Insects in the Mines

A mine of the fly, Pegomya rumicifoliae. The larva is still resident, and visible at the right end of the mine. The small black specks are fecal pellets. Photo from Priest et al. (2019)

Even very ubiquitous and diverse groups of organisms are sometimes so cryptic we’re barely aware of them. Leaf-mining insects, for example, are abundant where-ever there are leafy plants, but most of us are barely aware of them.  The leaf-mining life-style works so well, though, that it has evolved convergently in multiple insect orders; there are leaf-mining moths, flies, wasps, and beetles.  What they share is a core aspect of their life-histories; as larvae, they feed within the tissues of leaves, in the succulent layers between upper and lower epidermis.  As a group they constitute an ecological guild — a collection of species that, while not necessarily evolutionarily related, have similar approaches to making a living.

One of the main characteristics of the leaf-miner guild is their smallness; as larvae, they have to fit between the top and bottom of a leaf. That means they’re easy to overlook. Even though most of us who look at leaves at all are aware of they’re existence — we see the damage, the tunnels they leave within the leaves — not many naturalists can name even one species.

Adult of the leaf-mining micromoth, Cameraria aceriella. The total wing-span is about 8 mm or 1/3 inch. Photo from Priest et al. (2019) (see text)

A new publication opens a window on leaf-miner diversity in the Huron Mountains. From 2000 to 2012, Mr. Ronald Priest, associated with the Entomology Department of Michigan State University, collected sprigs of plants with active leaf-miners, tending them in his home lab until adult insects emerged. Priest, working with a number of taxonomic experts, identified these tiny, obscure insects to species, further documenting them with painstaking and extremely detailed microscopic photographs. Because microscopes afford an extremely narrow depth-of-field, Priest used ‘focus stacking,’ a technique combining many photographs of slightly different focal distance to produce an image that is sharp over a wider range of depths than is possible with a single image.

Adult of the leaf-mining beetle, Sumitrosis inaequalis. Photo from Priest et al (2019) (see text).

The new paper identifies 63 leaf-mining species of flies, beetles, and moths. Twenty of these are new to Michigan, and two new two science (these two were described in a previous paper; one — Scrobipalpula manierreorum Priest — is a micromoth named in honor of Dr. William and Anne Manierre). Taking a step even deeper into the cryptic diversity hidden in the foliage, Priest was also able to rear 42 species of tiny parasitoid wasps using leaf-miners as hosts (parasitoids, like parasites, live in the tissues of their hosts; unlike true parasites, they ultimately kill the host, emerging as adults from the leaf-mines instead of their hosts).

Priest and his co-authors, Robert Kula and Michael Gates, published the work in The Great Lakes Entomologist, volume 52 (2019). Because the journal is ‘open-access’, the entire 44-page paper is freely available at .

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