Answers from the Wild Side: The gall of some wasps

If your oak tree is sporting bright green orbs, read on. — Photo by Danielle Zerbonne

The great outdoors can produce baffling mysteries. MVTimes Wild Side columnist Matt Pelikan tries his best to solve them. Got a question for the Wild Side? Send it to

What’s this…thing growing on young oaks in the woods behind my house?

The Pelikan Brief:

Galls — actually pretty interesting — occur when an insect (generally a wasp) hijacks the tissues of a plant to produce a little wasp nursery. In effect, the wasp induces a tumor and then the wasp larvae mature inside the “tumor.”

The rest of the answer:

Galls are a catch-all term for abnormal growths on plants that are induced by insects. Generally, galls serve as nurseries in which insect larvae can mature. The structure of the gall protects the larva or larvae while it grows, and the maturing insect draws its nourishment from the tissues of the plant. To put it bluntly, galls are tumors that insects deliberately cause in plants to meet their own needs.

While they can be dramatic and (depending on your perspective) unattractive, galls rarely do any serious harm to the plant they’re on. The insect that forms the gall, clearly, has an interest in keeping the affected plant healthy, because the plant is nurturing the insect’s offspring. One exception to the “do no harm” rule, though, has been very much on the radar of Vineyard horticulturalists: a wasp named Bassettia ceropteroides apparently colonized the Island recently, and the lumpy little galls it forms on oak twigs block the flow of fluids through the oak’s transport system. The leaves and growing tips of the oak die, and without the ability to produce leaves, entire oaks die as well if they are heavily infested. In my Oak Bluffs neighborhood, several oaks about three feet in diameter, which must be hundreds of years old, succumbed after just two years of infestation by this tiny pest.

Beyond the basic commonalities of gall formation, this is an incredibly complex phenomenon. Many thousands of insect species are capable of forming galls; in most cases, each type of insect requires a specific plant species in order to form galls successfully. Often the plant needs to be at a specific stage of growth in order for a gall to form. Galls can form on roots, leaves, or stems. What triggers a gall to form varies depending on the insect: chemicals in adult insects, eggs, larval insects, or insect secretions or saliva can all be the active agent in triggering gall formation.

Gall formation is an especially common strategy among a wasp family called Cynipidae, which has many hundreds of species in North America alone (the villainous Bassettia ceropteroides is a Cynipid). You might say that this family evolved with gall-formation as its central reproductive strategy. But thousands of insect species across six different insect orders — from mites to moths — have been shown to make galls. Gall formation is a common ability in insects, and the ability to do it must have arisen multiple times during the course of insect evolution.

Which is pretty remarkable when you think about it: to form a gall, an insect species must develop the ability to hijack the biochemical processes that govern tissue production in a plant, producing a structure that perfectly suits the insect’s needs. Galls are a great example of the highly specialized interactions that govern the lives of many types of insects.