Worries about pesticides are always in the back of our minds when we buy fruit and vegetables. “Mmm, these strawberries sure look good — but they are probably full of pesticides. Should I feed them to my child?” We choose organic produce when we can find it and can afford it, and wonder whether eating no fruit is better than eating fruit with traces of pesticides. Food safety is a growing concern in our industrialized society. Food testing can show us levels of contaminants, but what if there were a better way? What if we could grow our crops in sufficient quantities without the use of chemical pesticides?
Plants Fight Back
Apparently, there is a way – a way we can learn from plants. When they’re being eaten by pests, plants don’t just sit there and take it. They respond in two ways: by building their own defenses and by calling for help. Understanding, learning, and using the language of plants can be a wise way to control pests naturally without spraying massive, ever-increasing amounts of industrial pesticides that the pests eventually grow resistant to.
Here is how Slate magazine describes plant communication:
When a caterpillar or a beetle starts chewing on a leaf, the plant responds by synthesizing its own defense chemicals in an attempt to drive away the insect. It also releases a chemical plume into the air, a message that can be intercepted by creatures nearby. Other plants respond to these alerts by producing their own chemical weapons, substances that repel leaf-eating insects. Predators that eat plant pests also detect the signals, using them like beacons to locate their prey. This chatter is going on all the time, all around us. A moth or wasp or a bird flitting through a field is barraged by constant status updates about which plants and creatures are nearby and what they’re up to. It’s the biggest and most important conversation on Earth.
What language is used in this conversation? Thirty years ago it was suggested that willow trees use airborne pheromones to warn the neighbor trees of attack by caterpillars and webworms, and as a result of such communication (which happens in the absence of root connections), the neighboring trees produce a change in foliage that makes their pests grow slower.
In addition to changing the chemical composition of leaves, plants call on predatory insects, which respond to emitted chemicals through their antennae, i.e., the electrical activity of odor-sensing cells. For example, a wild cabbage releases volatile compounds (which can be analyzed by gas chromatography-mass spectrometry, GC-MS) to attract parasitic wasps when larvae are deposited by herbivorous insects.
The situation becomes even more complex as the enemies of the enemies — secondary parasitic wasps (or ‘hyperparasitoids’) — enter the battle. They can also sense (and prefer!) the presence of the primary parasite, which fights the caterpillars and helps the plant in the first place.
Learning the language of plant communication may allow the development of a biological pest-control strategy, but first the complexity of communication between the plants, pests, primary and secondary predators must be completely studied. Timing is important, as predatory insects have to be called early enough to protect the plants from damage. Sensitive analytical methods, such as gas chromatography, are especially useful to study volatile compounds used by plants and insects for communication.
Image by goatling.
Source: “Listen to the Plants,” by Kat McGowan, slate.com, April 18, 2014.
Source: “Responses of Alder and Willow to Attack by Tent Caterpillars and Webworms: Evidence for Pheromonal Sensitivity of Willows,” by D.F. Rhoades, in Plant Resistance to Insects, Chapter 4, pp 55–68; Chapter DOI: 10.1021/bk-1983-0208.ch004.
Source: “Plant Volatiles Induced by Herbivore Egg Deposition Affect Insects of Different Trophic Levels,” by N.E. Fatouros et al. Plos ONE, August 2012, Volume 7, Issue 8, e43607, www.plosone.org.
Source: “Hyperparasitoids Find Their Parasitoid Host With Help of Plant Volatiles,” by E.H. Poelman et al. PloS Biology, November 2012, Volume 10, Issue 11, e1001435 (open access).