Early last spring, driving along the road past “The Elbow”—that sharp bend in the East Walker River that touches the northeast edge of the Bodie Hills—I was struck by the abundance of large galls on the branches of big sagebrush (Atremisia tridentata subsp. tridentata) beside the road. They were the size of walnuts to small apples.
A bit of searching about galls on sagebrush led me to a post by Jonathan Neal on the Living with Insects Blog, which identified these as galls of the sponge gall midge (Rhopalomyia pomum, in the gall midge family, Cecidomyiidae).
These tiny, delicate flies lay their eggs on the stems of big sagebrush. When the larvae hatch, they chew on the plant and chemicals in their saliva induce the growth of these large, soft-spongy galls on the stems. The larvae continue to grow and feed on the interior of the gall. With some luck, adult midges will emerge and renew the life cycle. Unlucky midge larvae may become hosts for tiny wasps. Wasps in Eulophidae and Platygastridae (and probably other families) are parisitoids (parasites that ultimately kill their hosts) that will lay their eggs in the developing midge larvae within the galls. The wasp larvae consume the midge larvae, and adult wasps, rather than adult midges emerge from the gall.
Welch (2005) surveyed the literature and found mention of at least 42 midge and aphid species known to produce galls on big sagebrush. Many other insects and arthropods are associated with sagebrush in various ways. Not to mention the birds, mammals, fungi, and even lichens that will live in an area only because sagebrush is a dominant species. This all illustrates how sagebrush can be a “foundation species“—one that has a strong role in structuring a community and makes it possible for many other species to exist wherever it grows in abundance.
Reference: Welch, Bruce L. 2005. Big sagebrush: A sea fragmented into lakes, ponds, and puddles. Gen. Tech Rep. RMRS-GTR-144. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 210 p.
Copyright © Tim Messick 2016. All rights reserved.
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