Oak Sawflies

Oak Sawflies are tiny fly-like, nonstinging wasps. The short-lived adults feed on pollen and nectar and do not cause any feeding damage, but their caterpillar-like larvae, on mass, can create extensive damage to oak canopies. There are several species of Oak Sawflies such as scarlet oak sawfly (Caliroa quercuscoccineae) which is native to the eastern United States and feeds on white oak, scarlet oak, pin oak and black oak and can be found in Canada in the prov. of NF, ON and QC as well as many American states (Eiseman C. S., Smith D. R., and Woods P., (2022)). There has been some recent talk that Caliroa quercuscoccineae does not feed on white oak and that species has been misidentified (Boggs J. (2022)). Caliroa annulipes also feed on oak as well as several other species and can be found in Canada in the prov. of BC and ON as well as Europe (Eiseman C. S., Smith D. R., and Woods P., (2022)). Caliroa fasciata feed on pin oak, northern red oak, and black oak and can be found in Canada in the prov. of ON and QC and several American states (Eiseman C. S., Smith D. R., and Woods P., (2022)). Caliroa lobata (which is almost indistinguishable from Caliroa quercuscoccineae), feed on pin oak, northern red oak, and black oak, and bladder nut; they can be found in Canada in the prov. of BC, NS, ON and QC as well as several USA states (Eiseman C. S., Smith D. R., and Woods P., (2022)). Caliroa lorata feed on northern red oak, American chestnut, dwarf chestnut, and can be found in Ont. and several American states Caliroa obsoleta feed on white oak, swamp white oak, bur oak, iron oak, sand post oak aka dwarf post oak, Chestnut oak, swamp chestnut oak and can be found in Canada in the prov. of AB, NB, ON, and QC as well as many American states (Eiseman C. S., Smith D. R., and Woods P., (2022)). Caliroa petiolata feed on pin oak and northern red oak and can be found in Canada in the prov. of ON and in the American states of MA, MD, MS, NC, NJ, PA, TN, VA, and WV (Eiseman C. S., Smith D. R., and Woods P., (2022)). Caliroa cinxia Caliroa varipes, and Periclista lineolata also feed on oak.

Meet the Pest

Larvae have a caterpillar like appearance and are covered in a slimy substance, that helps them adhere to the leaves. Due to this slimy covering this insect is often referred to as oak slug sawfly. They have semi-transparent bodies that are yellowish or pigment free (depending on species), with a wide greenish stripe that appears to run down the length of the larvae, which in essence is their gut and its contents. The head region is wider and somewhat flattened and tapers towards the bottom. Each instar stage looks alike, growing slightly bigger with each molt, measuring about 12mm (1/2″) in length at maturity. Larvae can be found on the underside of leaves, usually beginning at the top of the canopy working their way down.

  • Caliroa quercuscoccineae commonly called Scarlet Oak Sawfly larvae have either a black or amber coloured head and yellowish tint on the thorax, on front and sides. The female larvae pass through 6 feeding instar stages, while the males pass through 5, (Nordin, G. L., & Johnson, E. L. (1984)). They can have up to 3 distinct generations per year, depending on geographical location. There appears to be no overlapping of generations, (where many stages of the insect are present at one time) (Boggs J., (2021)). They initially feed in large groups side by side but as they get older, they tend to spread out a bit more and feed independently) Boggs J., (2018)).
  • Caliroa lobata is almost indistinguishable from Caliroa quercuscoccinea. Caliroa lunata is also very similar to these two species but they feed on bladder nut.
  • Caliroa lorata
  • Caliroa annulipes early instars have a pale head capsule which becomes reddish brown in older instars but retaining a dark bar between the temples (Sawflies.org.uk, (2021)). Their bodies tend to be unpigmented.
  • Caliroa obsoleta larvae have an amber, brown, or black head and either unpigmented body or may have yellow on the sides of thorax (Eiseman C. S., Smith D. R., and Woods P., (2022)). By the final instar stage, the head is cream colored and the sides of thorax may or may not have a yellowish colouring. Larvae feed either solitary or in small to large groups (Eiseman C. S., Smith D. R., and Woods P., (2022)).
  • Caliroa petiolata larvae are unpigmented except for black eyespots. They feed in small groups.
  • Caliroa fasciata no information available
Oak Slug Sawfly larvae (likely Caliroa sp.) feeding on columnar oak (Quercus robur f. fastigiata), located in Southern Ontario.
Oak Slug Sawfly larvae (likely Caliroa sp.) feeding on columnar oak (Quercus robur f. fastigiata), located in Southern Ontario.

Adults are tiny fly-like, nonstinging wasps that are black with yellowish legs measuring about 6 mm (1/4″). They are quite inconspicuous and may be found on flowering plants as well as oaks.

Eggs are inserted in a row along the midribs and larger veins of the underside of oak leaves.

Cocoons can be found in the soil (about 2-3″ in depth) and leaf litter, from late fall to spring, where the mature larvae overwinter then pupate in spring before their re-emergence as adults.

Feeding Damage

Damage caused by The Scarlet Oak Sawfly and White Oak Slug Sawfly is caused by the larvae who feed on the lower side of leaf surfaces eating the outer leaf tissue between the veins but leaving a thin layer of the upper epidermis and the veins intact. This type of feeding damage is called ‘window paning’. Gradually the remaining leaf tissues dry out, turn brown and the dead leaf tissue between the veins begin dropping out, eventually leaving only the veins. Feeding damage typically begins at the top of the canopy and gradually moves downwards.

Feeding damage to columnar oak (Quercus robur f. fastigiata) caused by Oak Slug Sawfly larvae (likely Caliroa sp.) located in Southern Ontario.
Feeding damage to columnar oak (Quercus robur f. fastigiata) caused by Oak Slug Sawfly larvae (likely Caliroa sp.) located in Southern Ontario.
Feeding damage to columnar oak (Quercus robur f. fastigiata) caused by Oak Slug Sawfly larvae (likely Caliroa sp.), located in Southern Ontario.

Life Cycle

Oak Sawfly overwinter as mature larvae in a cocoon in the soil or leaf litter. In spring the larvae pupate and emerge from the ground in May and June, as fly-like, nonstinging wasps. The adults are short lived and feed on pollen and nectar. They quickly mate, then the pregnant females use their saw-like ovipositor to make tiny slits in the underside of major leaf veins and insert their eggs in single rows. The eggs hatch in about 1-2 weeks. Scarlet Oak Sawfly (Caliroa quercuscoccineae) female larvae pass through 6 feeding instar stages while the males pass through 5, (Nordin, G. L., & Johnson, E. L. (1984)).

Management and Control

Oak trees can handle some feeding damage with little harm to the tree. Ensuring the tree is adequately watered, fertilized and properly pruned is always the first step, and often enough to manage this pest. Hand picking is an option for smaller trees with small populations. There are also natural enemies, microbial diseases, and parasites that help to control this pest’s population (United States Department of Agriculture Forest Service Northeastern Area (n.d.)). For outbreak years the young larvae can be suppressed with horticultural oil sprays. For mature larvae Spinosad is effective, although it is not permitted in Canada (still under review from what I understand). Oak tree sawfly outbreak years appear to be followed by years of low populations and thus even heavily defoliated trees may have time to recover (University of Kentucky, COLLEGE OF AGRICULTURE, FOOD AND ENVIRONMENT, (n.d.)). However repeated defoliations will weaken the tree and make it vulnerable to other pests and diseases. Oak sawflies are said to be a forest pest and most likely to occur in residential areas bordering woodlands (Rettke S. K., (2021)).

Photo credits: all photos taken by the author.

References:

Boggs J. (2022). White Oak Slug Sawfly Damage. Ohio State University. https://bygl.osu.edu/node/1980

Boggs J., (2018). Scarlet Oak Sawfly Damage is Underway. Ohio State University. https://bygl.osu.edu/node/1068

Boggs J., (2021). Bladdernut Slug Sawfly Mystery Solved. Ohio State University. https://bygl.osu.edu/node/1862

Bugwood wiki, (last modified 14:15, 2 February 2010). Caliroa quercuscoccinae. https://wiki.bugwood.org/Caliroa_quercuscoccinae

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), 225-244. https://doi.org/10.4289/0013-8797.124.2.225

Forestry Images, (Updated October 2018). oak sawfly Caliroa petiolata Smith. https://www.forestryimages.org/browse/subthumb.cfm?sub=828

ISU Extension and Outreach, (n.d.). Oak Sawfly. https://hortnews.extension.iastate.edu/oak-sawfly

ISU Extension and Outreach, (n.d.). Pin Oak Sawfly. https://hortnews.extension.iastate.edu/pin-oak-sawfly

Liesch P.J. and Mahr S., University of Wisconsin – Madison, (n.d.). Sawflies. https://hort.extension.wisc.edu/articles/sawflies/

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 Society57(4), 569–579. http://www.jstor.org/stable/25084565

Rettke S. K., (2021). Summer Pests on Oaks: Oak Sawflies, Oak Spider Mites & Oak Lace Bugs. PLANT & PEST ADVISORY. Rutgers Cooperative Extension. https://plant-pest-advisory.rutgers.edu/summer-pests-on-oaks-oak-sawflies-oak-spider-mites-oak-lace-bugs/

 Royal Horticultural Society, (n.d.). Oak slugworm. https://www.rhs.org.uk/biodiversity/oak-slugworm

Sawflies.org.uk, (2021). Caliroa annulipes (Klug, 1816). https://www.sawflies.org.uk/caliroa-annulipes/

Tree Canada, (n.d.). Scarlet oak (Quercus coccinea). https://treecanada.ca/resources/trees-of-canada/scarlet-oak-quercus-coccinea/

United States Department of Agriculture Forest Service Northeastern Area (n.d.). Scarlet Oak Sawfly. Pest Alert. NA-PR-06-98. https://www.forestpests.org/acrobat/sawfly.pdf

University of Kentucky, COLLEGE OF AGRICULTURE, FOOD AND ENVIRONMENT, (n.d.). Scarlet Oak Sawfly. https://forestry.ca.uky.edu/scarlet-oak-sawfly

 University of Wisconsin-Milwaukee, (2012). College of Letters and Science Field Station. Slug Sawfly: A Skeletonizer. https://uwm.edu/field-station/slug-sawfly-a-skeletonizer/

Updated Jan 23, 2023

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