Trouble Maker of the Month: Oak Lecanium Scale

The following is an article from the UMASS Extension Hort Notes Volume 30.5

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 name suggests) is found on trees and shrubs in the eastern US and may be confused with the oak lecanium scale.   

Oak lecanium and European fruit lecanium are so similar that identification to species requires examination of adults under high magnification or by analyzing DNA. However, even while employing both of those techniques, entomologists may still have a difficult time discerning the two species. Beginning in the summer of 2018 and again in the late spring and early summer of 2019, a widespread outbreak of lecanium scale activity on oak was reported on much of Cape Cod and Martha’s Vineyard. Samples in 2019 from Cape Cod (collected by the Elkinton Lab) and Martha’s Vineyard (originally submitted to the UMass Plant Diagnostics Laboratory) were given to Dr. Joseph Elkinton’s Laboratory (UMass) for analysis using DNA and samples from Martha’s Vineyard were sent to the USDA-ARS-Systematic Entomology Lab in Beltsville, MD for further identification. Jeremy Andersen of the Elkinton Lab reported the following: 

“We extracted DNA from the samples provided (immature females) [from Martha’s Vineyard] and compared them to adults collected from oaks on Cape Cod earlier this month.  The individuals from both locations were an exact match genetically.  Morphological identifications of the adult specimens using the key presented in Gill 1988 suggest that they are Parthenolecanium quercifex (oak lecanium).  However, there is much confusion in the literature as to whether or not oak lecanium is in fact a valid species or whether it should be considered the same species as European fruit lecanium (Parthenolecanium corni).  Both species have a tendency to reach high densities when trees are stressed by other insect pests, droughts, or lack of nutrients, and/or when natural enemies are affected due to non-target effects from insecticides.”

Dr. Gregory Evans of the USDA-ARS-Systematic Entomology Lab also used morphology to identify insects originating from Martha’s Vineyard as the oak lecanium, Parthenolecanium quercifex. 

The oak lecanium scale has one generation a year.  Eggs are laid underneath adult females that have a distinctive hemispherical or helmet shape (Fig. 1).  The number of eggs laid is dependent on many factors and literature reports 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 imbibe 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 (Fig.2).  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.  

Lecanium scales are known to have a large number of predators, parasitoids, 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.) 

An insecticidal oil application may be made as a dormant spray, targeting the overwintering 2nd instar nymphs on the host plant twigs during that time of year, at approximately 35-145 GDD, base 50°F. Such reduced risk active ingredients may help avoid unintended suppression of parasite/predator activity. In July, or August, following egg hatch, crawlers may be targeted on the leaves with a foliar applied insecticide if damage symptoms are seen. (Again, preserving natural enemies is very important, and broad-spectrum active ingredients can kill natural enemies as well as pest insects.) Systemic insecticides applied through the soil or bark are also labelled for this pest, and may be applied after bloom. 

Active ingredient options labelled for use against soft scales include, but are not limited to: abamectin, acephate, acetamiprid, bifenthrin, carbaryl, dinotefuran, horticultural oil, imidacloprid, insecticidal soap, neem oil, and pyrethrins. Read and follow label instructions for safety and proper use. Each active ingredient comes with different risks to the applicator and the environment. A summary of some of these risks may be found here, but this does not replace reading and following label instructions.

Russ Norton, Horticulture Extension Educator, Cape Cod Cooperative Extension and Tawny Simisky, UMass Extension Entomologist