Definition of willow borer
1 : any of several small bronzy longicorn beetles of the genus Agrilus that bore in the sapwood of the willow tree and often kill it: such as
a : a beetle (A. politus)
b : a beetle (A. anxius) that infests also poplars and birches
2 : poplar borer
3 : a European weevil (Cryptorhynchus lapathi) introduced and destructive to willows in the eastern U.S.