Autumn colours and aphids

In a previous life, before I became a successful, respected and well-balanced science writer, I did a PhD on the evolution of soldier castes in aphids. Happy days (not).

In this life — in 1996, to be exact — I travelled to an entomology conference in Florence. There, at a session on aphid biology, a fellow grad student and more talented researcher than myself called Sam Brown gave a presentation, coauthored with his supervisor Bill Hamilton, on the idea that the autumn colours of trees were a signal to marauding aphids of the strength of their antiherbivore defences.

Bright colours, their argument went, were an honest signal: only well-resourced trees able to make lots of the secondary compounds that plants use to make themselves unpalatable to herbivores would be able to make them. And aphids flying around in the autumn, looking for somewhere to lay their overwintering eggs, would shy away from such vivid displays.

The debate on the worth of this neat hypothesis has rumbled along for more than a decade now. Sceptics have pointed out that yellow pigments were there all along, but are only revealed when the leaf takes back it’s chlorophyll — leaves don’t turn yellow, they unturn green. Also that red pigments have other potential uses, centred on protecting the leaf as the plant takes it apart, chemically speaking. The latest update comes from Lars Chittka and Thomas Döring, two researchers on sensory ecology writing in PloS Biology.

To put it crudely, C and D’s piece plugs a forthcoming paper (this link wasn’t working when I checked) by them looking at colour vision in aphids. They find that Brown and Hamilton might have been deceived by their human’s-eye view. Aphids (and all other herbivorous insects looked at so far) don’t have good vision in the red — so red leaves wouldn’t look anything special to them. And yellow is powerfully attractive — it tickles their green receptors even more so than green itself, and brings insects flocking: ‘If trees wanted to deter herbivorous insects using color, yellow leaf coloration is about the worst strategy they could pick,’ they write.

The piece does such a good job of pouring cold water on the notion that autumn colours are signals that their final (inevitable) conclusion — the good old cry of ‘more research is needed’ rings a little hollow. Speaking as a rampaging adaptationist, who thinks that Hamilton has a good claim to having been the most important evolutionarybiologist since Darwin, I think that’s a bit of a shame, but there you go, beautiful theories, ugly facts and all that.

The essay also has some nice bits of history, and a lovely first sentence: ‘Most living things don't turn beautiful when they senesce’. But some editor should have stopped the use of ‘fascinating’ twice in the first paragraph. I can work out whether something is fascinating or not. (And while we’re about it, can we have a ten-year moratorium on the use of ‘intriguing’ in science journalism? It’s just ‘interesting’ wearing a false moustache.)