Chemical Calling Cards
Some plants release chemicals that resemble insect pheromones: volatile chemicals employed in communication between social insects. When set upon by aphids, wild potatoes cry out with a compound that acts as an alarm pheromone for aphids. The aphids flee the potato plants.
Corn seedlings ask for assistance when attacked by armyworms, releasing a pheromone that attracts female parasitic wasps which feed on armyworms.
A lima bean plant attacked by beetles has a 2-pronged response. 1st, leaves under attack spread the alarm to undamaged leaves to prepare for assault. Neighboring plants “leavesdrop” and steady themselves.
The lima bean’s leaf alarm is methyl jasmonate, a defense hormone in airborne form. In contrast, a plant under attack by bacteria exudes methyl salicylate, a gaseous equivalent to salicylic acid.
2nd, the lima bean plant has its flowers, which the beetles don’t bother, produce a nectar alluring to beetle-eating arthropods. Many insect-eating arthropods, as well as pollinators, coevolved with plants, and came to share chemical communiqués of mutual benefit.
Predators of plant pests can be picky, selectively flocking to the aromatic news of a menu option, while ignoring scents that signal species they don’t fancy.
A little wasp that injects its eggs into young caterpillars reacts to attacks when a plant’s panic aroma is of tender young caterpillars. But a wasp turns a deaf ear to a plant screaming from attack by geezer caterpillars.