JOURNAL ENTRY TWO: The Bug Ball

I would never have imagined the oak as a fruit-bearing tree, but that's certainly the impression it likes to give in these woods. If I were to reach to the branches or bend to the ground, however, and pluck one of these apple impersonators, I would be very disappointed to find what lives within.

As it turns out, the nutrients inside are not for me - rather, this object is at the same time a defensive reaction of the oak and a cozy place for a young insect to grow. If I knew what insect it was, I might understand this relationship even more.

SOLVE THIS GEOADVENTURE MYSTERY:

What type of insect is responsible for creating the apple-like object described in this journal entry?

(clue: Three words, eleven letters)