On May 12, I noticed a bunch of little yellow and black flies congregating on the leaves of some kind of Artemisia in the garden. Some of them were ovipositing, inserting eggs into the leaves, so that there was no visible sign of the eggs to the naked eye. They reminded me of some pictures of frit flies (Chloropidae) I’d seen, such as this one, but after reading up on them this didn’t seem to make sense, since the plant-feeding frit fly species apparently only feed on grass stems. I posted some photos on BugGuide.net, and Yurika Alexander suggested Liriomyza, a genus of leaf-mining flies (Agromyzidae). This made perfect sense, but it hadn’t crossed my mind because all the agromyzids I’d seen had been very plain-looking, like this one. Owen Lonsdale later confirmed her identification, adding that without knowing what the host plant is, species-level identification would require dissecting a male. That is to say, as is the case with a lot of these little flies, it is externally identical to other species in its genus, and apart from its ecology the only observable difference is in the male’s genitalia.
A female leaf-mining fly (Agromyzidae: Liriomyza) ovipositing in an Artemisia leaf. (1.7 mm)
Twelve days later, when I left for a road trip, I hadn’t seen any larval mines in the leaves, but when I returned on June 3 there were many in evidence. I might have never noticed them if I hadn’t been looking for them, because the plants had continued to grow and all the mines were on the lower leaves. I saw one adult fly hanging around that day, so it may be that several generations will appear throughout the season to mine the newly emerging leaves. This is the first time I have had the opportunity to connect an adult leafminer with its larval mine by finding it ovipositing, rather than by having it emerge from a leaf I collected.
Liriomyza mines in an Artemisia leaf.
