Kermes scale on bur oak
Defoliation of bur oak in northwestern Minnesota counties by Kermes scale insects of the insect order Hemiptera. One family, the Kermesidae, or gall scales are distinct for their feeding on oak host species. Most scale insects are small, 10mm or less, and may be found on only certain parts of the host plant.
Scale insects are among the most destructive agents on ornamental shrubs and trees. The first-instar nymph called a "crawler" emerges from the female egg sac and inserts its mouth parts into the leaf petiole and begins to feed by sucking plant juices from the petioles causing a disruption of normal sap flow from the leaves. Abnormal growth of the petiole causes a swelling or gall to develop resulting in wilting and discoloration of the leaves. Twig and branch die back are possible, even death of the plant if the scale population is high and the trees are stressed from other factors. Scales and aphids usually become numerous during periods of wet conditions preceded by droughty times. Natural enemies generally provide necessary controls unless the trees have been severely stressed by drought or flooding prior to becoming infested.
Chemical control requires use of a dormant oil such as horticultural oil or insecticidal soap are recommended, and should be applied in the spring during the time of bud-break and repeated after ten days following the first spray to affect the crawlers that hatched after the initial treatment.