The characteristic damage produced by Oak Slug Sawflies (Caliroa spp., family Tenthredinidae) is evident on their namesake hosts in southwest Ohio. Some of the damage occurred earlier in the season, while other damage is occurring right now as illustrated by the introductory image for this Alert showing slug sawflies and their damage on red oak (Quercus rubra).

Oak slug sawflies feed on the lower leaf surface; however, their damage is most obvious when the leaf is viewed from above. They consume the lower epidermis, spongy mesophyll, and palisade mesophyll, leaving behind the veins and upper leaf epidermis.

Oak Slug Sawfly Larvae on Chinkapin Oak (Quercus muehlenbergii)

The slug sawfly larvae first produce small areas that look like decorative glass (the “windowpane effect”) on the upper leaf surface. Eventually, entire leaves appear skeletonized.

Early Instar Oak Slug Sawfly Larvae on Burr Oak (Quercus macrocarpa)

Early Instar Oak Slug Sawfly Larvae on Burr Oak (Quercus macrocrpa) close up

Oak Slug Sawfly Damage on Burr Oak (Quercus marocarpa)

Oak Slug Sawfly Damage on Chinkapin Oak (Quercus muehlenbergii)

Oak Slug Sawfly Damage on Chinkapin Oak (Quercus muehlenbergii) 2

The glistening, slug-like larvae have semi-transparent bodies and 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.

Oak Slugs Sawfly Larvae on Burr Oak (Quercus macrocarpa) 2

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.

The last instars do not produce the slimy exudate, so they don’t stick to leaves. This causes them to drop from the ground where they pupate.

The slug sawfly larvae may feed side-by-side, creating feeding patterns that may appear as longitudinal stripes, fans, or circular patterns. Or the larvae may feed in a meandering fashion, as shown on red oak below. Of course, it should be noted that when the larvae that feed side-by-side are disturbed, they may start to wander around individually.

Oak Slug Sawfly Larvae on Chinkapin Oak (Quercus muehlenbergii)

Oak Slug Sawfly Larvae on Red Oak (Quercus rubra)

Oak slug sawflies are native to North America and seldom cause enough damage to rise above the status of being an oddity. However, there are occasional outbreaks.

The most notorious is the Scarlet Oak Sawfly (C. quercuscoccineae). In 1997, aerial surveys by the Ohio Department of Natural Resources revealed that the sawfly had damaged 174,197 acres of forest oaks in Adams, Scioto, and Lawrence counties. In 1998, 294,426 acres were damaged in the same counties. In the 1970s, over 1.5 million acres of oaks were damaged by the scarlet oak sawfly in Kentucky.

As its common name implies, scarlet oak sawfly larvae confine their feeding to oaks in the red oak group. Indeed, this is true for several Caliroa species.

However, over the past several years, oaks in the white oak group have been the most common target for oak slug sawflies in southwest Ohio. The larvae generally feed side-by-side, and I’ve observed this larval feeding behavior on Burr Oak (Q. macrocarpa), Chinkapin Oak (Q. muehlenbergii). White Oak (Q. alba), and Swamp White Oak (Q. bicolor).

Oak Slug Sawfly Larvae on White Oak (Quercus alba)

However, oak slug sawflies on Irish Oak (a.k.a. Sessile Oak, Cornish Oak, Durmast Oak) (Q. petraea) were observed feeding in a meandering fashion. As shown in the image below, the larvae on Irish oak are not fleeing; they are feeding.

Oak Slug Sawfly Larvae on Irish Oak (Quercus petraea)

Oak Slug Sawfly Larvae on Irish Oak (Quercus petraea) 2

Although I’m no expert on identifying the larvae, the only member of the Caliroa genus listed in the literature that feeds on oaks in the white oak group is C. obsoleta. Oddly, this species has also been observed feeding on Quaking Aspen (Populus tremuloides).

Another peculiar attribute of slug sawflies that I’ve observed feeding on oaks in the white oak group is the appearance of odd-looking black markings at the front of late instar larvae. The markings resemble sunglasses as shown on the larvae in the image below. They were feeding on Chinkapin Oak (Q. muehlenbergii) in a wooded park in Butler County, OH.

Oak Slug Sawfly Larvae on Chinkapin Oak (Quercus muehlenbergii) 1

Oak Slug Sawfly Larvae on Chinkapin Oak (Quercus muehlenbergii) 2

American bladdernut (Staphylea trifolia) and Caucasian bladdernut (S. colchica) may be skeletonized by C. lunata. The larvae strongly resemble slug sawflies that I’ve found on oaks in the white oak group; however, when I tried to feed the bladdernut sawfly larvae white oak (Q. alba) leaves, the larvae died rather than eating the foliage. Another key difference is that the bladdernut slug sawflies feed in a meandering fashion, much like those photographed on Irish oak.

Bladdernut Slug Sawfly (Caliroa lunata)

Bladdernut Slug Sawfly Damage

A considerable amount of work has been done in recent years on unraveling the taxonomy and lifestyles of members of the Caliroa genus. However, it remains apparent that more investigations are warranted. For example, the number of generations per season is unknown for some species, and more generations mean more damage.

Thus far, I’ve only come across small, sporadic populations of oak slug sawflies in southwest Ohio, with only a few leaves affected. The damage has been inconsequential relative to overall tree health.

However, these sawflies should be closely monitored for possible population outbreaks. As noted, the scarlet oak sawfly has a history in Ohio of producing significant defoliation on oaks in forests and landscapes. The slug sawflies that feed on oaks in the white oak group are probably governed by the same population dynamics.

Selected References

Eiseman, C. S., & Smith, D. R. (2020). New sawfly (Hymenoptera: Argidae, Tenthredinidae) host records from northeastern North America. Proceedings of the Entomological Society of Washington, 122(2), 299-307.

https://doi.org/10.4289/0013-8797.122.2.299

Eiseman, C. S., Smith, D. R., & 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), 225-244.

https://doi.org/10.4289/0013-8797.124.2.225

Nordin, G. L., & Johnson, E. L. (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, 305-314.

https://www.jstor.org/stable/25084416

Nordin, G. L., & Johnson, E. L. (1984). Biology of Caliroa quercuscoccineae (Dyar)(Hymenoptera: Tenthredinidae) in Central Kentucky II. Development and behavior. Journal of the Kansas Entomological Society, 569-579. https://www.jstor.org/stable/25084565