Leaf miners noticeable on leaves

by KEVIN STRAUSS

They have been with us since early summer, but they were so small that we probably didn’t notice them. Even now, we see their white tunnels on the leaves of large-leaf aster and goldenrod, but we don’t see the insect larva itself. We call the creatures that make these tunnels “leaf miners.” Although the tiny caterpillar-like larvae that eat these leaves from the inside could belong to many different species of insects from moths, butterflies, wasps, flies and beetles.

Unlike larger caterpillars that munch holes in leaves or defoliate entire trees, leaf miners hatch and begin eating through the “mesophyll” or middle layer of a leaf. Now it is hard for me to imagine that a paper-thin leaf has a “middle layer” but for very small creatures, that middle layer is big enough to contain a tunnel and to hold tasty green chlorophyll cells for food.

If you look carefully at the tunnels, you will notice that they start out very small and then slowly get larger. In some cases, you may be able to see where the leaver miner pupated (changed into an adult) and then emerged from the leaf to breed and start the cycle all over again.

While leaf miners do write their signatures on leaves, they seldom seem to hard the plants they feed on.

Most leaf-mining larvae are relatively small butterflies or moths. There are over 400 species of butterfly and moth leaf miners in North America. Another 200 are fly species, 50 are beetle species and 15 are bee and wasp species. Depending on the size of the species larvae may spend all or part of their time in side the leaf.

Leaf-miners feed on the rich organic material that makes up the middle of the leaf. Of course after they eat, they have to relieve themselves as well. Some miners leave their “frass” (droppings) behind in their tunnels while others make new tunnels when the original tunnel is too full of frass.

If you take a single-edge razor blade and cut open a leaf-mined leaf, you can use a magnifying glass to look at the tunnel, the frass and the larva itself. The larva is a worm-like organism with with a relatively large jaws and mouth for chewing. Be sure to return the larva to another leaf so it can continue its life cycle.

By spending its life inside a leaf, a leaf-miner avoids many predator insects as it grows and feeds in the relative safety of its tunnel.

The next time you are out for a walk, keep your eyes open for some of these less obvious forest insects and the signs they leave behind.