Stem borer etc. (Diptera: Agromyzidae) in Achillea

I observed agromyzid larvae to tunnel in aerial stems of the host and also in the petiole and crown of a rosette.

Larval activity in stems in early June sometimes caused the stem to wilt, due to the larva establishing a short, shallow tunnel around the upper stem in a spiral pattern, then penetrating into deeper tissues. A stem-boring larva I found on June 20 one year was nearly mature, and in another year, larvae in stems formed puparia as early as June 13, with an adult emerging before early August. Also, in both 2020 and 2023, I found a spent agromyzid puparium in a tunnel in a dead stem of the host during the winter, but was unable to determine the identity of these puparia beyond family level.

I discovered the activity in rosettes of the host by observing wilting and/or shriveling rosette leaves. Looking closer at one of the affected rosettes, I found that an agromyzid larva had hollowed out the crown of the rosette along with the base of the petiole of one of the wilted leaves. The straw-colored puparium was formed in the interior of the base of the petiole, and the adult emerged in September.