Sawfly (Strongylogaster tibialis)
If a niche exists, nature will fill it. This week’s crawly is one of those “find a niche, fill a niche” critters. Let’s meet a pretty sawfly with a tongue twister of a name and a fondness for ferns: Strongylogaster tibialis – we’ll call her “tibi” for short.
Tibi is one of 11 North American species in the Genus Strongylogaster; all 11 need ferns to raise their families.
Despite the fact that Oregon is home to at least seven different species of fern, there are only four recorded sightings of Strongylogasters on BugGuide for Oregon and only one of those is tibi. iNat has nine recorded sightings, none of which are tibi.
Tibi’s first cousin S. distans is quite distinct looking (as sawflies go, especially sawflies within a single family) and of the nine sightings on iNat in the genus, two are S. distans.
Fun fact: Both S. tibilis and S. distans favor the same ferns and their larvae have been seen dining side by side.
It’s difficult to say whether or not the scarcity of recorded sightings means tibi and her cousins are truly rare in our area, or they just go unnoticed.
While my sightings are not (yet) listed on BugGuide, our pictured tibi is the second I’ve found. The first was in Trail and our featured female was ovipositing into a bracken fern (Pteridium aquilinum) along the Rogue River Trail just a bit down river from The Gorge.
Quick refresher on why sawflies are named “sawflies.” In the photo Ms. Tibi is using her wicked cool, super stealthy ovipositor to saw a small opening into the bracken stem; she then will deposit an egg in the hole.
What makes the tibi’s ovipositor so stealthy? When not being used to carve an opening for eggs, tibis (and other species of sawflies) keep their ovipositors neatly folded (thus hidden and protected) inside their abdomens. When it’s time to lay eggs, their ovipositors unfold like a serrated jackknife always honed and ready to slit some stems (see photo).
Unlike their bee and wasp cousins, sawflies cannot use their ovipositors for defense (AKA sting). However, their wasp-like appearance is often enough to keep would-be predators at bay.
Prior to stem sawing our particular tibi did quite a bit of exploration around the still unfurling fern fronds, possibly to determine if they would be a good food source for her impending kiddos.
Adult tibis enjoy a sweet diet of nectar (so they are native pollinators), but for their young, it’s an all fern diet. Bracken ferns are tibis’ number one choice for egg laying and family raising and they’ll use it almost exclusively. There have been a couple recordings of tibis ovipositing in lady ferns (Athyrium filix-femina) and giant chain ferns (Woodwardia fimbriata).
Still, if you’d like to see a tibi up close and personal, your best bet is to keep your eyes on bracken ferns.
Some sawfly species are gregarious, with many larvae hanging out together and feeding. Tibis lead a more singular childhood. With the eggs spread out on separate stems the larvae will pick a frond and spend their days hiding on the underside their bodies flattened and looking very much like “just another stem.”
When they’re frightened, the larvae will either whip their abdomens back and forth, hoping for a startle effect a la “the fern jut came to life!” or drop to the ground and head for better cover.
When it’s time for them to morph into their final pupal form, they’ll drill a one-half inch long hole into some nice deadwood then crawl in head first and seal up the opening with poo. Yup.
Poopily protected, they’ll overwinter and hatch out the next spring as the ferns in their location begin to unfurl and the cycle begins anew.