Hickory Gall Interlopers

And speaking of beetles associated with galls (which we were, briefly, several weeks ago in my previous post), here’s an odd thing I noticed last May while wandering in the woods of northwestern Connecticut. A lot of the leaves of shagbark hickories (Juglandaceae: Carya ovata) were covered with little round, flattened galls:

Nothing too odd about that; galls like these are a common sight on hickories, and they are caused by aphidlike insects called phylloxerans (Phylloxeridae: Phylloxera spp.). Here’s a larger (7 mm across) gall of a different species that I cut open 14 years ago to get a look at the Phylloxera nymphs within:

The odd thing was that I kept seeing little weevils on the leaves with the galls. I didn’t think much of the first one I saw, but after the second or third weevil, I started to wonder, “What is it with you guys and these galls?” As I wondered this, I had a vague recollection of having seen weevil larvae come out of phylloxera galls at some point. And I wondered if the dark blemishes I kept seeing on these galls had been caused by the weevils I was seeing, maybe chewing holes in which to lay eggs. The one weevil I photographed in the field was certainly doing something to one of the galls:

So naturally, after I ate the sandwich I had brought, I put several gall-laden leaves in my sandwich box along with a few weevils, including a mating pair. At home I got some better photos of the weevils, including this one:

And when trying to get better photos of the galls, it was hard to find any that didn’t have the dark blemishes on them—they were made from the lower surface but the damage was often visible from the upper surface as well.

On May 29, twelve days after I had collected the galls, a couple of squirming, legless weevil larvae appeared in my sandwich box.

The galls, to my surprise, had continued to develop on the detached leaves, and were now nearly spherical in some cases.

Some galls had been visibly munched, presumably by the weevil larvae, although in some cases this may have been from a few caterpillars that had hatched on the collected leaves and were now devouring them.

One of the caterpillars was this little inchworm, which I first noticed on May 19:

By May 27 it looked like this, and was recognizable as the larva of a “half-wing” (Geometridae: Phigalia titea):

There were also a couple of owlet moth caterpillars, which looked like this on May 27:

By May 29 the second one had molted, confirming my suspicion that these were larvae of the “ruby quaker” (Noctuidae: Orthosia rubescens):

Anyway, I moved the weevil larvae to a jar with soil in the bottom for the larvae to burrow into. On June 11, two adult weevils appeared in the jar. One looked much like the ones I’d photographed previously…

…and one was plain black, without the red markings on the elytra.

Knowing what these weevils did for a living, it took only a quick internet search to identify them as Anthonomus suturalis (Curculionidae), which has been a known associate of phylloxera galls since 1866. The account by Gates & Burke (1972)* makes it sound like the munching on the last gall shown above was in fact caused by the caterpillars rather than the weevil larvae: “By the time a larva has completed its development, the tissue of the inner wall of the gall has been consumed. There is no tunneling by the larvae as in cases of some other Anthonomus inquilines; instead, the inner tissue of the galls is rather smoothly gnawed away. The frass resulting from the activities of a larva is packed in one end of the gall.”

It’s just as well that I didn’t discover anything new about these weevils, because evidently no one is willing to review my papers about weevils anymore. I submitted a two-page manuscript on leaf-mining weevils to a journal a full year ago, and another one-page manuscript half a year ago, and I’m still waiting for reviews on both of these. This makes me a bit grumpy since I’ve never taken more than a few weeks to review a manuscript, and I only take that long if the paper is several hundred pages long.

As for my vague recollection of having seen weevil larvae come out of phylloxera galls before, I checked my files, and what I was remembering was actually a single gall on a shagbark hickory catkin that I found on the ground on June 1, 2018, possibly having been nipped off by a squirrel. I didn’t take a picture of the gall until eight days later, when things had started emerging from it; at this point the plant material had all turned brown, but you can still get a general idea of what it looked like:

Two identical weevil larvae emerged from this gall, one on June 5 and one on June 9. They were identical with one another, but looked nothing like the larvae of Anthonomus suturalis pictured above.

Also on June 9, this fly larva appeared in the jar with the gall, apparently having emerged from it along with the weevil larvae. I assume it is one of the hover flies (Syrphidae) that are predators of aphids and other insects.

I put these three larvae, along with the gall, in a jar of soil with the hope of rearing adults, but all that ever emerged—in March of the following year—was this blastobasid moth, which Aaron Hunt identified as Asaphocrita busckiella (which he believes to be a species complex):

I’m not sure if anything is known about the larvae of A. busckiella, but I see that A. aphidiella was reported in 1886 to have been “reared from the larvae feeding on the contents of Phylloxera hickory galls.” Blastobasids are known as “scavenger moths”, and I presume the larva was feeding on the damaged plant tissue and maybe on the weevil larvae’s frass rather than preying on whatever phylloxerans weren’t eaten by the fly larva. As for what kind of weevil the weevil larvae belonged to, I have no idea!

* Gates, Danny B. and Horace R. Burke. 1972. Review of the gall-inhabiting weevils of the genus Anthonomus, with description and biology of a new species (Coleoptera: Curculionidae). Annals of the Entomological Society of America 65(5): 1215-1224.