January 22, 2004 at 2:12 p.m.
Spider mites are among the most serious houseplant pests. Left unattended, they can multiply rapidly, causing defoliation and death of the plant. They are not true insects, but are more closely related to spiders and ticks.
Spider mites are oval shaped and yellowish or greenish in color. They are so small that they are difficult to see with the naked eye. Magnification of their eyes shows them to be amber colored. They have whitish cast skins and shiny black fecal matter.
The mites thrive in dry, warm conditions. They make their way indoors in summer, from other plants brought into the home, and even from Christmas trees and greenery.
Mites first feed on the underside of leaves, then move from stem to stem and onto nearby plants by means of fine webbing. People can also accidentally spread them with their hands, clothing, tools and watering cans.
Spider mites damage plants by piercing leaf tissue with needle like mouth parts that feed on the sap. Usually the first sign of spider mites is a mottled or pin-prick yellow discoloration of the underside of leaves.
Control includes washing and using insecticidal soap. It usually takes two applications, spaced 7-10 days apart.
Scale insects don’t look like the 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 the growth of and stunting the plant.
Control options include washing, insecticidal soap, resmethrin, and disulfoton. Treatments need to be repeated 2-3 times with 10-14 day intervals between treatments. Adding a few drops of liquid dish soap to the insecticide will help it to slide under the protective shell of these insects.
Mealybugs are soft-bodied insects about 3/16 inch long, easily visible without magnification. Their bodies have white, waxy filaments protruding from the tail end, and look like they have been dusted with flour. They can be confused with powdery mildew. Sometimes they are observed as tiny cottony clusters on stems and leaves.
They are most commonly found along veins on the underside of leaves and where the leaves join the stems. Some mealybugs may also be found below the soil surface on the main stem. They pierce plant tissue with sharp mouthparts and suck out the sap, which results in yellowing, leaf drop and poor growth.
They can be controlled by washing, physical removal, disulfoton, resmethrin, tetramethrin, and pyrethrins. Several applications may be necessary for control. Be sure to read and follow the label instructions whenever you use a chemical for insect control.
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 droplets of black droppings on their backs. They damage leaves and flowers by scraping tissue with their mouths and then feed on the released juices. Damaged foliage will show irregular silver blotches that are 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 and badly damaged, the best course of action is to throw the plant away, so that other plants are not exposed to the same pest problem. If you are reluctant to dispose of the plant, prune it practically to the soil. Watch the new growth carefully for signs of infestation.
Reminder: The new number for your gardening questions is 651-237-3080. The new email address is: [email protected]
Brochures are available for Bonanza 2004 “Garden Fever” which will be held at Maranatha Church, between Wyoming and Forest Lake, March 6. Pre-registration is strongly encouraged. Call or email if you would like to receive a brochure or are interested in being a vendor. Forms are also now available to preorder bare root strawberry, raspberry, blueberry and grape plants. Many of these are cultivars developed at the University of Minnesota and used by commercial growers, but hard to find for home use.