It seems not much is known about how insects induce gall formation (perhaps someone with more of an entomology can jump in).

But we do know quite a bit about Agrobacterium tumefaciens, which causes crown gall in roses, grapes, and other plants. Agrobacterium invades cell tissue and inserts a piece of it's own DNA into the plant cell. The plant cells incorporates the DNA into its genome and begins expressing a number of new genes, including genes that lead to an over production of the plant hormones auxin and cytokinin. These hormones cause the undifferentiation of the plant cell and uncontrolled cell division, leading to the formation of a tumor-like gall.

More on Agrobacterium at Wikipedia. I can recommend some journal articles if you're interested.

It seems not much is known about how insects induce gall formation (perhaps someone with more of an entomology can jump in).

But we do know quite a bit about Agrobacterium tumefaciens, which causes crown gall in roses, grapes, and other plants. Agrobacterium invades cell tissue and inserts a piece of it's own DNA into the plant cell. The plant cells incorporates the DNA into its genome and begins expressing a number of new genes, including genes that lead to an over production of the plant hormones auxin and cytokinin. These hormones cause the undifferentiation of the plant cell and uncontrolled cell division, leading to the formation of a tumor-like gall.

More on Agrobacterium at Wikipedia. I can recommend some journal articles if you're interested.