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.