Household Pests: Scale Insects
by Rob Sproule
Scale insects are the clams of the garden, clinging to your plants and protecting themselves under thick armour. They flourish in hot, dry household air (infamous during our long winters), and have evolved a clever protective measure that makes them difficult to spot.
There are over 3,000 species of scale in North America. The species we deal with most often look like oval shaped bumps on plants’ stems.
While some species secrete honeydew, which can sometimes lead to sooty mold growth, the ‘clamshells’ are the most common. Eradicating them takes persistence, mostly because they typically unnoticed until there are lots of them, but it’s not difficult.
Scale Life Cycle
Females, safe under their protective coverings, lay eggs which hatch 1-3 weeks later. The nymphs, called ‘crawlers’, search for a suitable place to sink their tiny teeth into your plant and suck the juice out of its cells. This is the only life-cycle stage when scale are mobile.
Once they nymphs start sucking they secrete a protective covering that forms over their bodies and hardens into a shell. It’s usually about an eight to a quarter of an inch long. It renders them immobile but they don’t complain; they will live out their days happily sucking the life out of your beloved plant.
Their armour is a clever evolution that acts as camouflage as well protection from predators. Enclosed adults look like natural bumps on the stems (or rough bark on larger plants) until they start overlapping due to numbers, which is typically what sets off the alarm bells.
The crawlers are too small to look for, but if you develop an eye for the bumps you’ll be able to avoid a major infestation. Watch for legless ovals clustering together; the color may vary due to different species.
If you see strange bumps appearing on your plants, don’t assume it’s natural. Give them a stiff thumbnail scrape. If they come off easily (ie. not part of the anatomy) then you’ve probably got scale.
Fortunately, scale will take a while to seriously injure your plant. Left untreated, however, it will suck enough fluid to weaken your plant, turn it yellow, and eventually kill it.
Like any plant pest, control gets more difficult as the population increases. Wary human eyes are scale insects’ worst enemy, so be vigilant and try to catch them early.
When you see them, the first thing to do is isolate the affected plants. They are an invasive species and love to spread.
You can easily scrape off the shells with your fingernail or, if your nails are as short as mine, a twig works too. Don’t use knives or other sharp tools that might damage the plant tissue.
There are a variety of insect predators available for purchase with a quick Google search. Although often expensive, these critters will wipe out the population and, when there’s nothing left to eat, will die themselves.
Chemical cures are tricky, since their shells render the adults invulnerable to most products. The crawlers are vulnerable but only for a short time, so you’d have to spray repeatedly.
Horticultural oil tends to work on the adults. It coats the shell and smothers the bug inside by cutting off its air supply. Insecticidal soap will work on the crawlers but tends to be ineffective against the adults.
If you have a major infestation, look to pyrethrum as a last resort. Try to spray this outside your home and please note that although it claims to kill scale, the anecdotal results I’ve heard have been mixed at best.
Dead bugs aren’t courteous enough to plop off and disappear; they remain vacant shells on the plant. So after all the fancy controls, we still come back to scraping them off manually.