Local feeder (Diptera: Cecidomyiidae) in stems of Ageratina

In July 2023, I discovered numerous orangish or pale salmon-colored larvae living gregariously in the somewhat airy pith of two lower stems of the host. There was a limited brownish discoloration to the pith in the area where the larvae dwelled, but no external sign of their presence in the stems was observed. The larvae were identified to genus by R.J. Gagné from photos (pers. comm., March 2, 2024).

Later, in late July 2025, I collected a white snakeroot stem in my yard that showed an externally blackened region. Over the next few weeks, pale whitish and salmon-colored cecidomyiid larvae, which I tentatively identified as Resseliella sp. based on their resemblance to the 2023 larvae (including the presence of curved projections on their terminal segments, as is described for the genus in Gagné 1989), emerged from this portion of the stem and pupated naked (i.e., without a cocoon) in the potting mix at the bottom of the rearing container or in between layers of moist paper toweling appressed to the side of the container. When I believed most of the larvae had emerged, I dissected the blackened area of the stem and found a lone straggler in the stem interior. I wrote the following in my journal about this larva's subsequent behavior:

After I cut open the stem, which was nearly as black inside as it was outside, the larva started crawling along the stem interior a short distance, and then without even leaving the inner surface of the stem, it curled into a ball in preparation for vaulting. It then vaulted on to the tabletop, as you can see in the photos. I put the larva into the rearing container...with some moistened potting mix.

Several adult cecidomyiids emerged from this rearing in the final days of August and first days of September.

See also: Ageratina stem insects compilation