Have you ever noticed the following strange lumps on some leaves? These are leaf galls or abnormal growth of leaf cells due to stimulation by external agents. The external agents can be many things including insects. Galls may act as an external womb for some insects where their larvae develop. The interest in galls seem to be low here even among the botany community as I did not see much information on leaf galls in this country. Hopefully, there will be more insight to this topic pertaining to plants in this part of the world.
Galls are usually host plant specific. One can probably identify a plant by the type of galls found on its leaves although there are just a few types of galls around, at least those that are large or obvious enough to be noticed.
Common Yellow Stem Fig as its name implies has light brown stem and is commonly found in nature parks and forested area. The tree bears fruit, known as figs, all year round along its stem and provides the abundant food supply to a number of animals in the wild.
The galls found on the leaves of this fig tree are quite unique, having the appearance of mini rambutan. They are hairy with colour changing from green to red over time before they spilt open to release whatever that was supposed to be in there. At some leaves, the invasion of this gall can be very intense that the whole leaf is covered by the galls.
I was rather curious on what really caused the formation of these hairy rounded features on leaves. In March 2011, I decided to do a more thorough investigation on these gall. I opened up a few galls (3 to be exact) to take a peek at the inner chamber of these gall as well as to look around the leaves and plant to find the causative agent. What I saw in the gall was a tiny orange spot. After bringing it closer to my eyes, the orange dot appears to be moving, likely to be a kind of insect. This little creature was found in all the three galls that I had inspected.
As for the search for the causative agent, I first came across several groups of thrips at the underside of some leaves. Thrips are known to cause gall formation. This particular thrip looked similar to the Japanese Gall-forming Thrips (Ponticulothrips diospyrosi). The other possible suspect is a type of tiny fly called gall midges (family: Cecidomyiidae).
Later, at the leaves of another fig tree, I saw some very tiny orange creatures on the leaf surface that could be the main suspect. Upon close examination, they looked like bees with orange striped abdomen and dark brown head region. I could not be absolutely sure that these tiny bee-like creatures were the gall creator but they looked promising. The mystery was finally solved in December 2016. The galls are caused by a jumping plant louse, which is the tiny orange insect. In addition, this hairy gall was also found on the leaves of another fig tree, Common Red-stem Fig (Ficus variegata).
So far, I have encountered the following leaf galls. Unfortunately, I have little information on any suspected causative agent.
The galls have irregular surfaces and come in different sizes. When view from the underside of the leaf, each gall has an opened entrance and the gall appears to be hollow.
The galls are reddish in colour on the young leaves. Its appearance is similar to that of the galls of Litsea elliptica except for the colour. The host remained an unknown forest plant until December 2015 when I chanced upon a fruiting tree. The reddish galls make them quite easy to detect in the all-green surrounding of the forest. The back of the galls is sealed.
The galls are hairy and stand tall on the leaf surface. They tend to be located close to the main vein of the leaf. The hairy feature is likely inherited from the leaf itself. The galls do not have any opening at the back of the leaf.
The galls are slender and elongated in shape. In February 2011, I spotted a group of bugs, belonging to the Pentatomidae family, on its leaves. They included adult and young bugs. However, they were not commonly found on this tree and hence, unlikely to be the agent causing the galls.
For now, these are the 5 types of leaf galls seen. There may be more or smaller ones that are not obvious to me. More information on them will be added as and when available.
First written: March 2011; Last updated: 1 January 2017