The Collaborative International Dictionary of English v.0.48:

Jointworm \Joint"worm`\, n. (Zool.)
   The larva of a small, hymenopterous fly (Eurytoma hordei),
   which is found in gall-like swellings on the stalks of wheat,
   usually at or just above the first joint. In some parts of
   America it does great damage to the crop.
   [1913 Webster]