ORIGINAL: B.L.E.
On sheeted foam wings, it is the balsa skin that gives the wing its strength, the foam only gives the wing its shape. For even higher strength, the wing can be covered with glass cloth and epoxy or some other resin.
Not quite accurate. Byron originals had many large scale designs that were foam with low-temp covering covered directly over the foam. Their strength came from the use of hardwood spars inside the wing. Most guys cover the foam with balsa to make it easier to cover, and to make it dent resistant; not strength.