Houseplant Winter Critters

Sometimes pest issues on indoor plants continue into the winter. When home gardeners move houseplants back indoors as outdoor temperatures fall, they may inadvertently bring new pests inside. Pests can also sneak in on newly purchased plants or freshly added soil. Infestations of spider mites, scales, or mealybugs can cause branch dieback, stunted growth, leaf drop, or yellowing. Mealybugs and some scale insects produce honeydew, which causes the surface of leaves to become sticky and can encourage growth of sooty mold. Other pests, like fungus gnats, are primarily a nuisance rather than a threat to houseplant health.

MEALYBUGS

Mealybugs are sap-sucking insects commonly found feeding on succulents like jade plants. A common species found on houseplants is the citrus mealybug. Mealybugs are covered in white wax and are often found in colonies. They hide in protected areas like crevices or on the underside of leaves.

Management: Individual mealybugs can be removed with tweezers or a paintbrush. Small numbers can be wiped off ornamental plants with a cotton swab dipped in rubbing alcohol, but first test this on a small section of the plant to determine if it burns or harms the tissue. Contact insecticides such as insecticidal soaps and horticultural oils, or systemic insecticides with the active ingredient imidacloprid, are used to manage heavier infestations. To avoid killing pollinators, do not use imidacloprid products on flowering plants which are moved outside in summer.

Citrus mealybugs

Photo: John A. Davidson, Univ. Md, College Pk, Bugwood.org

Preparing to remove mealybugs with alcohol and a cotton swab

Photo: Abi Saeed, MSU Extension

SCALE INSECTS

Adult female scale insects produce fluffy wax or shell-like protective coatings, under which they feed, lay eggs, then die. Once they find a place to settle on a plant they do not move and are often mistaken for unusual plant growths. The dead females can remain stuck to the plant for several years. A common species that infests houseplants is brown soft scale.

Management: Small numbers of adult scales can be picked or gently scrubbed off, but for larger infestations, wait until the crawler stage of the life cycle to treat the plant with horticultural oil or insecticidal soap. Crawlers are tiny and mobile, making the timing of application critical. Monitor for these pests by using double sided tape wrapped around a card on a branch, or by shaking branches over a piece of paper to make the crawlers easier to spot.

Brown soft scales

Photo: Lesley Ingram, Bugwood.org

FUNGUS GNATS

Fungus gnats are tiny, dark flies that are sometimes mistaken for fruit flies, midges, or mosquitoes. They can get stuck on leaves and be a nuisance when large numbers are flying around. The immature stage of their life cycle is a small, pale maggot with a shiny, black head. The maggots thrive in wet substrate (including soil and/or other growing media), especially if it’s high in organic matter like peat moss. The larvae feed on fungi and decaying matter and rarely significantly affect houseplant health.

Management: Adult fungus gnat flies can be trapped using yellow sticky cards, and the larvae can be detected using chunks of raw potato embedded in the substrate as a bait. Remove the potato periodically and replace with new potato chunks. Sometimes over-watering results in fungus gnat outbreaks, so correct any drainage issues and water the substrate only once the surface is dry. This is also a good time to remove decaying leaves. Stubborn fungus gnat infestations might require repotting the plant in new substrate.

Fungus gnat larvae

Photo: Chloe Rice, MSU Extension

Fungus gnats on a yellow sticky card trap

Photo: Chloe Rice, MSU Extension

SPIDER MITES

The appropriately named spider mite deposits fine, silken strands on leaves and stems and its feeding causes speckled discoloration and scorching. A common species is the two-spotted spider mite, which proliferates in warm indoor temperatures and low humidity.

Management: Due to their small size, spider mites can sometimes be difficult to see. Monitor plants with the aforementioned symptoms by gently shaking a part of the plant over a sheet of paper and observing if any small mites fall off and start to move. Consider quarantining infested plants in cooler or humid areas of the house such as a bathroom. Humidity around a plant can also be increased by spraying or washing the plants regularly with water. Horticultural oils can also be used to treat some ornamental plants, but badly infested plants may need to be disposed of before they infest others.

Two-spotted spider mites

Photo: Frank Peairs, Colorado State University, Bugwood.org

Before using insecticides on houseplants, identify the pest to be treated and carefully read the label to check that the product is suitable for use indoors and labeled for the type of plant to be treated. Some products can harm or stain plants, so test the product on a small section of the plant before it is applied to a larger area. Always read and follow the recommendations on pesticide product labels.

Chloe Rice is an MSU Extension Associate Specialist and Urban Arthropod Diagnostician.