Dock Sawfly is Mystery Insect

During apple harvest this year, some apple growers have noticed the presence of circular 2 mm holes in the side of the fruit. These holes are caused by the larvae of the dock sawfly (Ametastegia glabrata), a sporadic pest of apples in North America. Apparently dock sawfly likes ‘Honeycrisp’.

There are four generations of dock sawfly each year. Generally the insects feed on weeds such as dock, sorrel, knotweed, and bindweed that can be commonly found in the orchard. However, the later generations of this insect begin looking for over-wintering sites in late summer, and early fall, as the fruit begins to approach maturity. The bright green larvae (black head) burrow into the apple, leaving small round holes, which usually develop a brown discolouration around the entrance to the hole. After the larvae has tunneled into the fruit it begins to burrow out to pupate. Since the dock sawfly requires the presence of weeds (docks, sorrels, knotweeds) to survive, the best way to manage this pest is by using selective herbicides to manage these weeds in the orchard. Fortunately the adults are not strong fliers, so these insects are not likely to move into an orchard from nearby ditches and meadowlands. Also keeping heavily fruited branches from touching the ground may help to reduce damage from this pest. See attached figures.