The oak lecanium scale, Parthenolecanium quercifex, is a common pest of ornamental trees and shrubs in the eastern US. This soft scale insect (Hemiptera: Coccidae) is primarily a pest on oak but has also been recorded on other species including hickory (Carya spp.) and birch (Betula spp). This insect is very similar to other closely related lecanium scale species, and as such there is often question about their identity as well as whether or not a cryptic species complex exists. Another very closely related insect, the European fruit lecanium or P. corni (also apparently native despite what the common name suggests) is found on trees and shrubs in the eastern US and may be confused with the oak lecanium scale.
The oak lecanium scale has one generation per year. Eggs are laid underneath adult females that have a distinctive hemispherical or helmet shape. The number of eggs laid is dependent on many factors and literature reports a range from dozens to several thousands of eggs per female. Eggs are laid in late May and into June. Eggs hatch in June or early July and the crawlers (immatures) migrate to host plant leaves. (Historically, reports from New York indicate that crawler activity occurs in mid-July.) The crawlers feed on the undersides of the leaves, typically along the main veins, using piercing-sucking mouthparts to remove host plant fluids. The first instars are tiny <1 mm. and are pale yellow/brown in color. The insects molt and become second instars in the late summer/fall and then migrate back to woody tissues, such as host plant twigs, where they will overwinter. (Crawlers are also flattened in comparison to the adults.) In the spring, females begin to enlarge and mature, eventually taking on the distinctive hemispherical shape. Adult females are approximately 4-7 mm. long and 3-5 mm. wide and vary in color from light-dark brown/gray. Adult female oak lecanium scales almost always possess a pair of lateral humps on their dorsal (back) surface when mature.
Lecanium scales feed on the phloem of trees and shrubs generally reducing vigor. High populations can result in stunted foliage, chlorosis, twig death, and dieback, particularly due to the feeding female scales from April-May. Their feeding habits result in copious amounts of honeydew (excrement) that can cover plants and structures beneath infested trees. Honeydew, rich in sugar, may provide a substrate for sooty mold to grow. Sooty mold is black in color and it is not a plant pathogen; however, it can reduce the aesthetic value of the plant it covers. Honeydew and sooty mold in large amounts may both also be a significant nuisance on structures including outdoor furniture, patios, cars, etc.
If oak lecanium scale populations are expected to be high in an area due to a cycle of persistent outbreak, monitor for overwintering second instar nymphs (immatures) in the late winter/early spring on host plant twigs. If present in large numbers, dormant oil applications may be planned according to label instructions and when temperatures and abiotic conditions allow. Dormant oil applications are reduced risk and will help preserve important natural enemies. If excessive amounts of honeydew and sooty mold are found on oak in the spring, search branches for "poofing up" or expanding female soft scales which may be feeding abundantly at that time.
Trees under drought stress in managed landscapes or urban forests may have more trouble with oak lecanium scale infestations than those without stress. Practices such as adequate watering during these times may help support tree health and reduce additional stressors. Prune and remove and destroy heavily infested branches if possible without compromising tree shape or health.
Lecanium scales are known to have a large number of predators, parasitoids (ex. a tiny parasitic wasp, Encyrtus fuscus), and pathogens which often keep populations under control. Populations of lecanium scale are known to rapidly increase especially on urban trees or trees under stress. Monitoring trees for lecanium scale is an important part of management due to the ability for populations to rapidly increase as well as the likelihood of predators and parasitoids to control populations. (Therefore, management may not be necessary from year to year, but rather occasionally when population outbreaks occur. It is also important to preserve natural enemies.)
Some research shows that lightly infested trees, those with low-level infestations of lecanium scales such as the oak lecanium, are beneficial for their landscape because those trees help provide the surrounding environment with more natural enemies. These trees act as reservoirs for these important natural aids in pest management! So the goal in managing the native oak lecanium scale should never be to completely eradicate it. For more information, read "Scale Insects Support Natural Enemies in Both Landscape Trees and Shrubs Below Them" (Wilson and Frank, 2022).
Abamectin (NL)
Acephate (NL)
Acetamiprid (L)
Azadirachtin (NL)
Buprofezin (NL)
Carbaryl (L)
Chlorpyrifos (N)
Clothianidin (NL)
Cyantraniliprole (NL)
Cyfluthrin (NL)
Dinotefuran (NL)
Flonicamid+cyclaniliprole (N)
Gamma-cyhalothrin (L)
Horticultural oil (L)
Imidacloprid (L)
Insecticidal soap (NL)
Lambda-cyhalothrin (L)
Neem oil (NL)
Pyrethrin + sulfur (NL)
Pyriproxyfen (L)
Spinetoram + sulfoxaflor (N)
Active ingredients that may be applied systemically include: abamectin (injection), acephate (injection), acetamiprid (injection), azadirachtin (injection, soil drench), clothianidin (soil drench), cyantraniliprole (soil drench, soil injection), dinotefuran (soil drench), imidacloprid (soil drench), and neem oil (soil drench).
When used in a nursery setting, chlorpyrifos is for quarantine use only.
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
