Stem borer (Coleoptera: Cerambycidae) in Ageratina

I found a single larva of this borer in late March, overwintering in a dead stem of the host that was approximately 90-100cm tall and ~4mm in diameter at the base. The larva had tunneled out nearly the entire length of the stem, with the tunnel extending from the base of the stem up to about 10-15cm below the stem tip. Sections of the tunnel were filled with rods of compacted frass; two of these rods measured 8.5cm long and 7.2cm long. The compacted rods of frass alternated with stretches of open tunnel with variously smooth or slightly rough walls. The frass consisted of fine, neatly round pellets, reminiscent of the pellets produced by cerambycid borers in other plants I've examined. When it was ready to pupate, the larva prepared a chamber ~15cm long in the stem, the chamber itself completely cleared of frass but bounded on both ends by frass rods along with an accumulation of shavings. The adult emerged in early May, and Richards (2024) identified it from photos.

See also: Ageratina stem insects compilation