Winter is a great time to add to or start a collection of indoor plants, but sometimes plants carry unwanted hitchhikers with them. Mealybugs and scale insects are common on tropical plants this time of year, and may even be found on plants being sold. To avoid bringing these pesky insects home, look plants over carefully before purchase. Avoid infested or stressed plants. If insects are observed in the weeks following, act quickly to destroy populations before they become established and spread to other plants.
Mealybugs and scale insects are closely related. They are both easily overlooked on plants and difficult to control. Mealybugs are typically white or gray in color, which would seem to stand out on a green plant, but they may hide in the crown of the plant or the point where a leaf curls around the stem. Scale insects have a great range in size and color, but tend to look like part of the plant rather than something feeding on it.
Mealybugs are oval shaped and a few millimeters to about a quarter-inch long. Their bodies have lines or ridges running the short way across the oval. They have a soft, mealy coating of wax, and adult mealybugs typically have waxy filaments extending from the sides of the body. Newly hatched mealybugs are mobile and can easily move to a new plant, but once they settle in to feed, mealybugs move very little.
Scale insects produce a cover over themselves that looks like a tiny turtle shell. They are typically one-eighth to one-quarter inch long at maturity and only slightly raised from the leaf or stem surface. They may be round, oval or elongated. Scale insects are separated into two groups: soft scales and hard scales. Soft scales have a thinner, soft and/or waxy cover. Hard or armored scales have a thicker, more solid cover.
For small infestations on tropical plants in homes, mechanical removal is the best option for control of mealybugs and scale insects. Place plants in the sink or bathtub and wipe the insects from the leaves and stems. Rubber or latex gloves or the use of a cloth or sponge is desirable. The insects will be sticky. Use warm water and a soft cloth to remove residue and honeydew. Prune out and destroy heavily infested stems.
Insecticides are generally unwarranted for small populations of these pests. For large infestations and/or particularly valuable plants, seek out identification of the species of mealybug or scale insect to get better information about control. Then, apply based on recommendations for the species and its life cycle (generally making applications after an egg hatches).
Mealybugs and scale insects feed by sucking sap from plants, which reduces plant vigor. Continued feeding and growing populations add cumulative stress to the plant and when combined with other stressors can lead to plant death.
Mealybugs and soft scales also secrete honeydew – a sticky waste product that drips onto leaves and stems below where the insects are feeding. Besides the sticky spots (that are usually more noticeable than the insects themselves), honeydew may grow mold and/or attract ants.
Sometimes the covers of scale insects are noticeable on a plant, but when bumped with a fingernail, the scale covers pop off and appear dry and hollow. These are the covers of scale insects that have already died, possibly at the end of their life cycle or possibly from insecticide treatment. If scales are alive, they will be sticky and wet under the shell or soft cover.
In addition to tropical plants, mealybugs and scale insects may be found on landscape plants. Mealybugs are uncommon in the Kansas landscape and are usually minor pests when they occur. Scale insects are widely found in the Kansas landscape, especially on trees and shrubs. Control may be warranted for certain species in certain situations, but insects and plants should both be identified and treatment determined based on species and site-specific factors.
— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.