Maple eyespot gall (left) and the grape tumid gall (right)
If you spend any time around plants you will have undoubtedly come across a gall. In fact, once you know what to look for you quickly realize that galls are everywhere. They come in many different shapes and sizes and they vary as much as the species you will find them on. Galls are abnormal growths on plant tissues and their causes range from bacteria, fungi, and nematodes to insects and mites. Most of the galls we regularly encounter are caused by insects.
You can think of galls as a type of edible nursery chamber. A female insect will lay her eggs in the tissue of the plant and chemicals released by the eggs and subsequently the developing larvae trigger abnormal tissue growth in the plant. Every detail of each gall you see is the result of the insect housed inside, which has led some authors to consider gall formation a literal extension of the insect phenotype. Without the chemicals released by the developing insects, the plant would not form such elaborate growths.
Lime nail gall (Eriophyes tiliae). Public Domain.
As mentioned, galls act as an edible nursery chamber. Not only does the developing larvae gain physical protection, they also consume the swollen plant tissues on the inside of the gall. Despite the attention galls have received in the literature, very few studies have touched on one fact of gall ecology that becomes quite obvious to the casual observer - most of them are very conspicuous.
Oak apple gall (Cynipidae). Public domain.
The shape and coloration of different kinds of gall causes them to really stand out against the background vegetation. Why would a structure meant to protect the developing insect inside be so easy to spot? A handful of interesting hypotheses have been put forth to explain this phenomenon. For starters, the chemical compounds that give many galls their distinctive coloration are the result of hijacked plant pigments such as carotenoids, anthocyanins, as well as tannins and other phenolic compounds. These are thought to protect the insect inside. This certainly plays a role, but we will come back to that in a minute.
Cynipid gall (Diplolepis polita). Photo by Dean Morley licensed under CC BY-ND 2.0
Still, one would think being so strikingly obvious would have some serious drawbacks. Predators and parasitoids alike could easily hunt down a bright red gall. Even if potential predators can't see color, the outlandish shape of many galls certainly makes them stand out. There is another hypothesis that gets right to the core of this. Simply put, it is thought that the conspicuousness of galls serves as a warning to potential predators that eating them would be a mistake. In other words, galls very well may be aposematic.
You will be most familiar with aposematic coloring in bees and wasps. Bright colors such as red or yellow contrasted against a strikingly different colored background serve as a warning to anything that might be thinking of taking a bite. "Stay away, I will hurt you" is the gist of the message. The bright coloration and often outlandish shape of galls coupled with the defensive compounds mentioned above may be sending a signal to herbivores, predators, and parasites to stay away or risk injury or illness. Being easy to find also makes galls easier to remember and a bad experience with one gall may make a bird think twice before messing with one again. In this way, the insects inside can go unmolested until it matures.
Obviously there are many caveats to this idea. Certainly not all galls fall under this umbrella. The researchers behind this hypothesis have outlined a series of predictions that are thought to promote the evolution of aposematism as a strategy. What's more, this hypothesis will need to be tested on many different types of galls in many different habitats with many different potential predators if it is to hold up. Still, it is an interesting idea worth investigating. One can see the potential here.
Further Reading: [1]