From 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]
From WordNet (r) 3.0 (2006) :
jointworm
n 1: larva of chalcid flies injurious to the straw of wheat and
other grains [syn: strawworm, jointworm]