On a recent trip to an Ecuadorian cloud forest I was fascinated by the large numbers of diseases and deformities that riddle the plants. Many of these take the form of plants galls.
Galls are abnormal tissue growth on the surface of plants caused by parasites, such as fungi, nematodes, insects, mites or bacteria, the galls are tailored as the perfect place for the parasitic organism to live in. In the past I’ve looked for and found plant galls in England, but it seemed that they were more numerous and varied in Ecuador. I believe there are three reasons for this, all related to Latitudinal Diversity, which is the phenomenon whereby animal and plant species diversity increases the closer you get to the equator, it applies most notably to rainforests.
- With a wider variety of plant hosts, there will be a wider variety of parasites to take advantage of them
- A greater number and wider variety of insects leads to more varied insect galls
- It is not known for sure that there are more species of fungi and bacteria close to the equator, but it is likely since fungi and bacteria both benefit from a stable environment where light, temperature and humidity are fairly constant, all of which is more true at the tropics than further North or South
What makes galls particularly bizarre, is that these growths are not attachments to the plant, but the plant itself, made to alter its normal growing behaviour in order to benefit its parasite host. Although it is not clear how insects cause this change, bacteria is known to insert its own DNA into the plant cells to alter behaviour. (example here) Insect larvae have been found actually inside the cells of the plant, which might suggest similar interference. It is thought that one wasp (Cynipinae) works in conjunction with a virus (viruses reproduce by inserting their own DNA into the DNA of a host) that lives in the wasps saliva and gets into the plant as the wasp eats it. This is an example of mutualism, since the insect benefits from the virus by getting to make the gall and the virus benefits by getting to reproduce.
Insect and mite galls
These galls are formed by the insect or mite either feeding or laying eggs. When adults lay their larvae on a leaf, excreta or saliva from the insect affects the cambium and causes it to grow differently (more detail above). The larva then grows inside the gall, feeding on the gall itself, eventually eating its way out and escaping. Sometimes the insect control over the plant tissue extends beyond the gall and starches and sugars are drawn in from elsewhere in the plant to increase the food store for the insect.
Insect and mite galls in Ecuador
Insect gall on tree trunk
Underside and Upperside of Leaf
Insect galls
Insect galls on Columnea
Insect galls showing holes of escaped insects
Non Gall Insect Invasions
Sometimes I found insects that had taken over leaves, or even entire plants to make a home in. These were not galls, because while leaves were often distorted, the cells were not expanded or changed, but they were still quite bizarre to see.
Ants Forming a Home in a Plant
Wasp Swarm onto Leaf
Insect Colonise a Leaf
Note: for other insect photos I took in Ecuador, see here
Other Galls
Although some galls I was clearly able to determine as being caused by insects or mites because I could find the animal or see its exit point, others I am just not sure about. The following are those less easy to decipher galls that may be caused by fungi or bacteria.
Leaf Spot Gall
Leaf Spot Gall on Decayed Leaf
Leaf Distortion
The plant in the next photo is a puzzle, the fluffy looking outgrowths at the base of the leaves (and in between) may be a normal part of the plant, perhaps even be the flowers, but they also look similar to the growths in the above picture, which are definitely galls.
Plant Galls or Plant?
This final gall, I believe, is caused by insects because I think it is possible to see them, the black mass at the heart of the distortion.
Severe Leaf Distortion
Close up of Leaf Distortion
Some interesting and useful websites on plant galls:
Some books I used for reference:
- British Plant Galls – M. Redfern and P. Shirley
- The Kingdom Fungi – S. L. Stephenson
- Parasite Rex – C. Zimmer
Tentative Plant Scientist 