long. Magnification is usually needed to see mites.

Two spotted spider mite David Cappaert, Bugwood.org
Twospotted spider mite
David Cappaert, Bugwood.org

Spider mites typically feed on the underside of foliage. This is where their amber-colored eggs are laid and hatched. Mites shed white exoskeletons several times before maturity and may also leave behind black fecal specks. A fine, silken-like webbing may appear. Webbing is produced as the mites move across leaf surfaces. It is often most visible where leaves join stems. Tapping branches with a sheet of white paper underneath causes mites to drop. They appear as tiny, moving specks on the paper. A generation of mites may be completed in seven to ten days.

Spider mites suck sap from plants causing leaves to appear stippled. Feeding injury to the plant can cause yellowing leaves and leaf drop. Severe infestation can cause leaves to look bleached.

How to Get Rid of Pests

If you suspect a pest infestation on any plant, isolate the plant immediately. Keep the plant separate from other houseplants until the pest is completely controlled.

Non-chemical Control

Here are some non-chemical pest management solutions. Do not expect the problem to be solved with one application. These treatments require patience and persistence, but they can give good control.

  1. Wiping plant leaves with a moist sponge or paper towels can remove insects and mites. Rinse the sponge or change towels often to prevent spreading the pests.
  2. Spraying plants with water will knock some pests off. Doing this several times can help manage aphids and spider mites if they aren’t as firmly attached to plants as mature scale is. Small plants can be sprayed in a sink. You may need to use the shower for larger plants. Be sure to spray all plant surfaces.
  3. Handpicking can control larger houseplant pests such as earwigs and slugs. Physical removal can also be effective with small numbers of scale and mealybugs. If you have a few scale insects on a plant, scrape them off with a fingernail file or similar object. Mealybugs can be removed by using tweezers or a cotton swab that has been dipped in alcohol.
  4. Pruning can also be effective. If pests are isolated on a few leaves or stems, these can be removed by pruning. For more severe or widespread infestations, heavy pruning or entire cutback may be needed depending on the type of plants. Monitor new growth for signs of pests.
  5. Throwing a plant away in the trash may be the best and simplest solution for severely infested plants. Heavy infestation often requires lengthy and extensive efforts to control pests. Discarding the plant may also prevent pests from spreading to other plants. If an isolated section of the plant is healthy, taking a cutting and starting a new plant may be an option. Start with a clean pot and sterile potting mix. Monitor the plant cutting for signs of pests.

Insecticidal Soap

Spraying a plant with insecticidal soap can often eliminate a pest infestation in its early stages. Spraying is best done outdoors. These soaps are contact insecticides and are only effective when they make direct contact with pests. Once the soap solution dries, it has no effect against pests. Insecticidal soaps are most effective against the common soft-bodied pests like spider mites, aphids, mealybugs, and immature scales (crawlers). 

Houseplants that have spent the summer outdoors should be washed and soaked to get rid of pests on the plants and in the soil before they are moved back indoors. Lightly spray the leaves and stems of each plant with a hose. Prepare a solution of water and insecticidal soap according to label instructions. Read the insecticidal soap label for the list of plants for which the soap can be safely used. Fill a spray bottle with the solution. Pour 2-3 inches of the solution into a tub. Put plants that can be washed safely with the solution in the tub. Gently splash the solution on the soil surface and spray the leaves, upper and lower surfaces, and stems. Allow plants to soak in the solution 30-40 minutes. Soaking can prevent pests, such as pillbugs, millipedes, and slugs, from infesting root balls. Ants, which may have nested within the potting soil, can also be controlled with soaking. Remove the plants, spray with clear water, and allow them to dry.

If insecticidal soap cannot be used, use diluted dish soap. Follow the same process described above.

Chemical Control

Pesticides are also an option when managing pests on houseplants. However, using pesticides on houseplants requires extensive preparation and precaution. This treatment option should be carefully considered. If you should choose a chemical control, make sure to select a pesticide specifically labeled for use on houseplants. Read and carefully follow all label directions. 

Best Prevention

Keeping plants healthy is key to managing pests and often preventing infestation. Ensure the plant is receiving adequate light, don’t overwater, remove yellowed or dried leaves, prune spindly stems, and check leaves, stems, and soil for signs of pests. A strong healthy plant is the best way to protect it from pests and minimize any pest infestation that may occur.

Sources

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