What’s bugging our oaks?

by | Oct 2, 2023

If you’ve recently driven through Johnston, Wayne or Duplin counties, you may have noticed oaks along the highway with brown or wilted branches. No, it is not the early signs of fall. Upon closer examination, these trees appear inundated with galls. These abnormal growths that form on leaves or twigs are in response to egg-laying and feeding by aphids, wasps, mites or midges. However, the culprit is the kermes oak scale, a group of insects commonly mistaken for galls.

Kermes oak scale where twig dieback is occurring.

Tree-branch flagging on water oak.

With approximately 8,000 described species worldwide, scales are a large, diverse group of piercing-sucking insects closely related to aphids and white flies. In North America, there are approximately 30 described species of kermes oak scale. Although, their taxonomy remains unresolved because they have not been well studied.

Scales in general are more of a specialist, and typically feed from one or a few tree species. Kermes oak scale is a soft scale that inserts its tiny, straw-like mouthparts into the bark of a tree to feed on sap (phloem). As their name suggests, Kermes oak scales feast on oak trees, primarily the red oak group which includes willow oak (Quercus phellos), water oak (Quercus nigra) and black oak (Quercus velutina). Due to a sugar-laden diet, soft scales like the kermes oak scale excrete a sweet, sticky substance called honeydew. Interestingly, ants and other insects are drawn to this sugary liquid and will guard scale insects from predators to farm the honeydew produced by the scales.

The exact timing of their life cycle is species dependent. Generally, adult males and females die soon after mating and laying eggs. Two to four generations are common in the southeast, where temperatures are warmer, while only a single generation occurs from Virginia northward. Females are sedentary, spending nearly all their lives in one place. Fertilized eggs are deposited into the female’s broad chamber and then covered with a protective wax. Depending on the species, scales may overwinter as eggs within this protective chamber, or as late-stage nymphs concealed within furrows of the bark. Nymphs, also called crawlers, have antennae and legs and are only mobile for a few days after hatching. Crawlers typically pass through three instars before reaching sexual maturity. Following the second molt, females migrate to a feeding site (typically new shoots near a leaf axil), insert their piercing-sucking mouthpart, lose their appendages and create a waxy “shell” as they mature. At the same time, males pupate and emerge with a single pair of wings. Males do not feed as adults as they lack mouthparts.

Adult female kermes oak scales are wingless with reduced legs and antennae. They are tan to reddish brown and range in size from three to six millimeters in diameter, resembling galls present on stems where stem dieback is developing. Adult males are small, red to light brown and have a single pair of delicate looking wings. Males are rarely observed in nature but can be found on the trunk of their host tree or in leaf litter.

Kermes scale moth (Euclemensia bassettella). Photo by Mark Dreiling, Bugwood.org

Kermes oak scale guarded by ants.

Though kermes oak scale insects may seem destructive, removal is not usually necessary as healthy trees can generally endure moderate infestation. Common symptoms of infestation include twig dieback, or flagging, and the accumulation of sooty mold, a dark colored fungus that grows on honeydew excreted by piercing-sucking insects like scales, causing twigs to blacken. Natural predators such as lady beetles, lacewings, parasitic wasps and predaceous moth larvae (Euclemensia bassettella) typically keep scale populations in check. However, successive years of heavy infestations combined with other stressors such as tree damage from other insects, disease or environmental may result in reduced growth rate or death.

Limbs of oak trees heavily infested with kermes oak scale used in home landscapes can be pruned. Individual scales can also be removed by hand. Dormant oil sprays containing paraffine can be used on females during the dormant season, while common insecticides containing imidacloprid can be applied when crawlers are active before they form their protective “shell.” The best way to detect active crawlers is to hold a white sheet or piece of paper under a suspected limb. Shake the limb, knocking the crawlers loose. If crawlers are present, they will appear as small, dark specks on the move. A hand lens can be handy for confirmation. Remember, any insecticide must be used in compliance with its label. Misuse may result in the unintended death of natural predators, resulting in increased scale populations.

One cause for concern relative to scale insects is that feeding sites create a point of entry for fungal diseases. The added stress can also trigger more complex issues for already unhealthy trees. In the Midwest, northern red oaks (Quercus rubra) are experiencing a ‘one-two punch’ from a combination of kermes oak scale and fungal canker/dieback caused by a Botryosphaeria bacteria. However, this does not seem to be the case in North Carolina. If you suspect an infested tree in your area showing signs of excess dieback or flagging, contact your NCFS county ranger. To find contact information for your county ranger, visit https://www.ncforestservice.gov/contacts.