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Alaska Science

Willow rose hosts insect drama within


July 21, 2010

From the more-you-look-the-more-you-see file, I present the willow rose.

The willow rose is lovely, green and unexpected, its whirled petals gracing the top of Alaska willows like the most delicate blossom in the cooler of a flower shop. But this rose is cultivated by an insect that manipulates the poor willow for both food and shelter, often at a price to the bug that seems worse than death.

jpg Willow rose hosts insect drama within

A willow rose, formed by an insect.
Photo by Tommi Nyman, University of Eastern Finland

Willow roses often appear on Barclay willows, one of 33 species of Alaska's most numerous trees. The Barclay, named for an English botanist who sailed the west coast of America in 1835-1841, is a common willow on riverbanks from the Yukon River southward in Alaska. Because it's hard to tell one willow from another, the presence of willow roses helps botanists know they are looking at a Barclay.

A fly about the size and shape of a small mosquito is responsible for altering the willow to its liking to create the willow rose. In springtime, after the snow is gone but before willow buds burst, a female willow gall midge lays an egg at the tip of a willow branch. That egg hatches into a wormy little grub, which then burrows into the bud. The bud, containing compressed new leaves that are awaiting the flush of moisture, is the waxy cap at the end of a willow branch that formed late the previous summer.

The tiny orange grub augers into the new plant cells that were to become the willow's summer leaves. Nestled within, the little worm starts munching on the same source of energy that has sustained moose all winter. This action halts the growth of the willow stem. Instead of the leaves forming in an orderly spiral long a new stem of the willow, they bloom in a pattern called, for obvious reasons, a rosette.

This rosette becomes the grub's apartment, in which other insects sometimes crash. Slice a willow rose in half and you will see the tiny orange grub at its heart.

Within the protected chamber, many grubs then mature over several seasons to become flies. But a good number of them do not. Some orange midge grubs suffer a fate you wouldn't wish on any organism but a mosquito - a creature implanted within the grub's guts eats it from the inside out.

The actor in this midge tragedy is a species of metallic green and purple wasp. In July and August, people sometimes see these wasps hovering over willow roses. The wasps land, and appear to sting the rose, depositing their eggs beside the grub in the chamber. This isn't good news for the grub. The wasp eggs hatch and a tiny translucent larva slimes over to the grub and bores its way into the grub's skin.

Over a few months that can't be much fun for the orange midge grub, the wasp larva consumes it from within and kills it. Given its miniscule brain, the grub of the willow midge probably never ponders a grisly irony ­ the beauty it creates with the willow rose may also attract its angel of death.

This column is provided as a public service by the
Geophysical Institute, University of Alaska Fairbanks,
in cooperation with the UAF research
Ned Rozell [] is a science writer at the institute.


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