Oak Slug

On June 23, 2012, I was working in northern Maine when I noticed some leaf mines I didn’t recognize on a red oak (Quercus rubra) sapling.  I paused to photograph them, and within a few minutes it was raining.  While I stood there waiting for the shower to pass, I noticed a couple of small sawflies resting on the undersides of the leaves, likewise waiting out the rain.


I collected them to photograph that evening, hoping at least one of them was a leafminer–at that point I didn’t have any good photos of adult leaf-mining sawflies, and I knew there is a species that mines in oak leaves.

As it happened, the smaller sawfly (3.5 mm long) was in fact a leafminer, but a birch-feeding species. That wasn’t too surprising, since there was a paper birch (Betula papyrifera) sapling right next to the oak.  The sawfly in the photo above (6.3 mm), however, turned out to be Caliroa lorata.  Members of the tribe Caliroini are the only sawflies in the subfamily Heterarthrinae that are not leafminers; instead they skeletonize patches on the (usually lower) surfaces of leaves.  It is likely that surface feeding of this sort gave rise to leaf mining in sawflies, as is thought to be the case with leaf-mining chrysomelid beetles.  The Caliroini (Caliroa and Endelomyia) are known as “slug” sawflies because of the apparently slimy and legless appearance of their larvae.  The best known species are the “pear slug” (C. cerasi) and “rose slug” (E. aethiops), but a number of species feed on oak leaves.  Sawfly specialist Dave Smith tells me that although C. lorata was originally only known from chestnut, there are also records from oak.  Not much is known about this species.


Anyway, in addition to the two sawflies, I collected some of the mined leaves to photograph too.  I determined that they were old mines of Japanagromyza viridula (Agromyzidae), the “oak shothole leafminer.”  Although the fly larvae were long gone, I noticed an egg (0.6 mm) inserted in the edge of one of the leaves.


I suspected it belonged to a sawfly, since most sawflies insert their eggs in plant tissue, and since it was conspicuously swollen as sawfly eggs are supposed to be after imbibing water from the plant.  Sure enough, after several days there was a “slug” sawfly larva feeding on the leaf.  Here’s what it looked like on July 6 (6.5 mm long):


In this more lateral view, you can see all three true legs, some stubby prolegs, and the very front of the head, including an eye and antenna.


Four days later it had become pale and had stopped feeding.


Knowing that most sawflies burrow into soil to pupate, I put it in a small jar of soil, and the next time I looked it had disappeared.  There was no sign of an adult at the end of the season, so I put the jar in the fridge to overwinter.  On May 8, the adult appeared!


Dave Smith informed me that this is a male and therefore not identifiable to species, but given the association with the female I had collected from the same plant, it seems likely that this is likewise C. lorata.  It’s ironic that after repeatedly failing in my deliberate attempts to rear whole batches of sawfly larvae, I managed to rear out this single larva from an accidentally collected egg.

About Charley Eiseman

I am a freelance naturalist, endlessly fascinated by the interconnections of all the living and nonliving things around me. I am the lead author of Tracks & Sign of Insects and Other Invertebrates (Stackpole Books, 2010), and continue to collect photographs and information on this subject. These days I am especially drawn to galls, leaf mines, and other plant-insect interactions.
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