SEVERAL species of mealybugs attack orchids. The adult mealybug is oval in shape, about 2-5 mm (.08 -.2 inches) long, and has a cottony, white to grayish-white appearance. Mealybugs use their mouthparts to suck the sap from plants: they feed under bracts and sheaths, and on the roots. Severe infestations weaken and even kill plants. Mealybugs arc commonly called “mealies.”
Signs of an Infestation. Small whitish. powdery or cottony deposits on plants can indicate mealybugs are present. As the damage progresses, leaves may curl. yellow, become stunted or drop. In addition, flower production and stem growth may be reduced. Like aphids and some scales, mealybugs also secrete honeydew, which attracts ants and promotes the growth of sooty moulds.
Methods of Control. The first line of defense, if a plant is not heavily affected by mealybugs, is to pick the bugs from the plant and squash them. Pests have yet to develop a resistance to this type of control.
Alcohol. 70 percent isopropyl alcohol (or witch hazel as a gentler alternative) can he used to control mealybugs. These insects can be physically removed in the same manner as scales. A cotton swab, soaked with isopropyl alcohol to dissolve the insect’s waxy covering, is a good tool to reach the mealybugs hidden deep down in the sheaths and leaf crevices. A series of applications, several days apart, may be needed. Alcohol can damage plants; do not use near fire. Alcohol can be combined with insecticidal soaps, but not with horticultural oils.
Growth Regulators. Products such as Enstar (active ingredient is S-Kinoprene) include a growth regulator that kills eggs and prevents insect maturation. These need a spreader-sticker (silicon works best) to be effective. Water plants 24 hours before applying growth regulators. These products are not for outdoor use. Read the labels for toxicity information.
Horticultural Oils and Insecticidal Soaps. When mixed with water and a few drops of liquid dish detergent, horticultural oils suffocate insect pests and their eggs. Insecticidal soaps also work by smothering pests. For a heavy infestation, the affected plant(s) must be completely covered. Environmentally gentle, both methods only affect pests while they are still wet, and must contact them to be effective.
Insecticides. There are many commercially available insecticides for control of mealybugs. Make sure the one you choose is registered for use on orchids, and includes mealybugs in the listing of susceptible pests. To control a severe infestation, it is probably necessary to remove the plant from the container, clean the mealybugs from all of the roots, treat with an appropriate insecticide and repot using a clean container and mix. Because the fluffy, waxy covering on mealybugs repels water-based insecticides, it is necessary to mix a wetting agent in with. the insecticide when spraying. A systemic insecticide has the advantage of reaching insects feeding on the roots.
Some general rules of thumb in control of mealybugs:
- Ridding a plant of mealybugs will require at least two applications of the chosen pest-control product. Three to four applications at four-to five-day intervals should be effective in eradicating mealybugs at all stages in their lifecycle.
- If spraying plants is necessary, it is best to do so early in the morning, when temperatures are coolest, and there is less chance of burning the plant in the sun with the liquid on its leaves.
- Always follow the directions from the product’s manufacturer, and take appropriate precautions when handling materials hazardous to humans, pets and livestock.
- Among the advantages to using horticultural oils, insecticidal soaps or alcohol is that these products are easier to handle safely, less hazardous to humans and the environment, and insects do not develop a resistance to these treatments.
Susan Jones, Assistant Editor, Orchids, American Orchid Society. Used with permission of the American Orchid Society, Orchids, May 2001