Callirhytis seminator

Callirhytis seminator, the wool sower gall wasp, is a gall wasp in the family Cynipidae. It is named after the puffy, white gall it makes on trees.
The adult wasps are about 1/8" in long. They are dark brown in color. The larvae (grubs) are white and fat and have no legs.
The wasps only lay eggs on white oak trees and only in the spring. They lay the egg in a gall, a round, white structure. It later changes color. The larvae live and grow inside the gall. People who open up the galls to look at them see things that look like seeds. They are not seeds. They are hard plant material. They protect the larvae.
Gall wasps have a two-generation system. This means that one set of parents makes children that look different from themselves. Then those children make children that look like the parents. That means grandparents and grandchildren look like each other, but parents and children do not. Among gall wasps, one generation makes stem galls, and second makes leaf galls. Scientists do not know what the alternate wool sower wasp gall looks like.
The wasps do not sting humans. They do not hurt the white oak trees.