gall wasp

US /ˈɡɔːl wɑːsp/
UK /ˈɡɔːl wɑːsp/
"gall wasp" picture
1.

a small wasp that lays its eggs in plants, causing galls to form

:
The oak tree was covered in galls caused by gall wasps.
Scientists are studying the life cycle of the gall wasp.