Gall Wasp

Chemical architects

Gall wasps are master chemical engineers. A female lays an egg in or on oak tissue — bud, leaf, stem, root, or acorn — and simultaneously injects chemical signals that hijack the tree’s growth hormones. The oak responds by growing a gall — a precisely shaped, species-specific structure of plant tissue that completely surrounds the larva, providing both food (the gall wall) and protection (hard or sticky outer layers). Each species produces a uniquely identifiable gall.

Oak apple

The oak apple (Biorhiza pallida) is the most familiar British gall — a large, spongy, apple-like growth produced on young oak shoots in early spring. Inside is a central chamber containing the larva. After the larvae pupate and the wasps emerge in summer, the empty galls persist on the tree, turning from pale green to brown. Oak Apple Day (29 May) was established in 1660 to celebrate the restoration of Charles II, who hid in an oak tree.

Alternating generations

Many gall wasps have complex life cycles with alternating sexual and asexual generations, each producing a different gall on a different part of the oak. For example, Andricus kollari (marble gall, introduced from the Middle East) produces round marble-like galls on shoot buds from sexual generation females; the asexual generation produces completely different small galls on turkey oak catkins. Without knowing the life cycle, the two galls would appear to belong to separate species.

Inquilines and parasitoids

Each gall supports a community beyond the wasp that created it. Inquiline wasps lay eggs in the gall, feeding on gall tissue alongside the rightful occupant. Parasitoid wasps drill into galls with long ovipositors to lay eggs on the gall wasp larva. Predators including great tits chip galls open to find the fat larvae inside. A single oak can support 50 or more gall wasp species and hundreds of associated organisms.