In autumn, as green hues give way to yellows and oranges, some leaves develop mysterious green islands, where life apparently holds fast against the usual seasonal decay. These defiant patches still continue the business of photosynthesis long after the rest of the leaf has withered. They aren’t the tree’s doing. They are the work of tiny larval insects that live inside it – leaf-miners.
The larvae were laid within the leaf’s delicate layers by their mother. They depend on it for shelter and sustenance, and they can’t move away. If their home dies, they die, so they have a vested interest in keeping at least part of the leaf alive. These are the miniature landscape architects that create the green islands, and they don’t do it alone – to manipulate the plant, they wield bacteria.
Wilfried Kaiser and scientists from Rabelais University discovered this partnership after realising that some bacteria and fungi can also cause green islands. He reasoned that microbes might be helping insects to achieve the same ends. So he searched for them in one particular species, a tiny moth called the spotted tentiform leaf-miner, Phyllonorycter blancardella. Its larva makes its home in the leaves of apple trees.
Kaiser found that the leaf-miners are host to just one detectable type of bacteria – Wolbachia. That’s hardly surprising. Wolbachia infects around 60% of the world’s insect species, making it a strong candidate for the title of world’s most successful parasite. Without exception, every leaf-miner that Kaiser tested, from all over the Loire Valley, carried Wolbachia in their tissues.
Autumn is a time of incredible beauty, when the world becomes painted in the red, orange and yelllow palette of falling leaves. But there may be a deeper purpose to these colours, and the red ones in particular. In the eyes of some scientists, they aren’t just decay made pretty – they are a tree’s way of communicating with aphids and other insects that would make a meal of it. The message is simple: “I am strong. Don’t try it.”
During winter, trees withdraw the green chlorophyll from their leaves, and textbooks typically say that autumn colours are produced by the pigments that are left behind. That’s certainly true of yellows and oranges, but reds and purples are a different story.
They are the result of pigments called anthocyanins, which trees have to actively make. That uses up energy, which is lost to the tree when the leaf falls. An investment like that implies a purpose, and that’s what scientists have been trying to uncover.
Shortly before he died in 2000, the great William Hamilton (he of kin selection fame) suggested that autumn colours are a warning to insects. Many species, such as aphids, lay eggs in trees during autumn and their larvae feed off their host when spring arrives. That’s bad news for the tree, which defends itself with insecticidal poisons. Those that are particularly well-defended would benefit from advertising themselves as inhospitable hosts, and Hamilton suggested that they do this through red leaves.
Hamilton found some support for the idea – for example, he showed that trees that have the strongest autumn colours are also those that are plagued by the widest array of aphid pests. But his former student, Mario Archetti from the University of Oxford, has truly championed the theory and his latest findings provide the strongest support for it yet. They show that aphids avoid red-leaved apple trees, that they fare better on trees without them and that wild trees have far redder leaves than domesticated ones, which are less troubled by the challenges of insects.