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The blues. A leaf that's been chewed (bottom) shows reduced photosynthesis (blue) in a halo around the holes, in contrast to an intact leaf.

Hidden Damage of Insect Bites

MADISON, WISCONSIN--Unless a plague of locusts is on the loose, a plant typically loses about 5% of its leaves to hungry insects. But the plant may be losing more than meets the eye, according to work presented here 6 August at a meeting of the Ecological Society of America. A new technique that maps photosynthesis across a leaf shows that some plants shut down the food-making process in areas around the chewed parts as well--decreasing the remaining plant's productivity by as much as 25%.

On a large scale, lost productivity might make a difference in how much carbon forests can absorb during photosynthesis. Since carbon dioxide is a major greenhouse gas, scientists are trying to figure out how much atmospheric carbon dioxide forests can consume from the air.

That's why forest ecologist Evan De Lucia of the University of Illinois, Urbana-Champaign, and colleagues explored what happens to the rest of a leaf when it gets into an insect's maw. The researchers let caterpillars nibble wild parsnip leaves for a day, then recorded the amount of photosynthesis in patches of 20 to 30 cells near the bites and throughout the leaf. They did this by shining quick pulses of calibrated light and measuring the amount of light reemitted by the chlorophyll, which indicates how much photosynthesis is going on. The fine-scale picture showed that photosynthesis was dampened in halos around holes in the leaf.

The researchers also found that the more holes a parsnip leaf has, the more of a bad-tasting chemical it produces. Instead of photosynthesizing, suggests De Lucia, the plant puts its energy into churning out the chemical to stave off more insect damage.

"I love that explanation. That's really novel," says ecologist Anurag Agrawal of the University of Toronto, who studies plant-insect interactions. But he cautions that some plants actually boost photosynthesis when chewed upon, so the parsnip's is not a universal plant response.

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