Andrew Holtham is a longstanding Trust volunteer and tree specialist. Here, he shares his knowledge on the bizarre lumps, bumps and growths that occur on plants when they are invaded by different organisms - and how late summer is the perfect time to track them down....

In 1660 Charles II was restored to the English throne after eleven years of republican government. He entered London on 29th May, and in celebration this date became a public holiday right up till the mid nineteenth century. It was known as Oak Apple Day – but what is an Oak Apple?

Oak Apples are unusual growths on Oak trees that bear a passing resemblance to small apples. The story begins with a small wasp that lays its eggs in Oak buds at the end of winter. Somehow the eggs stimulate the buds to grow abnormally in spring – instead of producing leaves, shoots and flowers the bud grows into the Oak Apple with up to 30 wasp larvae safely living inside, each within its own separate chamber. They feed on the ‘apple’, grow, and eventually pupate inside their chambers. The adult wasps then chew their way out and fly off to continue their life cycle.

The Oak Apple is an example of a gall, an abnormal growth on a plant resulting from the stimulus of another organism that uses the growth for food and shelter. For every gall there is a gall causer (in this case the wasp), a host plant (in this case the Oak) and the abnormal growth (in this case the Oak Apple).

There are around 3000 different galls in the UK and they come in a huge variety of shapes and sizes, occurring on almost every type of plant you can think of. They are caused by a wide variety of organisms, including bacteria, fungi, mites and insects. Most are small, befitting the size of the creature that causes them, but crown gall (caused by bacteria) can be the size of a beach ball!

Most galls are host specific, that is to say the gall causer will only associate with one species of plant. This means that if you can identify the host plant it is often straightforward to identify the gall. Oak trees are exceptional in being host to around 50 different galls – most species of plant only have a few each.

Although the gall offers protection to the organism living inside, it is not always enough. Many parasites have evolved to target galls; they lay their eggs in or on the gall and the larvae burrow in to kill the original inhabitants. There are also inquilines, freeloading lodgers who use the gall for food and shelter alongside the original gall causer. Then we have parasites of the inquilines, parasites of the parasites…Over 20 species of parasite and inquiline have been found in Oak Apples, making them a mini ecosystem all of their own!

In almost every case there is no benefit to the host plant from having a gall, so in this sense the gall causer is a parasite. However there rarely seems to be much harm to the host plant, particularly in the case of trees where only a small proportion of the buds, roots or leaf area is ever affected.

Of course not every unusual feature on a tree is a gall. Bracket fungi grow on trees and feed on them but they do not stimulate abnormal growth in the tree. The same is true for leaf miners – they eat the existing leaf tissue without producing any extra growth. Unusual tree growth can be produced by a range of physical triggers such as animal damage to bark and buds – but unless the organism causing the damage goes on to live in the growth and feed on it, it does not count as a gall.

Many of the galls caused by wasps have fascinating life cycles. For instance, after emerging from Oak Apples the male and female wasps mate and the females then burrow down to lay their eggs on the fine roots of Oak trees. These cause small, spherical galls to grow on the roots. After 16-17 months adult, wingless, asexual wasps emerge and crawl up the tree to lay eggs in the buds – starting the whole cycle again.

Although galls occur on all types of plant tree galls are perhaps best known since they often become woody and can persist for many months, even years. Common examples are the hard, round, marble galls, and the extraordinary knopper galls – which always look to me as if an alien has exploded from the inside of the acorn! In both cases look for the small round exit holes that show that the inhabitant has left. Probably best known of the galls that do not grow on trees is Robin’s Pincushion, found on wild rose.

If you want to search for the galls the easiest way is to head for the nearest Oak tree and start looking carefully at the leaves and twigs. Late summer is best, giving the leaf galls plenty of time to grow. Otherwise just keep your eyes open for something that looks out of the ordinary and have an enquiring mind!