Leaves
fell like glitter on the sun-showered
path. These tiny, yellow hearts of quaking aspen fluttered wildly as they descended, eventually ending up
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 captured between the first and
second veins on one side of the leaves’ midribs. The very top of the leaf’s
stem was thickened, too, just a little wider than normal. And where the thick
stem and bright green met, a small patch of brown frass (caterpillar poop)
filled in a section between veins. The trees are not responsible for the
variegated leaves—some insect had created a gall.
Splashes of green add color to the yellow aspen leaves of fall. Each little green island holds a tiny moth larvae. Photo by Emily Stone. |
Back
in July, a small, brown moth with white-fringed wings laid an egg on the leaf
petiole. She did this without the help of a male. This species of moth is parthenogenetic,
which means that the embryo developed even though the egg wasn’t fertilized.
Males hardly ever occur in the population.
If
you read my article last week, you may remember that I’m trying to avoid
calling living things “it” because it reduces them to an object. In the case of
this moth, not only does using the “grammar of animacy” acknowledge the
sentience of this creature, it also helps us learn more about the moth’s
biology. As I continue to use the pronoun “she” through the rest of the
article, you’ll be reminded of this moth’s interesting reproductive strategy.
By
September a translucent larva hatched and bored into the leaf’s petiole,
causing the stem to swell into a small gall.
Munching her way up inside the leaf under the
cover of darkness (she retreats into the thickened part of the stem during the
day), the leaf-mining larva interrupted the mechanisms the tree normally uses to draw chlorophyll out of the
leaf during the waning days of autumn.
How
does she do this? One hypothesis goes like this: the caterpillar hosts a common
bacterium, called Wolbachia, in her
body. Perhaps through the caterpillar’s frass, or other bodily secretions, Wolbachias enter the leaf, where they
manipulate plant hormones that control when a leaf dies, and cause one part of
the leaf to retain its chlorophyll.
The
result is a “green island” in the yellow aspen leaf.
Such
a tiny caterpillar would dry out in the summer heat if she tried to pupate high
in the tree canopy. Instead, she takes advantage of pleasant fall weather and then hitchhikes on the falling leaf
down to the damp forest floor. Once there, she steals a few more bites of the
green energy she’d hoarded in the leaf and then pupates in relative safety and
an agreeable microclimate. The
soon-to-be-moth spends the winter in her 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 her winter sleep.
While
not native to the United States, this drab
moth and her tiny caterpillars have traveled
far. No doubt this was facilitated by their ability to reproduce using
parthenogenesis. They live across all the
continents in the Northern Hemisphere, and have become quite common here and in
eastern Canada. As a result of their wide distribution, some of the information
I have presented here was translated from
Swedish and Dutch! 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 to this
particular species.
Whatever
you call it, the vibrant green islands those moth larvae preserve are a lovely
part of fall.
Editor’s
note: this article is an expansion on a Natural Connections article from 2012.
Emily’s second book, Natural
Connections:
Dreaming of an Elfin Skimmer, is now available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.
For more than 50 years, the Cable Natural History Museum has
served to connect you to the Northwoods. Come visit us in Cable, WI! Our new
Curiosity Center kids’ exhibit and Pollinator Power annual exhibit are now
open! Call us at 715-798-3890 or email emily@cablemuseum.org.