Early Leafminers

On a walk in the Mount Holyoke Range on April 18, I came across a patch of golden ragwort (Senecio aureus) with many linear mines in the young leaves.  These are the only leaf mines I have seen so far this year; leaf mines become much more common later in the growing season.

Leaf mine in golden ragwort, with the yellow larva clearly visible through the leaf cuticle.

The first thing that came to mind was the Bucculatrix staintonella mine pictured at the bottom of this page, and I wondered if I had discovered a new host plant for this species.  When I got home and looked at that photo, however, I realized that the Bucculatrix larva was white, whereas the larvae in the leaves I’d collected were bright yellow.  Several days later, the larvae widened the ends of their mines to pupate, confirming my second guess: these were Phyllocnistis mines, and because the host plant was in the aster family, they were P. insignis.  I had never seen P. insignis mines with a conspicuous trail of excrement down the middle before–I had raised this species from immaculate mines in pilewort (Erechtites hieracifolia) last year, and the mines I normally see in older golden ragwort leaves (like the one shown on p. 333 of my book) are always similarly clean and white.  Apparently larvae feeding in young, tender ragwort leaves either produce darker droppings or the droppings are concealed by a thicker membrane (the leaf’s cuticle and upper epidermis) in the older leaves.

Anyway, the first of the adult moths emerged yesterday, and I knew from last year that I was going to see something beautiful when I trained my macro lens on this tiny creature.

Adult Phyllocnistis insignis (3 mm).

The light areas on the top of its wing are partly glare from the flash and partly a result of some of its gray scales having rubbed off as it walked around inside the plastic sandwich bag.  Compare with this one from last year, which has the gray scales intact but has chewed-up wingtips.  Now that I know they’re emerging, I’ll transfer the other leaves to less constraining containers.

As is the general rule when a tiny caterpillar pupates inside a leaf, the pupa was thrust out of the leaf when the moth was ready to emerge, and the pupal skin was left behind as evidence of successful emergence.  If you look closely you can see the form of the moth’s antennae and legs in the pupal skin.

The moth's empty pupal skin protruding from the leaf.