Stem borer (Diptera: Agromyzidae) in Verbena

This borer tunnels in upper stems of Verbena spp. in early- to midsummer. The larva's activities may result in drooping or dieback of the tip of the stem, at least sometimes because the larva has mined shallowly around the circumference of the stem in a spiral pattern before entering deeper tissues. The stem may swell where this shallow mining occurs, resulting in an externally visible spiral ridge. In V. stricta, larval tunneling may extend into the flowering stalk, with the tip of the spike turning black; this sign can be even be found on the dead stems during the subsequent winter, as shown in the photos below.

I have observed many plants of V. stricta and V. urticifolia affected by this borer, but so far only a single stem of V. hastata. The latter plant seems to be less abundant or less widely distributed in my area, and I have had few opportunities to examine it for sign of this fly.

Late-stage larvae may generate relatively large quantities of solid granular frass that fill portions of their tunnels, which is somewhat unusual among the dipteran borers observed in the current study. The puparium is formed in the stem, with adults emerging a few weeks later, in late July or early August. I reared adults in 2017 and 2021. The species was described in Eiseman et al. (2021).