Scale Insects of Woody Plants

When you were a child you may have been forced to take piano lessons, like some of us, and you had to learn music scales. Now, we want to discuss another type of scale – scale insects and their impact on your cut flower operation.

I recently received a package from a cut flower grower in Maryland which contained branches of willow, plum, quince, yellow-stem dogwood, curly willow, lilac and star magnolia. Each sample was infested with soft or armored scale. This operation has a wonderful and thriving cut stem business, and they were alert enough to catch this problem before it really blew up on them. 

It’s easy to overlook scale insects since their covers (teste), for most species, blend in with the color of the bark of the plant. Their covers often look like part of the plant. The scales are pretty sneaky in this survival adaption. Because they are hard to spot they build up over several years until you either see the massive buildup of covers, or the plant stops producing  vigorous growth.

The least expensive control for soft and armored scale is applications of 3-4% horticultural oil applied when temperatures reach 50-55F for at least 4 or 5 days. Scale insects reduce their respiration in cold weather but when temperatures reach 50-55 respiration increases. The oil applications act as suffocants, so you want the insects’ respiration rate to be up to obtain successful suffocation. The application should coat with a fine mist the stems and trunk of the woody plant receiving the application.

The other method is to apply an insect growth regulator during the crawler period. To be successful with this you need to identify the scale species to determine when crawlers will be active. The two best insect growth regulators that have minimal impact on beneficial insects are Distance and Talus. They are great products but rather expensive. You may have to go this route if you have really let the scale population build up and the oil applications are just not sufficiently reducing the population.

Contact the Extension entomologist in your state to help identify the scales on your plants. You are welcome to send me good, close-up digital pictures of the scale covers for identification. Send them to [email protected]

Check out these photos of soft and armored scale on cut woody stems. They should help you identify similar insects in your own operations.
Also helpful is a 15-page fact sheet I developed for the Maryland landscape and nursery industry. It includes several photos, describes plants and their damage, as well as the insects’ life cycles. It provides control method recommendations.

You can find it online at www.IPMNET.umd.edu. The title is “Scale Commonly Encountered in Maryland Landscapes and Nurseries.”