Wednesday, September 15, 2010

Turning over a new leaf

The hoi polloi views insects as pesky fellow passengers on this crazy ball in space and science degrees hardly have an effect on most people. However insects are perhaps one of the most interesting fellow passengers around and more than make up for the annoyance of a few biters and blood-suckers. They came out of the waters a long time before the branching of life-forms with internal skeletons. Which means that they have had that much more time encountering the problems of life and solving them using "evolutionary algorithms". What interesting answers and questions might they have figured out ? (apart from perhaps finding that the answer is 42!)

Galls on Pongamia leaves
How does one find food on a planet like this ? There is so much green stuff and yet humans in a forest find so little that is edible. What has gone wrong here ? For someone who thinks about insects and plants, the real questions are not about how to keep the insects away from plants but about how plants keep them away and stay green in spite of the onslaught. Not every plant-eating insect feeds on every plant species and the few that are found in numbers on any plant has evolved such an array of tools to deal with their dinner and dining table.

Here are some Pongamia pinnata leaves I found not so long ago. One usually finds little outgrowths on the leaves. These are mostly insect galls, but galls may also be formed by fungi. If you cut an insect-gall you will likely find a central hollow, perhaps an exit hole on the base or tip and if you are lucky, a tiny larva somewhere. That could most likely belong to one of numerous wasps or flies and you will surely be hard-pressed to find anyone who can identify the species. Galls are not always shaped like this - they can be spherical, formed at the tip of a branch and botanists have been known to sometimes describe the gall as the characteristic fruit of a new found tree species. What exactly is happening is something to consider - is the plant trying to isolate the insect like a cyst forms to engulf a foreign body or is the insect manipulating the biochemistry of the plant to make it produce tasty tissue around it ? Turns out that the latter is often the case and sometimes the gall-inducer seems to be capable of inducing galls with very specific shapes, colours and structures (this article with illustrations is particularly worthy of reading).

Ficus at Hebbal with leaves eaten away
Anyone who has worked with plant-tissue culture can tell you what a complex and sensitive cocktail of auxins, cytokinins and other compounds have to be delivered to get a bunch of cells to grow into something that resembles a plant. Now, given that the structure is essentially an insect induced "fruit", there must be a few that people can eat and after some research I came upon the "Mulga apple" (apparently on Acacia aneura) of Australia but the number of cases of edible galls seems rather low. I have a rather vague recollection of seeing swellings on nettles in the Uttaranchal Himalayas and some comments on their edibility (if anyone has eaten this or knows more please do let me know). Now plants are not taking such damage lying down and they have come up with their own defences. (some years ago some of us worked on this article on Wikipedia and hopefully it is still readable) Turns out that the biggest human use of galls is in the extraction of tannins - and tannic acid - a mixture of potent chemicals used to treat leather (tanning leather), at least in the past. And printing ink was once made by treating rust with gall extracts ! Tannins are essentially anti-insect chemicals and in some plants, the tannin content goes up rapidly when the plant is physically damaged by insects this can have an effect on the insects. Now if the insects are such experts in plant biochemistry, perhaps the plants have in turn figured out the most critical insect biochemical gears into which their molecular spanners should be thrown. Given that these are potent chemicals tailored through evolutionary time-scales, there must be something in these Chinese cocktails worthy of some serious scientific attention.

Leaf mine - note widening with age of miner
The birdwatcher's of Bangalore hold regular field outings at Hebbal and Lalbagh on the first and second Sundays of each month. On one of the Hebbal outings around June and July we came across this Ficus, the leaves with holes, the ground below covered in tea-like frass and you could hear the fall of caterpillar droppings. The caterpillars were extremely pretty, moving along the trunk in large numbers, presumably the last instars looking for a place to pupate. Unfortunately we have been unable to obtain any further identification of this lepidopteran caterpillar. With the monsoon rains, the tree has now shed all its leaves. Now leaf-shedding is actually a bit of a story of its own and there are numerous theories on when a tree "ought" to shed its leaves "if its purpose" was to achieve something and one such function, teleologically speaking, is to get rid of pesky insects. But surely dropping leaves is not a good strategy to get rid of that eat leaves . At least certainly not without withdrawing all the best nutrients.

The caterpillars on the Ficus (unidentified)
Perhaps the easiest insect targets to get rid off by leaf-shedding would be things like galls and leaf miners. Life miners, usually moth or fly larvae live in the layer of the leaf and feed under the protection of the leaf surface, no rain, no drying and perhaps some protection from parasites and predators.
So, next time you have something to ponder over when you see a fallen leaf...

Turns out that some studies suggest that leaf dropping does not kill many leaf-miners and so seems unlikely while others point out that gall infestation induces early leaf shedding.

Postscript

Also discovered Huitlacoche caused by Ustilago maydis

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