Glistening, slug-like sawfly larvae along with their characteristic skeletonizing damage are appearing on oaks belonging to the white oak group in southwest Ohio. The upper leaf surface appears bleached with leave veins remaining apparent. Flipping the leaves over reveals the slug-like culprits stuck to the underside of the leaves.
The odd-looking larvae have semi-transparent bodies that are flattened towards the front and tapered towards the back. The larvae coat themselves in a slimy exudate thought to be produced by glands near their head. The coating causes them to glisten in the sun.
Research suggests the sticky mucoid-like slime allows the larvae to adhere to the underside of leaves where they feed. These types of sawfly larvae are generally referred to as “slug sawflies” owing to their shape coupled with their shimmering coating.
An Identification Conundrum
The slug sawflies feeding on oak were long identified in past BYGL postings as being the Scarlet Oak Sawfly (Caliroa quercuscoccineae (Dyar)), family Tenthredinid). However, two pivotal papers on the biology of the scarlet oak sawfly (see Literature Cited) note the sawfly larvae only feed on oaks in the red oak group.
This presented an identification conundrum considering that I’ve only ever observed oak slug sawfly larvae skeletonizing oaks in the white oak group. In fact, I’ve never observed oak slug sawfly larvae feeding on oaks in the red oak group in southwest Ohio.
Adding to the identification confusion, in 2015, I observed and photo-documented similar-looking slug sawflies skeletonizing American bladdernut (Staphylea trifolia) leaves in southwest Ohio. Unlike those observed on some oaks, the larvae were feeding in a random, meandering fashion.
Thanks to some serious sleuthing by others, it was eventually found that the true identity of the larvae on bladdernut is C. lunata MacGillivray. I summarized the discovery in the BYGL Alert titled, “Bladdernut Slug Sawfly Mystery Solved,” (September 7, 2021). You can access the Alert by clicking on this hotlink: https://bygl.osu.edu/node/1862
Further clarification of the host range of various slug sawflies was provided in a paper published in 2022 (see “Literature Cited”). Indeed, the authors cite the observations published in the 2021 BYGL Alert.
However, it remains clear that while it’s likely the “white oak slug sawflies” noted in this Alert belong to the genus, Caliroa, there is a distinct possibility that there are several species involved that are yet to be identified and linked to specific oak hosts. For example, the image below shows sawfly larvae sporting “sunglasses.” I’ve only observed these odd markings on late instar larvae while others have reported the markings in early instar stages.
Clarifying the exact species satisfies more than academic interest. Research data on the biology of one species, such as the number of generations per year, may not apply to another even if both belong to the same genus.
Keep in mind that these observations may apply to more than one white oak slug sawfly species. As noted above, the exact species complex may not yet have been identified.
All larval instars of the white oak slug sawflies reported in this Alert feed gregariously on the lower leaf surface consuming everything except for the veins and upper leaf epidermis. Some feed side-by-side creating feeding patterns that may appear as longitudinal stripes, fans, or circular patterns. I’ve observed this larval feeding behavior on Burr Oak (Quercus macrocarpa), White Oak (Q. alba), and Swamp White Oak (Q. bicolor).
However, oak slug sawflies on Irish Oak (Q. petraea, a.k.a. Sessile Oak, Cornish Oak, Durmast Oak) were observed feeding in a meandering fashion much like the bladdernut slug sawflies. Of course, it should be noted that when the larvae that feed side-by-side are disturbed, they may start to wander around individually. However, as shown in the image below, the larvae on Irish oak are not fleeing, they are feeding.
Whether the larvae are feeding side-by-side or randomly, initially, the upper epidermis has a faded, whitish appearance. Eventually, the epidermis dries out, turns brown, and drops from the leaf leaving behind the veins to produce the characteristic skeletonizing symptom associated with this sawfly.
Some of the white oak slug sawflies in Ohio have at least 2 generations per season. I’ve taken pictures in the past of early instar larvae appearing in mid-May and again in early July on the same tree. On the other hand, as shown in the pictures of the sawfly larvae on sessile oak, with early instars feeding alongside later instars, the generations may overlap, or egg hatch may occur over an extended time. Either way, the damage could escalate as the season progresses.
Thus far, I’ve only come across small populations of white oak slug sawflies in southwest Ohio with only a few leaves affected, and sporadic with few trees affected. The damage has been inconsequential relative to overall tree health.
However, these sawflies should be closely monitored for possible population outbreaks. The scarlet oak sawfly has a history in Ohio of producing significant defoliation on oaks in forests and landscapes. It’s probable the white oak slug sawflies are governed by the same population dynamics.
Nordin, G.L., and E.L. Johnson. 1983. Biology of Caliroa quercuscoccineae (Dyar) (Hymenoptera: Tenthredinidae) in Central Kentucky I. Observations on the Taxonomy of Principal Life Stages, Journal of the Kansas Entomological Society, Vol. 56, No. 3, pp. 305-314
Nordin, G.L., and E.L. Johnson. 1984. Biology of Caliroa quercuscoccineae (Dyar) (Hymenoptera: Tenthredinidae) in Central Kentucky II. Development and Behavior Journal of the Kansas Entomological Society, Vol. 57, No. 4, pp. 569-579
Eiseman, C.S., Smith, D.R. and Woods, P., 2022. Nearctic “Slug” Sawfly Larvae of the Genus Caliroa Costa (Hymenoptera: Tenthredinidae): New Rearing Records and a Summary of Hosts, Descriptions, and Distribution Records. Proceedings of the Entomological Society of Washington, 124(2), pp.225-244.