The Collaborative International Dictionary of English v.0.48:

Cankerworm \Can"ker*worm`\, n. (Zool.)
   The larva of two species of geometrid moths which are very
   injurious to fruit and shade trees by eating, and often
   entirely destroying, the foliage. Other similar larv[ae] are
   also called cankerworms.
   [1913 Webster]

   Note: The autumnal species (Anisopteryx pometaria) becomes
         adult late in autumn (after frosts) and in winter. The
         spring species (Anisopteryx vernata) remains in the
         ground through the winter, and matures in early spring.
         Both have winged males and wingless females. The
         larv[ae] are similar in appearance and habits, and
         belong to the family of measuring worms or spanworms.
         These larv[ae] hatch from the eggs when the leaves
         begin to expand in spring.
         [1913 Webster]