The elm spanworm is a native insect which is widely distributed over the
eastern half of the United States and Canada from Nova Scotia south to
Georgia and west to Colorado. The most widespread outbreak on record
occurred during the period 1954-1963 when over one million acres of
hardwood forests were defoliated in the mountains of western North
Carolina, eastern Tennessee and northern Georgia. The elm spanworm feeds
upon hickories and a variety of hardwoods; white oak, chestnut oak, and
northern red oak are the species most heavily defoliated in the
Appalachians. Repeated defoliation causes growth loss, reduces mast
crops, and will eventually kill the tree.
[Illustration: Sixth instar elm spanworm larva.]
Eggs are laid in masses of 12 to 200 on the undersides of host tree
branches in early July. Winter is spent in the egg stage. Larvae hatch
in early spring when foliage opens. The larval stage is an “inchworm” or
“looper”, approximately 1½ inches long when mature. Larval coloring
varies from green or light brown to black, depending upon population
density. The typical color of the larva in heavy populations is dark
brown to black, with a dark-red head, legs, and anal shield. The larvae
feed for about 1½ months, and then pupate in a loose cocoon for six to
ten days. The adult, a snow-white moth, emerges in late June or early
July. There is one generation per year.
Natural enemies help keep populations of the elm spanworm in check. One
of the most important is _Telenomus alsophilae_, a tiny wasp which
parasitizes eggs. Persistent outbreaks on high-value stands may require
treatment with chemicals.
[Illustration: Elm span worms feeding on oak.]