Many different sawflies, midges, mites and a rust fungus induce galls on the stems and leaves of willow (Salix) species. In the same way that willows are taxonomically complex, the insects that produce these galls are difficult to identify. Much work has yet to be done in the field of cecidology (the study of insects and the galls that they induce).
Galls are tumor-like growths that develop because of chemical or mechanical stimulation of an invading organism. Galls can occur on every part of the plant and can take many shapes or forms, depending on the particular plant/organism relationship. Usually the gall-inducing organism requires a specific host plant. Between the different groups of gall formers the mechanisms of induction vary widely and are influenced by many environmental conditions. Generally the galls do not harm the host.
Walking along Ash Creek (Lassen County CA) on an early fall evening I noticed many pea-shaped galls on the leaves of a willow growing along the creek bank. The galls were all on the undersides of the leaves and ranged in color from yellow to pink to red. Growing along the midrib, the galls were monothalamous, that is, having a single chamber. One larva inhabited each gall. I believe a sawfly induced these galls – specifically the willow redgall sawfly (Pontania viminalis), although another Pontania species may be the “culprit”. A common name for this gall is a midleaf pea gall.
I was so involved with the pea galls that I did not bother specifically identifying the willow.
In my next post more on the willow redgall sawfly.