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A sometimes severe pest of oaks, oak lecanium scale is recognizable by the very convex shape of the adult females. The brown to reddish-brown “shield” that covers the insects is nearly hemispherical, and protects the insect from many pesticide sprays. Lecanium scales feed by sucking plant juices from leaves. The feeding activity of large populations of lecanium scale is very taxing to the affected tree and can cause stunting and leaf drop.
Lecanium scale also produce large amounts of honeydew, creating a sticky mess under affected trees. The honeydew can then cause a problem with sooty mold which can affect underlying plants and even surfaces like outdoor furniture, exterior walls or other objects and structures.
The best time to treat lecanium scale is in spring after the eggs have hatched and the vulnerable juvenile form (called a crawler) is moving around the tree to find a location to settle permanently and feed. Small trees can be sprayed with any pesticide labeled for soft scale control; larger trees can only be treated with systemic pesticides applied as a drench. Dormant oil sprays in winter can help reduce lecanium scale populations, but the pest can be difficult to control and generally takes several consistent pesticide applications that may span a few years.
To reduce the chances of lecanium scale affecting your landscape, minimize stresses to your trees and shrubs, including regular (adequate, not excessive) fertilization and supplemental irrigation during drought.