Stem and petiole borer (Coleoptera: Brentidae) in Heracleum

I noticed larval damage in petioles of the host in 2021 or 2022 but did not photograph it. Later, I photographed adults crawling on and apparently ovipositing in stems and petioles of cow parsnip in my yard on 17 May 2023 and 9 June 2025.

On 28 June 2023, I observed several larvae in their tunnels in petioles, as well as what appeared to be a parasitoid wasp pupa in a petiole tunnel. I wrote the following notes to accompany the photos I took that day:

A couple weevil larvae in a petiole of cow parsnip, Heracleum maximum, along with their brownish tunnels running just under the inner wall of the hollow petiole. There were multiple larvae per petiole, each one inhabiting a mostly straight brownish-discolored tunnel that was was maybe 40-50mm in length. The larvae seemed to be of different ages; the largest I found was at least half again as large as the smallest (I looked at maybe 5 larvae).

On 7 July, three adult weevils emerged from these petioles, and I found and photographed two more adults and at least one pupa that were still inside their pupation chambers in the petioles. A typical pupation chamber consisted of a portion of the larva's tunnel perhaps 8-15 mm in length, sealed off from the rest of the tunnel on either end by a short plug of brown frass. Upon its emergence, one of the adults left a small hole in the outer wall of the petiole.

On 8 August, I examined another petiole in the field and found it to contain tunnels that appeared to be vacant, suggesting the weevil had already completed its life cycle and abandoned the plant material. This was consistent with the phenology of the individuals I had reared from other petioles a month prior.