Top 10 List of Pests That Affect Houseplants

Houseplant problems often arise when a new plant is brought into a home that previously was free of any problems. We enjoy giving houseplants as gifts or we want to sustain one that was given to us. Upon acquiring any new plants, whether it’s one you purchased or a gift, isolate that plant from others in the house for at least 30 days. 

How many of us have brought plants indoors to awake a week later with an infestation of fungus gnats or whiteflies? With a few precautions we can eliminate many of these pests. 

Overfertilizing, particularly in winter, can produce succulent growth that is very weak and prone to infestation. For my plants, unless they are under grow lights I don't fertilize them at all during the winter. Those under lights occasionally get a dilute solution during the winter. Those in the windows do not get enough light to support healthy new growth, so I don't encourage it. 

Here are the 10 most common pests that you might encounter and how to control them. (The ingredients listed in the control measures are active ingredients, not brand names. Look under active ingredients on the label). The images are magnifed many times.

Mealybugs are soft bodied insects that resemble tufts of white cotton. Usually they go unnoticed until they are adults, because that's when they produce their white cottony covering. They are also extremely adept at hiding on a plant. They are sucking insects, piercing the stems and leaves to ingest their nutrient rich sap. Often affected plants are sticky, as mealy bugs produce honeydew. Honeydew is essentially what the mealy bugs excrete after ingesting the plant sap. It is high in sugar and tends to attract a black powdery fungus called sooty mold, which feeds on these sugars. Heavy infestations of mealy bugs can distort new growth in plants. Small infestations can be controlled by handpicking or by dabbing with a cotton swab dipped in rubbing alcohol. Mealy bugs suck out the plant juices, often stunting or killing the plants. When mealy bugs’ numbers are large, ants may be found feeding on the honeydew excreted by these pests.

Control options: Washing, physical removal, disulfoton, bifenthrin, permethrin, imidacloprid resmethrin, pyrethrins (at least two to three applications sprayed once every 10 - 14 days are usually necessary).

Mealybugs

Aphids are another of the sucking insects that produce honeydew. They are small soft-bodied insects, ranging in color from nearly colorless to green, to yellow, or to nearly black. Most of the time they can be controlled by washing or wiping them off affected plants. In extreme cases they will also distort new growth. 

Control options: Washing, bifenthrin, permethrin, resmethrin, insecticidal soap, pyrethrins, neem oil, plant oil extracts, disulfoton, imidacloprid.

Aphids

Spider mites are among the most serious houseplant pests. Left untreated they can multiply rapidly, causing injury, defoliation, and plant death. They’re not true insects, but are more closely related to spiders and ticks. These mites are oval shaped and yellowish or greenish in color. They’re difficult to see clearly with the naked eye, measuring only 1/50th of an inch. Magnification can reveal amber-colored mite eggs, whitish cast skins, and black fecal specks. To verify spider mite presence, place a sheet of white paper under discolored leaves. Tap the leaves, and then watch for tiny moving creatures on the paper. Spider mites thrive in dry, warm conditions. They make their way indoors in summer and sometimes as hidden guests on Christmas trees and greenery in December.

Mites first feed on the undersides of leaves and then expand their territory as populations increase, moving from stem to stem and onto nearby plants by means of fine webbing. People can also spread them accidentally on their hands, clothing, and watering cans. Always wash your hands and any tools you’ve used after working with infested plants.

Spider mites damage plants by piercing leaf tissue with needle-like mouthparts, feeding on sap. Usually the first sign of spider mites is a mottled or pin-prick yellow discoloration on the undersides of leaves.

Control options: Washing, bifenthrin, insecticidal soap, plant oil extracts (at least two applications sprayed once every 7 - 10 days are usually necessary).

Spidermites

Scale insects don’t look like typical insects. Adults secrete a waxy shell-like covering which gives them the appearance of brown or gray bumps. They’re round, oval, or oyster shell-shaped, roughly 1/16 to 1/8 inch in diameter. Tiny juveniles or “crawlers” have legs and are mobile; they’re more vulnerable to pesticides than the stationary adults, but are barely visible without magnification.

Scales are usually found on plant stems and the undersides of leaves, especially along mid-veins. They use needle-like mouthparts to feed on plant sap, secreting sticky honeydew as an end product of that process. Heavy feeding causes leaves to yellow and drop, slows growth, and stunts plants.

Control options: Washing, physical removal, bifenthrin, permethrin, resmethrin, insecticidal soap, pyrethrins, disulfoton, imidacloprid, plant oil extracts (at least two to three applications sprayed once every 10 - 14 days are usually necessary). Because their waxy covers are so impervious to insecticides, add a few drops of liquid soap or detergent to help the material slide under the edges of the “shells.”

Scale

Thrips are very small (about 1/16 inch long) and slender; usually tan or dark colored. Immature thrips are white, yellow, or orange. Adults can fly, jump, or run quickly. They are difficult to see without a hand lens, though they may look like little threads on the plant.

These pests may not survive long inside. However, some types of thrips are capable of living indoors on houseplants year-round. Thrips feed by scraping leaves or flowers with their rasping mouthparts and then sucking the fluid that’s released. Damaged leaves develop irregular silvery streaks or splotches. Flowers become streaked or distorted. Where feeding is heavy, you may also see small shiny black drops of excrement on the leaves.

Control options: Washing, physical removal, disulfoton, bifenthrin, permethrin, imidacloprid resmethrin, pyrethrins (at least two to three applications sprayed once every 10 - 14 days are usually necessary). 

Thrip

Springtails are very small (about 1/16 inch long), slender white or gray insects. They’re commonly seen in the soil or in saucers beneath potted houseplants. Springtails jump when they’re disturbed, so they’re particularly noticeable right after you water.

Springtails are scavengers, feeding on decaying roots and fungi. They prefer to live in damp potting soil, especially a mix containing a high percentage of peat. Often associated with houseplants that are kept quite moist, they rarely, if ever, damage the plants. They’re just annoying.

Control: Water thoroughly, but let soil dry as much as possible without letting plants wilt.

Springtail

Fungus Gnats: Adult fungus gnats are small (1/16 - 1/8 inch long) insects you may see flying around houseplants or resting nearby where they frequently are mistaken for fruit flies. The legless, worm-like larvae live in damp soil and are scavengers with habits similar to springtails, causing little or no damage.

Control options: Water thoroughly, but let soil dry as much as possible without letting plants wilt, Bacillus thuringiensis

Fungus Gnats

Whiteflies are not true flies. They’re more closely related to scales, mealy bugs, and aphids. These pests are more commonly associated with plants growing in greenhouses than those growing in homes. Adults are very small (about 1/16 inch long), white moth-like insects. They’re easily disturbed, fluttering up when you water or handle a plant they’re clinging to. Immature whiteflies are even smaller. They’re flat, oval, scale-like insects normally found on the undersides of leaves. It’s not easy to spot these pale insects.

All stages of whiteflies feed on plant sap, using their piercing-sucking mouthparts. Infested leaves can yellow and drop, reducing plant vigor, but usually not killing the plant.

Control options: Washing, bifenthrin, permethrin, resmethrin, pyrethrins, neem oils, and plant oil extracts (at least three applications sprayed once every 5 days are usually necessary).

Whiteflies

Cyclamen mites are not true insects but are more closely related to spiders and ticks. They inhabit protected portions of plants, especially young tender leaves, buds, and flowers, but are not as common as other pests previously described.

These mites are extremely small and semi-transparent; impossible to see without magnification. They’re spread by leaf to leaf contact and on hands and clothing. Infested leaves are stunted, brittle and often hairy; flower buds may be deformed and streaked. Sometimes injured leaves, buds and flowers may turn black. Stems of infested ivy plants are often leafless or have small hairy leaves. African violets typically have small, cupped hairy leaves in their centers.

Control options: Throw away infested plants 

Cyclamen mites

Leafminers are larvae (worms) of small flies that feed between upper and lower leaf surfaces, leaving narrow winding trails or mines. When plants are severely infested, leaves have a water-soaked appearance. The larvae are very tiny, but their injury to leaves is quite conspicuous. Adults can reduce a plant's value by their feeding. The adult makes small feeding and egg laying punctures in the leaf surface, which turn white in a short time, giving leaves a speckled appearance.

Control: Remove and destroy any leaves showing leafminer damage. 

Leaf Miner	Leaf Miner Damage

I normally don’t endorse products but in this case I definitely want to share a success story. I bring in a lot of plants to overwinter under lights. I have gotten infestations of fungus gnats a couple of weeks after the plants are in the house. This insect lays its eggs in the soil; after a couple of weeks they hatch thus the invasion. Last year I used Bonide Houseplant Insect Control, I applied a week before I brought my plants indoors. I did not see one pest. This is available wherever Bonide products are sold.

Stay tuned for my next article on diseases that affect houseplants

Photos courtesy Clemson University/ University of Minnesota/Bonide