Plants have evolved a great many defenses against herbivores. A common strategy is to produce trichomes (hair-like projections) that physically block small insects, and/or exude toxic chemicals. The wild tobacco (Nicotiana attenuata), uses its trichomes for a different purpose. These trichomes are composed of acyl sugars that label any caterpillars that eat them with an ant attractant.
After ingesting the acyl sugars, both the caterpillars themselves and their poop (frass in entomology speak) exude a distinctive odor. Alexander Weinhold and Ian Baldwin from the Max Planck Institute found that predatory ants are attracted to that odor. In short order, ants arrive on the scene and carry the hapless caterpillars back to their nests for consumption. Thus, by providing a small sugar meal to herbivorous caterpillars, the plants call in ants to rid them of the little invaders.
My favorite part of this story is an experiment in which the researchers marked grains of rice with an equivalent amount of the frass odor to see whether ants would be lured to the rice. Sure enough, ants rushed out to collect the rice and carry it back to their nests. Back in the ant nest, I envision the ants grumbling, “these are the worst caterpillars I’ve ever had!”
A freshly hatched Manduca sexta larva (tobacco hornworm) consumes trichomes of wild tobacco (Nicotiana attenuata).Credit: MPI Chemical Ecology: Ian Baldwin, Alexander Weinhold.