Gall.
Infested stems can usually be discerned by a somewhat abnormal thickening, though occasionally a badly infested stem may be nearly twice the usual size. The female appears to deposit eggs in a small slit in the stem, the larva usually tunneling the pith and frequently excavating a channel just beneath the surface

- John M. Clarke, EP Felt, Homer D. House: (1917) Report of the State Botanist 1916^©