You want to apply epoxy (slow set, 30min or longer) to the faces of the root ribs. That should be exposed wood. Then press the parts together and hold in place. I like to use masking tape to hold everything together until the epoxy cures. If you get a good, no-gap glue joint between the wing halves, and get enough epoxy on the plywood wing joiner, the wing will be strong enough. If you make a poor glue joint in the center, the wing could fail, the wood joiner by itself usually isn't strong enough.