Dusky Birch Sawfly (Croesus latitarsus) larvae and Spiny Oak Sawfly (Periclista albicollis) larvae are appearing on their namesake hosts in southwest Ohio. Both belong to the hymenopteran sawfly family Tenthredinidae. They seldom cause significant injury to the overall health of their host trees; however, both can produce noticeable damage, drawing the attention of horticulture professionals and home gardeners.
Caterpillar-like sawfly larvae are sometimes mistaken for caterpillars and targeted with the biological insecticide Bacillus thuringiensis var. kurstaki (Btk). This bacterium will kill moth caterpillars (order = Lepidoptera) if ingested. However, sawflies belong to the order Hymenoptera (e.g., wasps, bees, and ants). Thus far, no Bt strains have been discovered that will kill hymenopteran insects.
The simple way to tell the difference between lepidopteran caterpillars and hymenopteran sawfly larvae is to count the number of abdominal prolegs; the fleshy "extra legs" that are lost during pupation. Counting from front to back, the first 3 pairs of legs just behind the head are the hardened "thoracic legs." These legs will be found in the same position on the adults. Following the thoracic legs are pairs of fleshy legs called prolegs. The last pair of prolegs are called the “anal prolegs,” and they won’t be counted.
Sawfly larvae have 6 or more pairs of abdominal prolegs; caterpillars have 5 or fewer pairs of prolegs. A handy way to remember this identification tip is to compare the number of prolegs to the number of fingers on your hand.
Dusky Birch Sawfly
Participants in the OGIA/OSU Extension Diagnostic Walkabout in Wegerzyn Gardens MetroPark in Dayton came across aggregations of Dusky Birch Sawfly larvae munching on the leaves of its namesake host. The aggregations may include 10 - 20 individuals.
All instars have shiny black head capsules and distinct black spots on their bodies. When disturbed, the larvae form their bodies into an "S" shape (S for sawfly?). Early instar larvae are dark gray, middle instars are greenish-gray, and late instars are yellowish-green in color.
The larvae feed like a conga line along the leaf edges. Early instars consume interveinal leaf tissue, leaving behind the midvein and lateral veins. Middle and late instars consume the entire leaf blade, leaving behind stick-like midveins.
Dusky birch sawfly larvae seldom require control measures owing to their focused feeding behavior, population dynamics, and impacts from natural controls. The larval aggregations mean that feeding is confined to individual branches. The aggregations are easily dispatched using digital compression (= squeezing with your fingers). It’s extremely rare for an entire tree canopy to be affected.
The sawfly has a year-to-year history best described as "now you see them, now you don't." Seasons with high populations are usually followed by seasons with low sawfly populations. Indeed, I've never observed multi-season outbreaks in Ohio.
Finally, although dusky birch sawflies are reported to have two generations per season in Ohio, I’ve never observed increases in larval population densities and subsequent damage as the season progresses. I suspect that predators, parasitoids, and pathogens (the 3-Ps) that arrive on the scene during the first generation limit the development of larger populations in the second generation.
Spiny Oak Sawfly
I recently came across tattered bur oak (Quercus macrocarpa) leaves while checking for pest and disease issues in a local park in southwest Ohio. A close examination of the undersides of the leaves revealed sawfly larvae that were small, light-green, and semi-transparent with shiny black head capsules.
The larvae are covered with rows of short, forked (bifurcated) spines. Their small size, coupled with their coloration, allows the larvae to easily blend with their leaf-food background.
I’ve tentatively identified the sawfly as Periclista albicollis, which I called the “Spiny Oak Sawfly” in a past BYGL Alerts. My pictures match those posted in Bugguide [ https://www.bugguide.net/node/view/1455785 ].
On the other hand, iNaturalist has postings of a sawfly identified as the Brown Bordered Oak-Cutter (P. albida). The images also look remarkably similar to my pictures [ https://www.inaturalist.org/taxa/601923-Periclista-albida ]. Oddly, iNaturalist has no postings for P. albicollis, and Bugguide has no postings for P. albida. Unfortunately, I fell into a taxonomic black hole trying to sort things out.
In past years, I have found these sawfly larvae in southwest Ohio feeding on bur, chinkapin, swamp, and white oak. Early instar larvae feed primarily on the underside of newly emerging and expanding leaves. They produce small holes that get larger as the leaves expand.
Later instars consume interveinal tissue, leaving behind the main veins. The accumulated damage produced by all instars eventually gives oak leaves a tattered appearance.
Based on my past observations, I believe there is only one generation per season, with the larvae pupating sometime in May. Thus, this sawfly has the potential to produce holes in newly expanding leaves and then leave the scene of their crime before being discovered. This may create a diagnostic challenge because their small holes become large holes once leaves fully expand, but by that time, the sawfly has left the building.
