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Ask a Gardener - Oystershell Scale

January 10, 2014


Question: How do I eliminate Oyster Shell Scale from ash trees?
- Doug

Oystershell scales are one of the more damaging scale insects in Colorado, but the good news is that there are some steps you can take to get rid of them.  



Understanding the lifecycle of insects is the first step to control.  Oystershell scale spends the winter as eggs underneath the mother scale.  In mid Spring, May or June, the eggs hatch.  The CSU Extension Office says Memorial Day is often the approximate time when eggs hatch in most years.  The new scale larvae are called crawlers.  They crawl around the tree looking for a place to feed.  Then they molt and form the hard armored scale, feeding and growing throughout the summer.  In the late summer or fall, they lay their eggs and the whole cycle starts again.  

The key to control is breaking this cycle.  This generally takes a combination of a few control methods.  First, keep trees and shrubs well watered, feed and trimmed as scale tends to infest stressed or damaged trees first.  Then, prune off any extensively damaged branches.  If you have a smaller tree, old scale and eggs can be scrubbed off by hand with a soft plastic dishwashing pad.  This should be done before eggs hatch.

You can also apply horticultural oil in the winter before bud break.  Horticultural oil works by smothering the dormant eggs before they hatch.  However, because the eggs are protected by the scale, the oil might not penetrate and get all the eggs.  For complete control, you’ll need to target the crawlers in the spring

In mid-May, wrap a piece of double-sided tape around small branch with scales.  Keep checking the tape until you see little crawlers stuck to it.  You can also check for crawlers by shaking an infested branch over a white piece of paper.  The crawlers will be easy to see on the bright white paper.  Once crawlers are detected, you can spray them with a horticultural oil, insecticidal soap or pyrethroid insect killer, like Bonide’s Eight.  Sometimes, even just a strong jet of water is enough to kill the little crawlers.  Once you break the life cycle of the bugs, continue to feed and water your tree regularly to build up resistance. 

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