How Scales Spread
Probably the most common method of infestation can come from acquiring a new plant that already has a scale problem. Most scale species have a crawling stage and can migrate onto nearby plants if they are close together. Another way these guys spread is from getting blown from one plant to another. Some male scales of the Biosduval variety mature with wings and can fly to different plants as well. This is most common for outdoor plants, but they can also get spread around by heating and ventilation fans in nurseries. If this is how they are getting around you'll have the largest infestation in areas with a lower air circulation.
Life Cycle
Scales have a very short life cycle that is between seven to ten days long. There are three stages of life: eggs, Larva, and adult. Eggs are laid under the shell of female scales and hatch after she dies. The larva (also called crawlers) are mobile and go out to find a suitable place for them to begin feeding. Females will then form a hard outer shell and males (depending on the species) grow either a hard shell or wings.
Management
Once you have found scales on an orchid quarantine that plant quickly. I would also quarantine the plants surrounding it separately. You don't want to put your possibly infested plants with those that definitely are infested, but you also don't want it with your others if it has eggs or larva you can't see yet. Just a few plants are a lot easier to treat than a whole collection.
Once scales have matured into adults they are almost impossible to kill because of their hard shells. Their shells have a coating that keeps a lot of pesticides, oils, and other control agents from killing them. Soft shelled scales are much easier to kill for this reason. Since adults are hard to kill we attack the crawler or larva stage.
Rubbing alcohol is a commonly used remedy for scales. Only use isopropyl rubbing alcohol as other types of alcohol can penetrate the plant tissues and damage the plant. This is best used on stiffer-leaved orchids. Types like the Oncidium orchid who have soft leaves can be damaged from isopropyl. If this is what you've settled on, take a cotton swab and gently wash your plant. This will kill the crawlers and the washing will help remove adults from your plant. Make sure to do this once a week until you're sure the infestation is gone. The is because the life cycle is 7 to 10 days you want to keep attacking the crawlers until they are unable to produce another generation. Once you've washed your plant with alcohol you want to wipe off any access, because it evaporates very quickly and causes excessive cooling that can damage areas of your plant.
Oils and insecticidal soaps are also used to manage infestation. These are safer for people and pets than insecticide. Commonly used are neem and mineral oils - these can cover the scales and effectively suffocate the insects. Complete coverage is key and this might be a good way to go for a smaller orchid collections, but would be time consuming for larger collections. Don't apply oils in temperatures of 85 degrees or higher because they will overheat your plants and hurt them.
Overall key to getting rid of scales is persistence and patience. Some people will weigh the price of their orchid to the time it will take to get rid of scales and possible go out and get a new orchid. I have sentimental value to all of my orchid babies! I hope this post helps anyone out there trying to combat these nasty bugs!
~Happy Planting!
(I grabbed these photos from November 2001 issue of Orchids - The Bulletin of the American Orchid Society - www.aos.org)
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