Slug Sawfly on American Bladdernut

Published on

In 2015, I reported that I had found sawfly larvae skeletonizing American bladdernut (Staphylea trifolia) leaves in southwest Ohio (see BYGL Bug Bytes, September 3, 2015, “Scarlet Oak Sawfly on Bladdernut?”).  The title of that report was based on the appearance of the larvae:  they were the spitting image of Scarlet Oak Sawfly (Caliroa quercuscoccineae) which is sometimes called the scarlet oak slug sawfly or just oak slug sawfly.  The “slug” in the common names are based on their elongated unsegmented bodies and a covering of mucoid-like slime that presumably helps them stick to the undersides of leaves.


Scarlet Oak Sawfly


However, the scarlet oak sawfly appears to confine its feeding to its namesake host as well as a few other oaks (see the BYGL Alert! posted on June 14, 2017, “1st Generation Scarlet Oak Sawfly Larvae”)  At the time, I found no slug sawflies on several nearby oaks; prime fodder for the scarlet oak sawfly.


I’ve monitored the bladdernut trees since that time and failed to find any sawfly larvae in the intervening years, until earlier this week.  Once again, the larvae look identical to scarlet oak sawfly and they practice the same feeding behavior by munching side-by-side across the lower leaf surface to produce similar skeletonizing damage.


The slimy larvae consume everything except the veins and upper leaf epidermis causing the upper epidermis to initially have a faded, whitish appearance.  The epidermis eventually dries out and drops from the leaf leaving behind the veins to produce a skeletonized appearance.  Occasionally, the damaged bladdernut leaves simply turn brown and become paper-like.


Bladdernut Slug Sawfly


Bladdernut Slug Sawfly Damage


Bladdernut Slug Sawfly Damage


Bladdernut Slug Sawfly Damage


Again, I could find no sawfly larvae feeding on nearby oaks; even on a tree with branches crossing an infested bladdernut.  Based on these observations and several online reports about an undescribed slug sawfly feeding on bladdernut, I’ve concluded several things.


First, I agree with some of the online reports that this is an undescribed species, most likely in the genus Caliroa.  However, until larvae are reared to adults and examined by a taxonomist, this conclusion remains hypothetical.  Second, scarlet oak sawflies have 2 – 3 generations per season in Ohio; I believe the “bladdernut slug sawfly” also has at least two generations.  My 2015 bladdernut slug sawfly observation was made in early September and I’m currently seeing what appear to be middle-instar larvae; thus, it seems reasonable that this sawfly has two generations.  Third, although I saw extensive damage in 2015, the trees fully recovered indicating this sawfly may cause limited harm to the overall health of their bladdernut host.  Finally, the disappearance of the bladdernut slug sawfly indicates there may be a number of bio-allies (e.g. insect pathogens, parasitoids, and predators) that can regulate the sawfly populations.


Paraphrasing a well-known Marxism (Groucho):  those are my conclusions, and if you don’t like them … well, I have others.


Please let me know if you see the bladdernut slug sawflies.  Just drop me an e-mail message with the date and location.