If you've found your way here, chances are you've noticed some curious growths on an oak tree and want to know more. You're in the right place! I've assembled a user-friendly (and completely free!) guide to help you identify some of the most common galls found on oak trees. 

Get your guide HERE and get exploring!

Oaks as a Keystone Species 

Oak trees are keystone species, supporting thousands of other organisms within their ecosystems. Gall-forming wasps, predominantly from the family Cynipidae, are but a few of the species that call oaks home. These wasps are obligate parasites, meaning they cannot complete their life cycle without their oak host, making them more reliant on these trees than some.

A single oak tree can host a wide range of gall-forming wasp species. Interestingly, the galls they create don’t just serve the wasps - they can become microhabitats for other organisms too. For instance, inquilines (or tenants) are species that take advantage of these galls. Unable to form their own home, they simply move into that of another.  Galls can also attract hyperparasites - parasites that target other parasites - such as this striking metallic blue wasp, Ormyrus nitidulus. 

What is a gall?

A gall is an abnormal growth (often in plants) triggered by the presence of another organism. Typically, this is the result of a parasitic relationship where the organism that causes the gall gains both food and shelter from its host. These are sometimes highly specific, with some parasite species being bound to a single host species. 

Galls can be caused by a wide variety of organisms and occur on many types of plants. In this guide, however, we’re focusing on oak galls - most of which are created by tiny cynipid wasps. Often no more than 5mm long, these wasps are so small and similar in appearance that identifying them in the field is extremely difficult. Fortunately, their galls are far more distinctive and can be used to reliably tell species apart. Without the galls, you might never know these wasps were there at all.

The Lifecycle of a Gall-forming wasp

Most of the gall wasps found on oaks have two distinct generations within a year as part of their lifecycle, each with its own distinct gall:

• Sexual: This generation is made up of both males and females, which enables gene mixing. This generation hatches from galls present on catkins (flowers) or emergent leaves, which tend to be found in the Springtime.
• Asexual (Agamic): This generation is composed solely of females. Their galls are larger, more conspicuous and tend to persist on the plant (sometimes for several years). The galls of this generation are found Mid-late Summer onwards. The females of this generation overwinter in their galls and lay the sexual generation in the Spring. The cycle repeats. 

Before genetic sequencing, the Asexual and Sexual galls were often thought to be different species. To add to the confusion, galls from differing sexual generations can also be host-specific, sometimes with an alternating host. 

This guide focuses on the agamic (asexual) galls as they are, for the most part, easier to find and harder to confuse! 

Location is key

When it comes to identifying different gall species on oaks, the location of the gall on the tree can be diagnostic. This guide is divided into three sections based on where the gall appears: acorn, bud, or leaf. Each gall-forming species lays its eggs in specific parts of the tree, and understanding this can help distinguish between similar-looking galls.

You’ll notice that galls are often found on parts of the tree that are actively growing. That’s no coincidence - these areas are rich in sugars and growth hormones, making them ideal for parasites to exploit for their own benefit.

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