Abstract
The great web-spinning pine-sawfly, Acantholyda posticalis (Matsumura 1912) (Hymenoptera, Pamphiliidae), is a common and widespread species, occurring throughout the pine forests of Europe and Asia. In the recorded history of forest entomology of Estonia, no mass outbreaks of this species have been reported before. In 2008, an outbreak of A. posticalis was recorded on the island of Saaremaa, Estonia. About 250 hectares of Scots pine (Pinus sylvestris) stands growing on sandy soils were defoliated. In 2009, no flight of A. posticalis was observed, the larvae remain in diapause in soil. An overview about distribution and outbreaks of A. posticalis is given and its impact on pine stands is discussed.