Knopper Gall

Andricus quercuscalicis

Caused by the asexual reproduction of a gall wasp on English Oak and Sessile Oak. The galls enclose acorns and raised significant concerns when they were introduced to Britain in the 1960s, as it was believed that the impact on acorns could reduce future generations of oak trees. Initially, knoppers are green, sticky, and develop a reddish hue. Once mature, they turn brown and woody. They fall to the ground in late summer, and gall wasps typically emerge the following spring.