Stem borer (Coleoptera: Erotylidae) in Ageratina

I found several A. gracilis larvae overwintering in their tunnels in dead stems of the host (one larva per stem). These larvae had accomplished most of their feeding while the stems were still alive and/or in the process of senescing during the previous summer and fall. I also found one larva in a living stem in August. All of the larvae were orangish in color, with relatively well-developed thoracic legs and a pair of sharply pointed, curved urogomphi on the posterior end.

The tunnel of a typical overwintering larva often extended along much of the length of the stem. Stretches of the tunnel were smooth-walled and clear of frass, while other regions of the tunnel were tightly packed with long rods of compacted frass. I usually found the larva in the lower portion of the tunnel.

Though the August larva was still somewhat early in development when I found it, its stem tunnel was already extensive. A photograph of a portion of the tunnel shows that its walls were smooth. A leaf attached to this stem contained a brownish mine that began at an oviposition site in the midrib and traveled down the petiole into the stem, but I hypothesized that this mine was the work of a mordellid larva that did not survive, rather than the work of the erotylid larva (see records 0021 & 0727); at least in Leucanthemum, Medicago, and Lupinus, oviposition by the erotylid Languria mozardi appears to be mostly or entirely restricted to the main stem rather than in a leaf midrib or petiole (Vaurie 1948; Tangren & Frye 2020).

I reared multiple overwintering larvae to adulthood. These larvae pupated in their stem tunnels, and adults emerged in spring.

See also: Ageratina stem insects compilation