Scale Insects on Orchids

In the home orchid collection, scales are acquired by your plants in some combination of three methods. The most common method of acquiring scales is by purchasing an infested plant. On plants at home scales are easily transmitted from infested to clean plants when your plants touch each other and the crawlers to move from plant to plant. The final method is colonisation of your plants by windblown during the summer when your plants are outdoors, but it can also occur indoors in greenhouses and sunrooms.

Life Cycle

Scale insects have a three-stage life history: egg, larva (or nymph), and adult. Females lay the eggs, with the eggs usually retained in the body and under the hard scale when the female dies. These hatch into the mobile nymphs, called crawlers. The crawlers are the active stage that can move between plants. After finding a suitable place for feeding, the crawler will settle and begin feeding, and transform into the next nymphal stage. At this point it begins to form the hard protective “scale” covering. The covering enlarges as the insect grows. Nymphs often have a light yellowish scale, which darkens to tan or brown as the insect matures to an adult.

Management

Scale management is usually a protracted and serious effort, and rarely much fun. Light infestations restricted to one or a few plants can usually be treated with household products rather than concentrated insecticides. When possible, immediately isolate infested plants from others to prevent the scales from moving amongst them.

Because the life cycle of scales can be so short, in order to bring a serious problem under control you will need to do a treatment every 2-5 weeks, depending on the life cycle period of your particular problem scale species. Consequently, the key to scale control is persistence.

Management methods that are the least toxic to people, pets, and plants, are the most time-consuming and laborious. Insecticidal methods, including horticultural oils, soaps, and synthetic insecticides are progressively more toxic and more expensive, but less work. Regardless of method or chemical used, you must remain vigilant and expect to make at least 2-3 applications 10-16 days apart.

Because of plant cost, personal attachment to orchids by owners, and the overriding desire to avoid insecticides whenever possible a number of effective “home remedies” for scale control are available. Be aware that non-insecticidal treatments may not be highly effective for elimination of scales. Thus, they should be viewed as controls, not eradicators. Also, many common “home chemicals” are extremely toxic to humans, pets, and plants even in diluted forms, often being proportionately more toxic than the feared insecticides.

Rubbing Alcohol

Probably the most popular home remedy is to swab and daub plants with a cotton bud or ball of cotton dipped in isopropyl (rubbing) alcohol. Do not use other alcohols, such as ethanol or methanol that will penetrate the plant tissues and cause considerable damage! On hard-leaved plants, gentle rubbing with the fingers or a soft infants toothbrush is effective, with or without the alcohol massage. Remove all scales, large and small. Afterwards, you will still need to repeat the alcohol treatment to remove the tiny yellowish spots, which are the recently hatched crawlers. Pay particular attention to the midrib, other veins, and leave edge areas. Closely monitor your plants to get an idea of the life cycle of the particular species of scale that is your problem, but expect to repeat treatment against the immature every one or two weeks.

A potential problem with alcohol treatment that is occasionally reported may be chilling of the plant. The rapid evaporation of alcohol cools the plant tissues. Especially with air movement that increases evaporative cooling, this chilling is suspected of over-cooling tissues and creating zones of dead cells that can become necrotic with bacterial or fungal infestation. On warm days consider wiping any residual alcohol with a tissue instead of permitting it to evaporate off the plant.

Final Consideration

Whenever using oils, soaps, and harsher chemicals, be thorough, change solutions frequently, and never ever use more than a minimum concentration of mixture. Too, never use chemicals prophylactically, that is do not routinely use chemicals as a preventative as it is a waste of chemical (and money!) and such use allows resistant scales to develop. Finally, keep up the manal removal of all scales, if possible. Removing the egg laying adults is as important as killing the nymphs. Again, you need to monitor the cycling of your scales, this time to optimise spray effect and minimize total number of sprays.

 

Paul J. Johnson, PH.D.

Insect Research Collection

South Dakota State University

Box 2207A, Brookings, SD 57007, USA