6 Oct 2020: Cecidomyiid larvae found inside leaf petioles of white snakeroot, Ageratina altissima. In the example shown in the current series, the affected leaf had prematurely wilted, which was my cue to pluck it off the plant and examine the interior of the petiole for signs of insect shenanigans. This larva was present, along with the remains of 2 or 3 other larvae, inside the very base of the petiole, right where it joins the stem.

Here's a second example in which the leaf had not wilted/shriveled but its petiole did show some subtle discoloration at the place where the larvae were living:


In both of these examples, I believe that the larvae I found were primary consumers of plant tissue, as opposed to Johnny-come-lately secondaries that arrived after some other insect had already been at work inside the petiole.

In August 2018 I found what appeared to be the pupal exuviae of a cecidomyiid protruding from its cocoon inside the petiole of a white snakeroot leaf, but since the animal was gone, it was hard to know whether that identification was correct:


I have also found cecidomyiid larvae inside petioles of a woodland Eutrochium sp. in my area: