It’s late autumn, nearly winter, and, as you would expect, the leaves on this Beech tree have all now changed from vivid summer green to autumn brown … or have they? If you look more closely at this image, you’ll notice that some of the leaves have what entomologists, moth-ers and others in the know call ‘green islands’.
Butterfly Conservation’s Associate Director of Recording and Monitoring Richard Fox explained the reason for this most succinctly in a recent post on Twitter: ‘Thanks to Wolbachia bacteria in its body, the caterpillar uses cytokinin to maintain a green island of plant tissue in which it can feed in autumn.’
Fox was referring to leaf-mining moth caterpillars, like the Stigmella tityrella moth larvae that produced the mines in the leaves shown above, but other insects also manipulate the physiology of leaves so they can continue feeding. Another example from the same Beech tree can be seen below – here the larvae of the gall-inducing midge Hartigiola annulipes have also caused green islands to form in the leaves.
If you want to read more on the science behind this process, I found a paper entitled ‘Plant green-island phenotype induced by leaf-miners is mediated by bacterial symbionts‘, on the Royal Society website.