aphids – the heart thrills

Vienna, 21 November 2020

Just look at that maple! What a magnificent yellow its leaves turned!

My wife and I walked under it during a hike we did a couple of weeks ago. We were following the edge of a wood and lo and behold! there it was.

As I mentioned in an earlier post, as we have been walking the woods these last few weeks the trees have been putting on their autumnal colours. We have been bathed in yellows of all hues, turning to russet, and finally to dark brown.

But what we have not been bathed in is reds. We have not witnessed the wonders of a North American Fall

or the splendour of an East Asian Autumn.

“Why is that?” I asked myself as I sat there gazing at my photo of that yellow maple tree, “why is it that North Americans and East Asians have splendid red hues in their autumn colours and we in Europe do not?”

To answer this, we are going to use a version of Root Cause Analysis called the “5 Whys”. This was something invented by Sakichi Toyoda, the father of the founder of Toyota, who claimed that you had to ask “Why?” (more or less) five times before you got to the root cause of something. His son used it extensively in his car factories as a quality control tool, to discover the fundamental reason – the root cause – for a quality failure (and at a much more modest scale I have used it to discover the root cause of a source of pollution or waste). A simple example goes as follows:

“Why the hell isn’t my car working?!”
Because the alternator isn’t functioning.
“Well why is the bloody alternator not functioning?!”
Because the alternator belt has broken.
“Oh. Why did the alternator belt break?”
Because it was well beyond its useful service life but has never been replaced.
“Ah. Why wasn’t it ever replaced?”
Because you, idiot that you are, didn’t maintain your car according to the recommended service schedule.
“Ah, right, OK, sorry about that.”

OK, so now we can start using the method on our little problem:

“Why do the leaves of many species in North America and East Asia go red, whereas so few do so in Europe?”

We see leaves as green because of the chlorophyll they contain. But leaves also contain other pigments, which if the chlorophyll were not there would make the leaves look yellow, orange, or all hues in between. The chlorophyll simply masks them.

In Europe, when autumn comes and the chlorophyll begins to disappear, these other pigments are finally allowed to “express themselves”, giving the leaves the beautiful hues of yellow that we see. This explains the fact that the maple we came across went from green to lovely canary yellow.

In North America and East Asia, something else happens when the chlorophyll begins to disappear from the leaves. There, trees begin to produce – from scratch – a red pigment, anthocyanin, in their leaves. This pigment masks – or perhaps “mixes with” – the yellow or orange pigments already there, to give various shades of red. Thus do North American and East Asian maples go from green to red.

“OK, but why do North American and East Asian species produce this red pigment at the end of their leaves’ lives?”

Yes indeed, it does seem that the trees and bushes which do this are penalizing themselves. Just when their leaves are about to fall off, part of the general shut-down for their winter slumber, the trees start expending precious energy to pump their dying leaves full of red pigment. The reason for this apparently foolish behaviour has to do with pest control and especially control of aphids (which I happened to mention in an earlier post on wood ants). Aphids have this nasty habit (as far as trees are concerned) of sucking amino acids from them in the Fall season, and then laying their eggs on them; the eggs hibernate along with the trees and give birth to a new generation of aphids in the Spring. So the trees get hit twice: they lose precious amino acids to those pesky aphids, and then the next year they have to endure attacks by the next generation of aphids! Now, it so happens that aphids believe that a brightly-coloured tree is a tree that is chemically well defended against predators, so they tend to avoid laying their eggs on such trees. So of course trees in North America and East Asia have evolved to turn themselves bright red in the Fall, just when the aphids are laying their eggs, by pumping their dying leaves full of anthocyanin.

“Why do aphids think a brightly-coloured tree is a chemically well defended tree?”

I thought you might ask that. The answer is, I don’t know. Stop being a smart-ass and move on to the next question.

“A bit touchy are we? Well OK, why don’t European trees make their leaves go red then?”

Because they don’t they have aphids which prey on them.

“Why is that? How can it be that aphids prey on the North American and East Asian trees and not on the European trees? What’s so special about European trees?”

Yes indeed, this is where it gets really interesting. To answer this, we have to go back 35 million years. At about this time, the northern hemisphere began to go through a series of ice ages and dry spells. Most trees reacted to this by going from being evergreen to deciduous. They also retreated southwards when the ice sheets advanced and returned northwards when the ice sheets retreated. In North America and East Asia, their predators of course went with them, evolving to deal with the fact that trees now lost their leaves and went dormant during the winter. In turn, the trees evolved to fight off these predators by, among other things, turning their leaves red in the Fall. This struggle between tree and predator continued even as the trees moved northwards or southwards as the ice sheets advanced or retreated. Thus, still today, the trees in those parts of the world go a glorious red in the Fall.

But in Europe, there were the Alps and their lateral branches, which ran east-west. In North America and East Asia, the mountain ranges, where they existed, ran north-south, so the trees in their periodic advances and retreats could “flow around” these mountains. In Europe, though, as the trees moved southwards to escape the ice sheets they hit the barrier of the Alps; there, they could go no further and so perished in the piercing cold. And so of course did the predators which they harboured. Only seeds were carried southwards, by birds or the wind or in some other fashion, and of course these seeds harboured no predators. Thus it was that European trees did not need to make red leaves and so they give us glorious shades of yellow in the Autumn.

There is at least one exception to this rule, and these are dwarf shrubs that grow in Scandinavia. They still colour their leaves red in autumn. Unlike the trees, dwarf shrubs managed to survive the ice ages; in the winter they would be covered by a layer of snow, which protected them from the extreme conditions above. But that blanket of snow also protected the insect predators! So the plants had to continue their struggle with their predators, and thus evolved to colour their leaves red. We have here an example, the smooth dwarf birch.

Well, that was an interesting use of the 5 Whys method! I must see if there are other issues I could use it on.

Vienna, 22 August 2020

It was during the hike which my wife and I did in the Dolomites this year that we first noticed these large, cone-shaped piles of dead pine needles on the side of the path.

They mini Mt. Fujis were really quite arresting in their symmetry among the gentle anarchy of the forests.

A closer look at them told us that these were ant nests; there were columns of ants radiating out from them into the surrounding undergrowth and their surfaces were pullulating with ants.

A bit like the statues of John of Nepomuk that I have written about earlier, once we noticed one nest we began noticing them everywhere we went on our subsequent walks. We always came across them in wooded areas, mostly among conifers or mixed woodland. Sometimes the nests were modest mounds, at other times they were really quite large.

A little surfing of the web has taught me that these nests belong to wood ants, of which there are some 32 species distributed in the colder reaches of the northern hemisphere: 13 in the Eurasian continent, spread all the way from Japan to Ireland, and 19 in North America. My favourite of all these species has to be Formica lugubris, the lugubrious ant. I wonder what its namer had in mind when they came up with that name. This particular species seems no more lugubrious than any of the others. I throw in a close-up of another species, Formica rufa.

I must confess to putting in this close-up photo simply to gross out my wife, like I did with close-ups of crickets and dragon flies in earlier posts (so childish of me …). However, the first of these photos also allows me to point out the ants’ black and red colouring – although I must confess not to have noticed this colouring scheme when inspecting the ants milling about on top of the nests.

Coming back to Formica lugubris, I have to say if I were a wood ant I think I would feel pretty lugubrious. The great majority of the ants are – female – worker ants. They spend their whole short lives (a couple of months) looking after the queens and their babies (or grubs, to give them their more scientific name), feeding them, moving them from one good spot in the nest to another, watching over them as they finally pupate and metamorphose into adult ants, and generally fussing over everyone; marching off into the surrounding forest to collect food; building up the nest, mending its thatch (more on that in a minute) … and all this and more with hardly a moment’s rest (a power nap from time to time is all they get). No wonder they croak after a few months! As for the few males, they are of course completely feckless, doing bugger-all to maintain the nest or feed the kids (typical …). Mind you, they have even shorter lives than worker ants – a couple of weeks. They have only one role in life, which is to impregnate the queens. This they do with savage abandon, with these mating rites becoming a huge free-for-all. Once that is over, they expire – if they haven’t already become lunch for birds and other predators who hang around during the mating rites and pick them off. As for the even fewer queens, they only need to go through the mating rite once in their much longer lives (they can live up to 15 years or so); the sperm they so collect lasts them a lifetime. Thereafter, they bunk down in the nests, and spend the rest of their lives begetting children and sleeping. What a life, for all of them!

Of course, to think of ants in human terms is very silly: ants are ants, humans are humans. But this tendency of projecting human foibles onto animals has a very honourable history. Take the French poet Jean de La Fontaine, for instance. He wrote many animal-centered poems whose point was to skewer human weaknesses and stupidities. One of his best-known poems is La Cigale et la Fourmi, the Cricket and the Ant:

La Cigale, ayant chanté
Tout l’été,
Se trouva fort dépourvue
Quand la bise fut venue
Etc.

The point of the poem is that the cricket spent the whole summer singing the days away while the ant industriously spent it collecting food to see it through the winter.

Come autumn, the cricket comes piteously to the ant, asking it to give it some food, and the ant tells the cricket to bugger off (the moral of the tale for us humans is made clear in this old drawing, by dressing up both cricket and ant in humans’ clothes and having the ant live in a human house).

My French grandmother often quoted the poem in approving tones, making it clear to me that I should be the industrious ant and not the feckless cricket. Personally, I think the story lacks Christian charity, but perhaps in La Fontaine’s day, when most people lived very close to the edge, they simply didn’t have enough to be able to generously share with feckless idiots who had failed to lay in the necessary provisions.

But back to the wood ants.

Let me describe their nests, which are marvels of engineering. First let me insert a cut-away diagram of a nest.

The whole structure is designed to maintain optimum temperature and humidity levels for the grubs and pupae. So, the nests are somewhat flatter on their southern side, to have the sun’s rays hit the nest as directly as possible; the worker ants lay the pine needles and other debris which make up the nest’s thatch in the direction which maximizes the latter’s ability to heat up in the sun’s rays; the nests are often built around a tree stump – the heat given off by the rotting process adds to the nest’s heat; and if that is not enough, worker ants will “sunbathe” on the thatch and when they are nice and hot will go back into the nest and cool down where heat is needed. As for control of dampness, the ants carefully choose sites which are not damp in the first place. Then the same thatch will act as thatch on a human house, keeping out the rain. Since the bottom of the nest, which is in the ground, tends to be damper the worker ants will carry damp material from the lower floors to the upper floors to even out dampness differences.

These wonderful nests have attracted a number of hangers-on. Some are useful, like the worm Dendrodrilus rubidus, for instance. It gets (steals?) food in the nest but it keeps moulds and fungi in check. So it pays for its keep, as it were. Others are not, like several species of beetles, which spend their larval stage in wood ant nests. Most are just a nuisance, eating plant food they find there. Several species of beetle, though, are real little bastards. They eat the pupae, and to avoid being killed by the ants they produce chemicals which disguise their presence. Some of these little buggers go so far as to secrete a scent which the ants can’t resist. The poor ants then allow the beetle to roam freely about the nest unharmed. Little shits … In the case of other species, it’s not clear if they play a role – bad or good – in the nests. There’s the tiny shining guest ant, for instance. It has its own tiny nests and tiny broods in the wood ants’ nests. If a queen and a bunch of worker ants take off to set up a new test, a bunch of shining guest ants will go with them. But when the going gets tough – when conditions in the nest deteriorate – the shining guest ants get going: “hasta la vista, baby, been nice knowin’ ya!” And then there’s a species of woodlouse which has been cohabiting with wood ants in the dark chambers of their nests for so long that it has lost its eyes and colouring (I remember reading about the same thing happening to some species of fish which were discovered living in completely dark caverns off the coast of Mexico somewhere).

As I said, if you look at a nest you’ll see columns of ants marching off to forage – and marching back with what they’ve foraged. Wood ants play an incredibly important role in keeping in check certain species which are bad for the health of the trees – more on this in a minute. But they actually get most of their food from stroking the bums of aphids. This is an absolutely fascinating relationship, probably the only known example of farming by a species other than humans.

Aphids feed by sucking the sap from trees and shrubs. They extract what they need from the sap and excrete the rest as “honeydew” – the name gives one an idea of the taste of this stuff, which is packed with sugars, acids, salts and vitamins. Wood ants love this stuff, and it makes up the major portion of their diet. Over time, wood ants and aphids have developed a symbiotic relationship. Wood ants look after the aphids; they protect them from predators and they move them around to places with more or better sap.

In return, aphids will excrete their honeydew when gently stroked by the ants. It’s hard not to think of human beings and their cows when you read about this relationship.

The ants will fill themselves up with the honeydew, march back to the nest, disgorge it and feed it to the queens and grubs.

One reads lurid stories about ants biting and stinging people. Wood ants can certainly bite – they have the necessary mandibles – but they also have a secret chemical weapon. They keep a store of formic acid in their gaster (that bulbous end section of theirs), which they can spray at attackers or prey.

As the photo shows, they can shoot our their formic acid over quite a considerable distance, relatively speaking. If they were my size, they would be squirting formic acid over a distance of 20 metres – not half bad! As you can imagine, a concerted attack like the one in the photo would be enough to keep most predators away. But some birds have figured out how to turn this spray of formic acid to their advantage. They alight close to the nest, and use the resulting formic acid shower as a way of killing off parasites which they’ve picked up. This European Jay, for instance, is having its formic acid spray-over and seems to be quite enjoying the experience.

Of course, formic acid gets its name from the Latin name for the ant, formica. Formic acid was discovered by one John Ray, an English naturalist, in 1671. He obtained the acid by getting hold of a large number of wood ants, crushing them, and distilling off the acid from the resulting mess. Poor ants! sacrificed to the advancement of science. Here is the painting of the man about to do something awful to a foxglove.

I’m sure myrmecologists (which I have learned is what experts in ants are called) would find a thousand and one other things which are fascinating about the wood ant. But I’ll stop here. There is one final thought, though, which I want to leave with my readers because it goes close to the work I’ve been doing these last forty years.

The fate of wood ants is a great example of human beings thinking they are very clever and know everything when in fact they know very little. This is particularly true of the workings of the natural world. Thus, in the case of wood ants, people didn’t realize that they probably play a key role in the health of forests. I say “probably” because actually we don’t know all that much about the life and times of wood ants, so it’s difficult to judge their true role in forest health. Nevertheless, they certainly seem to keep down the populations of insects which would otherwise attack trees, like caterpillars of moths such as the pine looper and sawfly. Their farming of sap-sucking aphids also appears to affect tree growth. They help in distributing the seeds of plants. They of course provide food to a whole suite of animals. Yet we have thoughtlessly – and ignorantly – been destroying their habitat. As a result, wood ants are on the International Union for Conservation of Nature’s Red List, although thankfully at the milder end of that List. Some species are already extinct locally – the black-backed meadow ant, for instance, is extinct in the UK since 1988. Not only is it really troubling that these ants could be facing extinction (“extinction is forever”), but foresters are also finding that the health of forests has been impacted as a result of the drops in ant population. In this day and age, when we desperately need every tree we have to combat climate change, that is truly worrying. In fact, efforts are now underway to protect these ants and get them to help us protect our forests. I can only hope for the best.

Tag: aphids

YELLOW AND RED

WOOD ANTS