Tree's Got a Lotta' Gall was originally published in the BFEC Newsletter, Vol. 14/No. 3, Summer 2010.
White oak trees, while certainly not alone in having relationships thrust on them, do seem to invite more than their share of hangers on (see p.1 of original newsletter). Of the many organisms that live among their branches, some cause tissue abnormalities, or galls, to grow and insects are the most common culprits.
Saw flies are amongst the most pervasive of these. Many species cause galls to form on leaves, ranging from long-stemmed, to warty, to brightly-colored and nipple shaped. One species even causes a flat, scale-like gall during one generation that alternates with a woody root gall during the next.
Gall wasps form galls that range in size from the microscopic to the massive. The familiar spotted oak apple, a spherical gall that hangs from the undersides of leaves (pictured below) can grow to about an inch in diameter, while other wasp galls are nothing more than single-cells that develop inside leaf tissue.
Not all galls serve only their makers. In a twist, some actually allow the tree to selflessly provide an ecological support-service as they are consumed by visiting animals. Twig galls provide winter nutrition to finches and sparrows, squirrels and mice feast on colonial hedgehog galls, and even young oak apples are eaten by several species throughout the year. Covered in insects and sores and still gracious? Now that’s turning a frown upside down.