Tuesday, October 23, 2012

Green Islands

Leaves fell like glitter on the sun-showered path. These tiny yellow hearts of quaking aspen fluttered wildly as they descended, eventually ending in drifts built upon the wilted bodies of their companions. Placid raindrops beaded up on their slick surfaces, shining like jewels in the slanting rays of afternoon sun. A gentle sweetness wafted on the lukewarm breeze.

I do love fall.

And yet I already miss (just a little) the vibrancy of a buzzing summer day. Maybe if we could hold on to that green energy for just a little longer…

The golden leaves almost all had their own little hitchhiker hiding out between those slick, waterproof leaf-skins. My evidence? Bright green trapezoids of chlorophyll trapped between the first and second veins on one side of the leaves’ midribs. You may assume that the aspens are simply showing their support for the Packers. I have no evidence to discredit their status as cheeseheads, but the trees are not responsible for the variegated leaves.

A small, brown moth with white-fringed wings laid an egg on the leaf petiole (stem) back in July. By September, a translucent larva hatched and bored into the petiole, causing the stem to swell a bit into a small gall. Munching its way up inside the leaf under the cover of darkness, the larva interrupted the mechanisms the tree normally uses to draw chlorophyll out of the leaf during the waning days of autumn. The result is a “green island” in the yellow aspen leaves, and a forest carpet of Packers’ colors.

Such a tiny caterpillar would dry out in the summer heat, or if it tried to pupate high in the tree canopy. Instead, it takes advantage of pleasant fall weather, and then hitchhikes on the falling leaf down to the damp forest floor. Once there, it steals a few more bites of the green energy it preserved, and then pupates in relative safety and an agreeable microclimate. The soon-to-be-moth spends the winter in its cocoon, which is loosely woven to the surface of the now-brown leaf.
The receding snow and warming sun of May stimulate metamorphosis, and the new moth emerges from its winter sleep.

While apparently unstudied in the U.S., this drab moth and its tiny caterpillar have a Holarctic distribution. This means that they live across all the continents in the Northern Hemisphere. In fact, some of the information I have presented here was translated from Swedish and Dutch by the magic of the Google Translator! I can track this organism throughout the world by the universal language of scientific names. Ectoedemia argyropeza may not roll off your tongue, but scientists all over the planet use this one name to refer this particular species.

Whatever you call it, the vibrant green islands those moth larvae preserve are a lovely part of fall.

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