Over 10,000 acres of aerially visible western hemlock looper defoliation on grand fir was detected on State and Federal forests of central Idaho in 1972. This is the first report of this insect building up in Region 1 since a widespread outbreak in 1937-39. The western hemlock looper has the potential of being one of the most destructive forest insects in North America. It can cause tree mortality in 1 or 2 years of feeding. Extensive tree mortality has been associated with nearly all past outbreaks. Outbreaks usually persist for about 3 years at epidemic levels prior to collapsing from natural control agents. Insecticides have been effective in protecting large stands during some looper infestations.