On Friday, I took a walk through the woods. The sun was shining, and autumn was in full swing. A light breeze caused the occasional scatter of crisp leaves, which drifted to the floor around me, settling on the already-golden carpet…
Of course, this type of rhetoric is quite common around this time of year, but autumn is a biologically important time and it is sometimes easy to overlook that with the beautiful changes which occur.
In truth, autumn means vital adjustments for nature as the start of the cold weather sets in. The loss of leaves may be aesthetically pleasing, but there is an important reason behind the process, and I really love the fact that the process means you get to see parts of nature that you don’t always get to, and it was for this reason that I took a walk by the river.
The loss of leaves is an important occurrence for trees. It actually happens as a way of protection during the colder months. Deciduous trees tend to be broadleaved and such leaves are highly susceptible to damage during the colder months, and by shedding them, the leaves protect themselves, conserve energy and prevent water loss. The annual process begins around the same time every year as the temperature begins to decline. Trees will cut off certain cells which provide leaves with the vital supplements they need. Nutrients are reabsorbed and stored in the roots. As chlorophyll is one of the first nutrients to be broken down, this triggers the colour change that we love so much, as the lush greens drain out. As food and water leech from the leaves, so does the life, and eventually they die, causing them to dry out, crisp up and fall off. And with them, come some amazing sights that you don’t usually to see.
I was looking for something quite specific on my walk, and I was super excited to find several specimens. The first was a pair of perfectly round spheres stuck to an oak leaf. These alien globes are often mistaken for fungi, but they are in fact, gall wasps.
The wasps themselves, in this case likely the oak marble gall (Andricus kollari), aren’t seen that often so finding their galls is extra exciting. These balls form after the female lays her eggs inside the plant tissue. The larvae feed on the tissue, and whilst very little is known about the actual process which is occurring, it is thought that the grubs secrete chemicals which adjust the plants normal growth process resulting in abnormalities in the plant cells. The wasps are exploiting the plants to give their young a protective home, aiding them in the start of their life, pupating inside these strange but exquisite orbs.
The galls largely contain single larvae, but in some cases, such as with the oak apples, multiple larvae can be secreted. Sometimes, the galls will also contain other insects, some just feeding on the gall without being the cause, others parasitise on either the gall or the wasp larvae inside.
Continuing my search of the woodland, I was treated to a few other specimens. I came across a different shape of gall on an oak leaf, this time flat and in larger number. These are common spangle galls (Neuroterus quercusbaccarum). Another oak leaf treated me to a doughnut shaped gall, the work of an oak silk-button spangle gall wasp (Neuroterus numismalis).
Whilst the few I found have all been caused by wasps, not all galls develop due to insects. In fact, there are lots of different types of galls and they can be caused by bacteria, viruses, mites and fungi, as they are the result of a reaction within the plant cells. In fact, galls are not restricted to plants either, they can be found on animal bones or even on humans. Galls are so widespread that there is a form of study dedicated to them, cecidology, and the truth is we don’t know a great deal about them. However autumn is a fantastic time to study these nature phenomenon’s, so whilst you’re out walking, take a closer look at the leaves which fall, study the changes, and get more acquainted with some of the fascinating secrets which are going on around us all the time.