Insects on house plants

Gardening TipsJerry Vitalis
Chisago County Master Gardener

Gardeners want to get houseplant problems under control before they start with outside gardening. I am not an expert on houseplants, so I will refer to articles written by Jeff Hahn and Deb Brown.

Spider mites are among the most serious houseplant pests. Left unattended, they can multiply rapidly, defoliating the plant.

Not true insects, spider mites are related to spiders and ticks. The are oval-shaped and yellowish or greenish. They are so small that they are difficult to see. Magnification shows that they have whitish skin and amber eyes and produce shiny, black fecal matter.

Mites thrive in dry, warm conditions. They make their way indoors in the summer from plants brought into the home, and even from Christmas trees and greenery.

Mites first feed on the underside of leaves, then move to stems and onto nearby plants by means of webbing. People can also accidentally spread them.

Spider mites damage plants by piercing leaf tissue with needle-like mouthparts that feed on the sap. Usually the first sign of spider mite infestation is a mottled or pinprick yellow discoloration of the underside of leaves.

Control includes washing and using insecticidal soap. It usually takes two applications, spaced seven to 10 days apart.

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 are round, oval or oyster shell-shaped, roughly 1/16 to 1/8 inch in diameter.

The young have legs and are more mobile and 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.

Heavy feeding causes leaves to yellow and drop, slowing growth and stunting the plant.

Control options include washing with insecticidal soap, or treating with resmethrin and disulfoton.  Treatments need to be repeated two or three times, with 10 to14 days between treatments. Adding drops of liquid dish soap to the insecticide will help it slide under the protective shell of these insects.

Mealybugs are soft-bodied insects about 3/16 inches long, easily visible without magnification. Their bodies have white, waxy filaments protruding from the tail end, and they look like they’ve been dusted with flour. They can be confused with powdery mildew. Sometimes they are observed as tiny, cottony clusters on stems and leaves.

Mealybugs are most commonly found along veins on the underside of leaves and where the leaves join the stems.  Some may also be below the soil on the main stem.

They pierce plant tissue with sharp mouthparts and suck out sap, resulting in yellowing, leaf drop and poor growth.

Control methods include washing, physical removal, disulfoton, resmethrin, tetramethrin and pyrethrins. Several applications may be necessary.  Be sure to read and follow the label instructions when using a chemical.

Thrips are slender, barely visible insects that range from tan to black and may have lighter markings. Adults can fly, leap and run rapidly. Young thrips can be whitish, yellow or orange and carry black droplets.

Thrips damage leaves and flowers by scraping tissue with their mouths and then feeding on the released juices. Foliage will show irregular silver blotches speckled with little black dots of excrement. Flowers typically become streaked or distorted.

Thrips can be controlled by washing or using the insecticides Orthene or Malathion.

When a houseplant is heavily infested, throw it away so that other plants are not exposed, or prune it practically to the soil.  Watch the new growth carefully for infestation.