A plant that looks like it’s hurt
If you’re a plant, there are lots of things that want to eat you. The most obvious to us are big herbivores: deer, rabbits, and so forth, but for many plants, insects and other invertebrates actually pose a bigger danger. Some of the most interesting of these are leaf miners, flattened insect grubs that hatch and live inside the leaf, eating away between the two outer layers, until they’re ready to turn into adults and fly away. The term ‘leaf miner’ describes a way of life, not a group: they might be from one of a few groups, mostly moth larvae (caterpillars) and fly larvae.
On the surface of the leaf, this forms distinctive trails where the grub has eaten away parts of the leaf. In the last few years in the UK, there have been a lot of them on horse chestnut trees, as the horse chestnut leaf miner has been spreading across the country from the continent. It’s likely that female insects looking for leaves on which to lay their eggs prefer fresh, uneaten ones to leaves already covered in leaf miner trails.
Three scientists from Bayreuth, Germany have suggested that a plant living in the rainforest in Ecuador produces leaves with a pattern that makes them look like they’re already riddled with leaf mines, to deter their leaf mining moth from laying real eggs on them. This is a bit like ‘playing possum’, where animals pretend to be dead to dissuade predators that prefer fresh prey. Passionflowers also have a similar trick, making structures on their leaves that look like butterfly eggs, which dissuade butterflies from laying real eggs.
The idea is that this plant, in the arum family, uses variegation—white patches on the leaf—to mimic the pale leaf mines made by the caterpillar. Variegated varieties of many plants have been bred by gardeners, but it’s not so common in nature, because without the green chlorophyll, that area of the leaf can’t photosynthesise. For variegation to stay, it would have to give some benefit, and it seems that it does: the researchers found that only 1.6% of variegated leaves had leaf miners, compared to 7.9% of plain green ones.
Case closed? Not quite. There are a whole host of reasons why variegated leaves could be attacked less: perhaps they grow where moths are less likely to go anyway, or perhaps the leaves also smell slightly different. After ruling out one simple link (variegated leaves don’t grow in more open sites in this species), the authors set about a straightforward experiment: they painted plain green leaves with Tippex to look like the variegated ones. Of course, Tippex also produces a smell, so they did a control using a mixture of all the same chemicals except the white pigment, which should smell the same without leaving any markings. As predicted, there were fewer leaf mines on the white-patterned leaves than the unpatterned ones. (Aside: I think they’ve missed a statistic. I’d like to see a statistical comparison of just the two groups of painted leaves, whereas they have done a more complex comparison using both those and the naturally variegated/plain leaves.)
With those results, it’s pretty convincing, although still not absolutely conclusive. The variegation could dissuade the moths another way, for example by marking leaves that are tougher for the miners to eat. To show that it was deception, you’d have to check that the miners from eggs that did get laid on variegated leaves grew as large and as quickly as the miners on plain leaves. And perhaps moths are just less likely to recognise the oddly patterned leaves (only about a third of the leaves are variegated).
There’s also the question of how natural selection is balanced: if variegated leaves were always better, all of the plants would have them. Of course, as I mentioned above, white leaf area is also a cost, because it can’t photosynthesise. Perhaps, the more variegated leaves there are around, the less likely the moths are to overlook them when laying eggs, so if there are too many of them, the cost of variegation outweighs the benefit (for those who study such things, that would be ‘negative frequency dependent selection’). Or perhaps there are more moths in some areas than others, and the plants interbreed between areas where variegation is advantage, and areas where it’s a disadvantage.
Soltau, U., Dötterl, S., & Liede-Schumann, S. (2008). Leaf variegation in Caladium steudneriifolium (Araceae): a case of mimicry? Evolutionary Ecology, 23 (4), 503-512 DOI: 10.1007/s10682-008-9248-2