Some plants may use an odd, yet simple defensive tactic against insect herbivores: sand. New research suggests that some plants use sand grains as an unappetizing and abrasive armour.
Psammophorous (“sand-carrying”) plants have sticky surfaces to which sand adheres. The coating was thought to somehow protect the plants against herbivorous insects, but this was only formally tested in 2016. of the University of California, Davis and his colleagues confirmed that plants with a coating of sand are eaten less ().
However, they also found that the sand does not work by camouflaging the plant. They have now examined what the sand does to herbivores.
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The researchers raised caterpillars on beach plants called sand verbenas (), which were either sand-covered or “clean”. The caterpillars were either , which take big bites of leaves, or “” that devour the leaf interior. The team tracked the caterpillars’ growth and development, feeding, and plant choices.
The leaf miners showed no preference for sandy or clean plants, but more than 80 per cent of white-lined sphinx caterpillars preferred clean foliage. Eating sand hobbled their development, slowing their maturation and stunting their growth. Dissections revealed that their guts were full of indigestible sand. The rough sand also eroded the caterpillars’ mandibles, making it harder for them to feed.
“We were astounded by how worn down the caterpillars’ mandibles got after less than a week of feeding,” says LoPresti.
This blunting may also inadvertently kick off a plant’s chemical defense system, says at the University of Lausanne in Switzerland. “When insect mandibles are blunted, the insect starts to crush tissue instead of cutting it,” he says. “That in turn activates more defense responses.”
Many more questions remain outstanding about sand armour, says LoPresti. “In what plants is it and isn’t it defensive? What herbivores are very deterred by it? Which aren’t? The answers to these questions would really illuminate broader ecological and evolutionary patterns.”
Ecological Entomology
