By Donald Lewis
Extension Entomologist
Iowa State University
Tropical plants that are forced to survive through a long, dim and dry winter inside the typical Iowa home or office have two strikes against them from the start. Insects and mites then become another challenge for indoor plants, and if left unchecked, may even kill heavily-infested plants. The big three pest problems on indoor house plants are scale insects, mealybugs and spider mites. All feed on plant sap, causing a gradual weakening or decline in the plant health
Spider mites are tiny eight-legged arthropods related to the spiders and ticks. Like the spiders and the ticks, mites have no antennae and no wings. They get from place to place by crawling rather than flying.
Spider mites feed externally on the foliage of nearly all houseplants, though some, such as ivy and Norfolk Island pine seem particularly susceptible. Spider mites use a short, sharp beak to puncture the plant tissue and feed on the liquid within the cells. The piercing-sucking action and sap loss cause unique symptoms. In light infestations the foliage will appear to be speckled with very tiny tan spots. Heavy mite infestations can turn the foliage to greenish-yellow and eventually tan or brown. Heavily infested plants often drop their leaves. Examination of infested foliage may reveal very fine webbing produced by the mites.
The first step in spider mite control on houseplants is to inspect regularly, frequently and thoroughly for pests. Spider mites are tiny and may be difficult to see with the naked eye on the plant. A convenient detection technique is to hold a sheet of white paper under a stem and then shake or tap the stem against the paper. The mites, if present, will show up as tiny, slow-moving specks on the paper. Examine leaf undersides for crawling mites or their eggs that will look like small shiny spheres. The common houseplant spider mite is the twospotted spider mite, a yellowish green mite named for the two large dark spots, one on each side of the abdomen.
If mites are present, the next step is to determine the extent of the damage and the value of the plant. It is usually easier, quicker and more convenient to discard a small plant that is heavily infested than it is to return it to good health and appearance. If possible, prune heavily infested portions of larger plants before attempting mite control. Washing or syringing infested plant foliage may reduce light mite populations. Move plants outdoors or into the bathtub or shower and then use a forceful spray of water to dislodge mites from an infested plant.
Several miticides (pesticides that control mites) are available for houseplants. Check with your local nursery or garden center. Sprays available to homeowners usually contain insecticidal soap, horticulture oil, or pyrethrin. Read and follow all the label instructions. Thorough spraying, especially to the undersides of leaves is important for control. These sprays have no residual activity and only control mites and insects that are contacted directly and repeated applications will be needed. Keep plants watered and fertilized to promote health and vigor and to reduce the impact of mite feeding.
In years past we suggested a homemade dishwashing detergent spray for plant pest control. No more. Newer detergents that boast of grease cutters, higher concentration, or other ingredients for better cleaning action may be harmful to plants. In fact some detergent products are sold as herbicides because certain components of the detergent can kill or disrupt plant tissue. Soaps and detergents can be effective miticides, but to be safe use insecticidal soap products only according to label directions.
-30-