Willow Stem Gall Sawfly

Emigrant Wilderness, Huckleberry Lake, July 2014

Galls or cecidia are outgrowths on the surface of lifeforms. Plant galls are abnormal outgrowths of plant tissues and can be caused by various parasites, from fungi and bacteria, to insects and mites. Plant galls are often highly organized structures and because of this the cause of the gall can often be determined without the actual agent being identified. This applies particularly to some insect and mite plant galls. In pathology, a gall is a raised sore on the skin, usually caused by chafing or rubbing.

Gall-inducing insects include gall wasps, gall midges, gall flies, Agromyzidae aphids (such as Melaphis chinensis, Pemphigus spyrothecae, and Pemphigus betae), scale insects, and psyllids.

Willow Shoot Galls: These swellings on shoots, twigs, or leaf petioles, may be caused by small flies (midges) or small wasps (sawflies).The gall increases in size as long as the immature stages are active. They cause no significant injury.The infestation may be reduced by pruning and destroying the galled areas before the adult insect emerges, usually in late summer.

Sourced from: http://www.wildflower.org/plants/ -- and -- Wikipedia

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