Defense strategies for plants

What does it take for a leaf-eating beet armyworm normally satisfied with a vegetarian diet to turn to cannibalism? Apparently, a really bad tasting tomato leaf. Does this sound like the making of a B horror movie? The plant tastes so bad that the caterpillars would rather eat each other than the plant. University of Wisconsin Professor John Orrock states “at the end of the day, somebody gets eaten. It often starts with one caterpillar biting another one in the rear, which then oozes. And it goes downhill from there”. Very high gross factor if you’re the caterpillar. Score one for the plant though.

If you think about it, plants can’t get up and run away from predators, nor can they fight back. Have you ever seen a sunflower karate chop an insect? Me neither. Plants are continually under attack from insects, bacteria, and fungi.

One defense mechanism plants will use to ward off herbivores is to produce chemicals, like the one in our scenario above. That awful tasting tomato plant also was working like a neighborhood watch program in your garden. Professor Orrock likened this foul tasting emission to a “chemical scream” and said nearby plants then “detect this scream and begin to invest in their own defenses”. Plants can communicate with each other. How nifty is that?

Another cool example of chemical defense occurs with cotton plants under attack from caterpillars. These creative and clever organisms will release a mixture of customized chemical spirits (kind of like homemade moonshine, I imagine) into the air to attract parasitic wasps. If you aren’t familiar with these wasps, check out these creepy, but amazing, photos of parasitic wasp eggs on caterpillars: https://growingsmallfarms.ces.ncsu.edu/growingsmallfarms-parasiticwasp/.

Plants have physical defenses too. Bark helps protect trees and leaves have a waxy cuticle. Trichomes, hair-like structures on leaves, are sharp and can emit chemical irritants. Some plants have thorns, spines and prickles to protect themselves from predators. I know, because I got poked trying to harvest an eggplant the other day. Another interesting type of defense occurs with the mimosa plant. It will fold up its leaves when touched to avoid looking appetizing.

“Plants may not be able to flee a scene of attack or fight off predators with teeth and claws, but with sturdy armor, a well-stocked chemical arsenal, a neighborhood watch, and cross-species alliances, a plant isn’t always an easy meal.” This is how Valentin Hammoudi summarized plant defenses in this five minute TEDEd video: https://ed.ted.com/lessons/the-amazing-ways-plants-defend-themselves-valentin-hammoudi

Evolving with their environment, plants have adapted some interesting and effective survival tactics. A healthy plant is, indeed, its own best defense. I think I’ll step outside now and see if my tomatoes want to have a chat.

Kelley Rawlsky has an M.S. in horticulture and is the director of Bringing People and Plants Together, an organization dedicated to bringing horticulture education and therapy to the community. For more information: PeopleAndPlantsTogether@gmail.com or follow us on Facebook.

Originally published in Broomfield Enterprise - Sept 10, 2017