The Collaborative International Dictionary of English v.0.48:

Webworm \Web"worm`\, n. (Zool.)
   Any one of various species of moths whose gregarious larvae
   eat the leaves of trees, and construct a large web to which
   they retreat when not feeding.
   [1913 Webster]

   Note: The most destructive webworms belong to the family
         Bombycidae, as the fall webworm (Hyphantria
         textor), which feeds on various fruit and forest
         trees, and the common tent caterpillar, which feeds on
         various fruit trees (see Tent caterpillar, under
         Tent.) The grapevine webworm is the larva of a
         geometrid moth (see Vine inchworm, under Vine).
         [1913 Webster]