Observation Series
"Pimples" on Leaves: Galls

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 shared locally. 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 might be able to identify a plant by the type of galls found on its leaves. This page showed some of the galls that I had observed at my field trips.

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Common Red-stem Fig (Ficus variegata):

Common Red-stem Fig is a tall tree with 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.

photo photo photo The galls found on the leaves of this fig tree are quite unique, having the appearance of rambutan but in miniature size. 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 concentration of the galls may be so high that they cover entire surface of the leaf.

photo photo photo I was rather curious on what really caused the formation of these hairy rounded features on these 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 galls. At the same time, I searched around the leaves and plant thoroughly in the hope to find the causative agent. In opened gall was a tiny orange spot. After bringing it closer to my eyes, I noticed that the orange dot appeared to be moving and looked like some kind insect. This little creature was found in all the three galls that I had inspected.

photo photo 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 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 were caused by a jumping plant louse, which was the tiny orange insect observed.

So far, I had encountered the following leaf galls. However, I had little clue on any suspected causative agent.

Litsea elliptica

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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. They looked quite similar in appearance to the galls induced by Erythrina Gall Wasp (Quadrastichus erythrinae) which was originally found in Singapore that attacked leaves of trees from the genus Erythrina [1]. The adult wasp is less than 2 millimetres in length, making it them hardly visible to the naked eyes.

[1] Kim IK, Delvare G, Salle JL. A new species of Quadrastichus (Hymenoptera: Eulophidae): A gall-inducing pest on Erythrina (Fabaceae). J Hym Res 2004;13(2):243-249. | Read article |

Litsea accedens

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The galls are reddish in colour on the young leaves. Their appearance are similar to that of the galls on Litsea elliptica except for the colour. The host remained an unknown forest plant until December 2015 when I chanced upon a fruiting tree (Litsea accedens). The reddish galls make them quite easy to detect in the all-green surrounding of the forest. The back of the galls is sealed.

Canarium pilosum

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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.

Millettia pinnata (Pongam Tree)

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The galls are slender and elongated in shape. In February 2011, I spotted a group of bugs, belonging to the Pentatomidae family (Homoeocerus bipustulatus), 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 were the 5 types of leaf galls seen. There might be more or smaller ones that were not obvious to me. More information on them will be added as and when available.

Last updated: 24 December 2019

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