Willow Rosette Gall

Along the Lower Hat Creek Trail (Shasta County CA) is a cluster of willows heavily infected with two different gall forms. These galls appear along the length of approximately three-fourths of the stems. One form is a thick-walled, smooth, elongated, integral stem gall. The other is a multitude of squished-together bracts that resemble leaves.

The willow rosette gall is induced on several species of willow (Salix sp) by a midge (Rabdophaga salicisbrassicoides). The galls are detachable and occur when the midge lays its eggs in developing leaf buds. The larval chambers are located at the base of the galls. When fresh these willow rosette galls are pliable, leafy and green with a red blush at the base of the bracts.  In winter the galls with their brown, brittle bracts resemble cones. The larvae overwinter and adult midges emerge in the spring. There is only one generation of willow rosette gall midge per year.

Several parasitic wasps are associated with the willow rosette gall midge and can be recovered from willow rosette galls.

Even though the willows are heavily infected with galls, catkins are open on the twigs above and beyond the galls. I assume the heavy gall infestation has not killed the shrub. When I was young I called any willow with this type of catkin a “pussy willow”.  My willow identification skills remain primitive, especially in the winter when leaves and other trail markings are absent. Thus I will simply call this host plant a willow – Salix sp.

In my next post I will discuss the integral stem gall visible in the one photograph.

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