Finding willow roses during the winter

Southeast Wild

Posted: Friday, March 02, 2001

Roses in winter? Take a walk out by Mendenhall Glacier or anywhere else in Juneau where you're likely to find willow bushes, and you will probably see them: brown, papery growths that look like small roses at the ends of willow branches. Some grow as large as golf balls, and sometimes there are so many of them a bush looks like the start of a dried flower arrangement.

These "willow roses" are actually galls, the homes of tiny larvae that will develop into adult flies in April or May.

Each gall began forming last spring when an adult fly deposited a single egg into the swelling bud at the end of a willow branch.

When the egg hatched, a tiny, legless larva emerged and began feeding on the bud with its jaw-like mouth parts. The larva also began secreting powerful growth stimulating substances that prevented the bud from developing normally into a twig. Instead, the bud kept growing larger and larger, forming layers of stunted, overlapping green leaves around the larva.

By summer the gall had taken on its characteristic "rose" shape. By winter, the larva was settled inside this "winter palace," protected from predators and insulated against the cold.

Willow rose larvae in Southeast Alaska also have other protections against winter cold. These insects can withstand temperatures even below freezing because in fall they build up high concentrations of sugar or sugar alcohol in their blood. This lowers the freezing point of their body fluids much the way antifreeze lowers the freezing point of water in a car radiator.

This ability has been well demonstrated in the gall insects on arctic willows, which in Southeast are found primarily in alpine areas. Arctic willow gall larvae have proved able to withstand temperatures of up to 87 degrees below zero.

Mark Schultz, entomologist at the U.S. Forest Service Pacific Northwest Laboratory in Juneau, said the particular species of insect that causes willow roses around Juneau has not yet been identified, but we wonder if it might turn out to be one of the gnats of the genus Rhabdophaga that form galls on arctic willow and other willows found in northeastern and northcentral United States.

These gnats are tiny flies that look like small mosquitoes about a tenth of an inch long.

Each gall fly species (they make up the insect family Cecidomyiidae) feeds on only one or a few related species of plants. We wonder if the willow roses around Juneau are found specifically on Barclay willow. Leslie Viereck and Elbert Little, Jr. suggest in "Alaska's Trees and Shrubs" that the presence of willow roses may be used to help identify Barclay willows.

It seems at first that willow roses must be destructive to their host plants, but entomologists say the galls cause only minimal damage. Apparently the insects can enjoy their palaces without causing undue harm to our willows.

Marge Hermans and Bob Armstrong write regularly about the nature of Alaska. Bill Brown will speak on traveling in Mongolia and Sue Schrader will talk about legislative issues at Juneau Audubon Society monthly meeting beginning 7:30 p.m. Thursday at Dzantik'i Heeni Middle School library.



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