Orchid scale is an annoying problem at best and a plant killer at worst. It can also be difficult to get rid of these pesky bugs once they infest a collection. There are not many pests that can severely damage an orchid collection, but along with mealybugs and aphids, the scale is one of orchids’ most pernicious.
Scale range in size from one to five millimeters and, in certain life stages, can be difficult to see without magnification. Male soft scale appears grayish to white and can look similar to mealybugs. Soft scale damages the plant by sucking sap from the plant. It also leaves behind a sticky excretion called honeydew. The hard scale does not leave this excretion.
How Scale Got On Your Orchid
The most common way of acquiring scale is purchasing an infested plant. Scale is easily transmitted to clean plants when plants are crowded, and crawlers move from plant to plant. Colonization by windblown crawlers may occur when plants are outdoors, but this can also happen indoors and in greenhouses when crawlers float on currents from circulation and heater fans. This can produce pockets of infestation where air currents are weakest when crawlers settle on plants. If you suspect scale, immediately isolate the infected plant(s).
Scale management is usually a protracted and serious effort, and never fun. Scales have short life cycles but may cycle many times a year. Typically, a month or more is required for completion of a scale generation, but a mere two to three weeks is possible in favorable conditions. The overlapping generations create the biggest scale-management problem.
The non-insecticidal treatments may not be highly effective for eliminating scale—they should be viewed as controls, not eradicators. Also, many chemicals for home use are toxic to humans, pets, and plants even in diluted forms, often proportionately more toxic than the feared insecticides.
We always recommend trying the following methods first before buying a powerful pesticide. Light infestations restricted to one or a few plants can be treated with household products rather than concentrated insecticides.
For a small infestation, you can rub the scale with isopropyl alcohol on a cotton swab to remove the bugs. Make sure to get under the leaves and down in the crease where the leaf joins the stem. Wipe the plant completely clean. After treating, spray thoroughly with a diluted alcohol solution. Repeat this treatment weekly until the infestation is under control.
Another option for management suggested by the Canadian Orchid Congress is to carefully wash the whole plant in warm water with grated soap (be sure the soap is mild and not ammonia-based, as this will cause damage to the plant). The plant must be washed every other day for a month. Then it must remain separate from other plants for two weeks to ensure no new scale appears.
Oils, Soaps, and Sterilants
Horticultural, neem and mineral oils, and insecticidal soaps are generally considered safer for humans, pets, and plants than insecticides. None provide absolute control over pests, but frequent applications reduce insect populations in small orchid collections.
Oil solutions smother insects, so complete coverage of all sprayed plants is essential. These oils are mixed with water and a plant-safe detergent for enhancing spreading and sticking. Insecticidal soaps, while considered safe, may still damage some plants, particularly tender new tissues.
Finally, if the horticultural oil does not take care of the problem, you may have to use chemicals. Few insecticides are tested on or specifically registered for use on orchids, but several common, inexpensive, home-and-garden chemicals are labeled for ornamental plants. Always follow the label carefully and do not use a concentration anymore or less than suggested. Remember, insecticides can be harmful to people and pets. It is important to be safe when using insecticides indoors.
Some of the more effective insecticides available to home growers are Orthene (wettable powder), Malathion (liquid), Diazinon (liquid), and Carbaryl (water-based emulsifiable concentrate). Orchids are tough but sensitive to many chemicals. While certain species may not react to a given formulation, others might, so advance testing is advised.