“Our study shows that leaf shape physically deters an herbivorous insect from processing the leaf,” say Yumiko Higuchi and Atsushi Kawakita in a recent article Nature Plants. The finding is in contrast to previous work that suggested herbivores tend to use chemical cues to decide whether or not to attack a leaf. But this might be because of the insect and plant the Japanese duo examined.
The pair looked at Isodon plants, and an insect that lives in them, a leaf-rolling weevil, Apoderus praecellens. The weevil rolls the leaves when laying eggs to provide the larva with food and shelter. Higuchi and Kawakita looked at Isodon. In the field I. umbrosus tend to have fewer leaves lost to weevils folding material around eggs than I. trichocarpus. Yet the weevils were happy to eat I. umbrosus, so clearly there wasn’t a chemical or nutritional problem. But I. umbrosus is a lot more lobed in its leaves than I. trichocarpus. Was the leaf shape interfering with folding?
Back in the lab, Higuchi and Kawakita took some weevils and ran choice experiments. First, they offered the weevils a choice of I. umbrosus and I. trichocarpus. This experiment produced the result they expected that the weevils would prefer I. trichocarpus. But was this a chemical cue that the botanists had missed? They then conducted a second experiment.
Higuchi and Kawakita offered the weevils a choice of two I. trichocarpus plants. They should have been as identical as was possible, except for one difference. One plant had had its leaves trimmed to resemble I. umbrosus. “Although A. praecellens fed on the leaves of both types equally, female weevils cut and rolled non-lobed leaves more often than lobed leaves,” say the authors in their article, “indicating that female preference was changed solely by leaf shape. This finding, together with the fact that females can use I. umbrosus leaves if they are less lobed, suggests that leaf lobation functions as a major deterrent against oviposition by A. praecellens.”
Although the experiments show an advantage for having lobed leaves, Higuchi and Kawakita are not satisfied that I. umbrosus evolved lobed leaves purely as a plant defence. They refer to other work that shows leaf shape can be influenced by abiotic factors like, light or rainfall. They also note that I. trichocarpus hasn’t evolved similar leaves, despite demonstrably being under the same pressures. So while leaf shape is having a demonstrable effect on plant defence, it’s clear defence alone is not the explanation for leaf shape. Nevertheless, it is a trait that has a measurable impact.
“Given the generality of insects that manipulate leaves or recognize leaf shape by palpation, differing leaf shapes may act in various ways in the context of biological interactions,” conclude the authors.
See also Plants and Pipettes How the leaf got its shape