Stem borer (Coleoptera: Curculionoidea) in Ambrosia

In late September, I found a larva in an interior tunnel in the middle to upper stem of the host. The tunnel appeared to have been primarily constructed by a tephritid larva that was dwelling lower in the same stem, and it was unclear to what extent the beetle larva had contributed visibly to the excavations. The larva was legless, with a pale yellowish-white body and a slightly darker head capsule bearing sclerotized, triangular mandibles and two small dark eyes positioned just behind the bases of the mandibles. In some respects the larva appeared very similar to brentid larvae encountered in other hostplants in the current survey, and a brentid has been previously reared from stems of giant ragweed in the geographic area covered by this survey (Hatfield 2021). Interestingly, in the image from the Hatfield reference, the adult weevil is shown in a tunnel that appears to be mostly the work of a larger borer, similar to the situation in which the larva in the current record was found. Further study would be required to better understand the ecological role of these beetles in the internal stem ecosystem.