Two apples were submitted from Douglas County at the end of September that were infested with small, 1/8 inch long, brownish or grayish scale insects. This was not easy to identify at first as there aren't any common scales that attack the fruit of apples in Minnesota. However, after checking with a couple of colleagues, the problem was discovered to be oystershell scale.
This common scale is known to attack a wide variety of hardwood trees, including apple, typically feeding on branches and twigs. It uses piercing-sucking mouthparts to feed on the sap from plants. Small numbers of this insect are not likely to cause much damage, although heavy infestations can discolor and stunt the leaves and weaken and even kill branches.
The literature does describe oystershell scale as occasionally attacking the fruit of apples as well. Although this scale does not seem to damage the fruit too much, its appearance does mar the surface of the apple. When oystershell scale are found on apples at this time of year, there are not many options. You could try to remove scales from individual apples but that could still leave them unappealing to eat. There aren't any practical options to treat these insects at this time of the year.
Fortunately, this is not a common problem and it is unlikely a home orchardist will have an ongoing issue with oystershell scales. If a gardener is faced with a persistent infestation, the best management is to treat the scale when the immature crawlers hatch and are active which is generally in the spring about late May or early June.
Originally published in Yard and Garden Line News, November 2007