Our native Pine Needle Scale (Chionaspis pinifoliae) was once a common and troubling "key pest" back when Mugo pines (Pinus mugo) rivaled yews (Taxus spp.) and junipers (Juniperus spp.) as one of the most common landscape plants in Ohio and Scotch pine (Pinus sylvestris) was THE Christmas tree grown in our plantations. However, that's changed over the years.
As mugos have become less common in our landscapes and Scotch pines largely replaced by other conifers in our Christmas tree plantations, so has the occurrence of pine needle scale become a rare thing in both locations. In fact, I'm now down to only one landscape in southwest Ohio where I can monitor a scale-infested mugo. All of the other locations have removed their mugo "scale trees."
However, that doesn't mean we should turn our backs on pine needle scale; plant pests have a way of sneaking up on us when we do. Beyond Scotch and mugo pines, this native scale may be found on a wide range of conifers including eastern white pine (P. strobus); Douglas-fir (Pseudotsuga menziensii); hemlocks (Tsuga spp.); spruces (Picea spp.); junipers (Juniperus spp.); cedars (Cedrus spp.); and firs (Abies spp.).
Pine needles scale is a type of "armored scale" (family Diaspididae) meaning much of its life-cycle is spent under a hard protective covering. Armored scales insert their long piercing-sucking mouthparts into plant tissue to slurp-up the contents of ruptured plant cells.
This is unlike so-called "soft scales" (family Coccidae) that feed by inserting their piercing-sucking mouthparts into phloem vessels to extract amino acids that are dissolved in the sugary plant sap flowing through the vessels. They discharge excess sap from their anus in the form of a sticky, sugary, clear liquid called "honeydew;" a polite name for scale diarrhea. Of course, the honeydew is commonly colonized by unsightly black sooty molds. Armored scales do not produce honeydew, so infested trees do not become blackened with sooty mold.
As with all armored scales, pine needles scale 1st instar nymphs are the only mobile stage in this sucking insect's life-cycle; thus, the name "crawler." It is also the stage that is most susceptible to insecticide applications. The tiny, dot-like pine needle scale crawlers are dark pinkish-red to rusty-red. Once the crawlers settle to feed, they turn tannish brown. A 10x hand lens is helpful with detecting and monitoring the scale crawlers.
It's a Generational Thing
Overwintered pine needle scale eggs are hatching right now in southern Ohio. This scale has two generations per season with populations expanding considerably with the second generation; the so-called "summer generation." This means reducing the number of first-generation crawlers (the "spring generation") will have a significant impact on decreasing the overall infestation by preventing the population contribution by the second generation.
Also, first generation eggs typically hatch over a relatively short period of time meaning that if management includes the use of a topical insecticide, a single application may be sufficient. Second generation eggs hatch over a prolonged period of time often requiring multiple applications depending on the residual activity of the insecticide product.
Spring generation crawlers hatch from overwintered eggs when the accumulated Growing Degree Days (GDDs) reach 305. Although common lilac (Syringa vulgaris) reaches full bloom at around 315 GDDs, I've found this to be a pretty good indicator plant for the appearance of 1st generation crawlers.
Research has shown that a number of bio-allies such as lady beetles and other scale predators as well as parasitoids play an important role in holding scale populations below damaging thresholds. Insecticidal soaps and horticultural oils are effective against scale crawlers and will spare the beneficial insects. The downside is that both will only kill on contact meaning that thorough coverage is required since there is no residual activity. This also means pine needle scale crawlers should be closely monitored in case an extended egg hatch occurs and new crawlers escape the first application
Many standard insecticides labeled to control armored scales on the infested conifer species are also effective against scale crawlers. However, the downside is that these products may also kill bio-allies.
The systemic neonicotinoid, dinotefuran (e.g. Safari, Transtect, Zylam, etc.), has proven effective against armored scales and will have a limited impact on beneficials. However, applications must be made prior to egg hatch to allow time for the active ingredient to reach plant tissue in concentrations sufficient to kill the crawlers. The neonicotinoid, imidacloprid (e.g. Merit), is not effective against armored scales. In fact, some research studies have shown applications of this systemic insecticide actually contributes to scale outbreaks.