The Corn Plant and the Wasp
The corn plant (Zea mays) is a tall, grass-like crop that provides us with sweet corn, tortillas, popcorn, and more of our staple foods. First domesticated in southwestern Mexico nearly 10,000 years ago, its symbol of abundance now adorns many brands. But did you know that the corn plant is also intelligent, in its own unique way? It senses its environment and responds with precision. In fact, when under attack, it mounts a defense and even summons reinforcements in the form of wasps.
Corn Plant Male Flowers
H. Zell, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Predators vs Plant Defenses
Caterpillars, particularly the beet armyworm (Spodoptera exigua), are among corn’s most vicious predators. They chew through the leaves, reducing the plant’s ability to photosynthesize. During heavy infestations, armyworms can even damage the ears, chewing through the husks into the tender kernels inside, causing massive crop losses. But corn isn’t just a defenseless plant, unable to escape or fight back, as some of us may believe. It fights back using chemical signals in the form of airborne messages known as volatile organic compounds (VOCs).
Beet Armyworm Caterpillar
By Unknown author – Nicotine Keeps Leaf-Loving Herbivores at Bay. PLoS Biol 2/8/2004: e250. doi:10.1371/journal.pbio.0020250, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=1374967
Aroma as Armor: Meet VOCs
VOCs are the plant world’s version of pheromones. Many plants emit these naturally to attract pollinators, deter pests, or even communicate with neighboring plants. For example, linalool is a common floral volatile found in lavender, basil, and coriander. It plays a dual role: attracting pollinators like bees and butterflies while also deterring herbivores and inhibiting the growth of certain fungi and bacteria. It’s part of the plant’s front-line defense and is a major ingredient in the scent profile of flowers and herbs.
Another powerful VOC is (Z)-3-hexenyl acetate, which gives freshly cut grass its distinctive smell. This VOC is part of a group of Green Leaf Volatiles (GLVs) named after that distinctive smell, and this group is the most widely employed chemical defense in the plant kingdom. Produced within seconds of leaf damage, this compound warns nearby plants of an attack, prompting them to boost their own defenses.
However, VOCs are rarely emitted alone; they’re often part of complex, species-specific chemical cocktails that carry nuanced meaning in the plant-insect world. Despite their abundance, we cannot even sense them without complex laboratory experiments, specific to each compound.
Invisible Chemical Defenses
Plants, as well as trees, have a whole world of chemical signaling that is invisible to us. As I walk through the wilds, looking at leaves, I often imagine clouds of plant pheromones wafting through the sky, communicating with anyone with the knowledge and skills to listen.
When corn undergoes some form of mechanical damage alone, it emits those GLVs to warn its neighbors of an attack so they can mount their chemical defenses. However, that is not the only trick up its sleeve.
Corn has a special mechanism when it’s being chewed on by caterpillars. Not only does it detect mechanical wounding from the chewing, but it also recognizes specific molecules in the caterpillar’s saliva, like fatty acid–amino acid conjugates (FACs) such as volicitin (N-(17-hydroxylinolenoyl)-l-glutamine). Researchers have analyzed this oral secretion and curiously discovered that the fatty acid portion of volicitin is derived from the plant, while the 17-hydroxylation reaction and the conjugation with glutamine are carried out by the caterpillar using its glutamine.
These compounds act as herbivore fingerprints. Once the plant detects those insect-modified, plant-derived fatty acids, the corn plant initiates a response, which triggers the production of some very specific VOCs.
Airborne SOS: How Plants Call for Backup
One of the key VOCs released is (E)-4,8-dimethyl-1,3,7-nonatriene, better known as DMNT. This compound doesn’t repel the caterpillars directly. Instead, it acts like a distress beacon, which specifically alerts parasitic wasps.
DMNT Chemical Diagram
By Scott Kildall
Wasps (Cotesia marginiventris) are natural enemies of caterpillars. But they have their own tricks up their sleeves. Not only can they detect VOCs, they can identify DMNT specifically. Studies have shown that they can distinguish between all the VOCs to differentiate between mechanical damage vs. herbivore attack and even determine which plants with a heavier herbivore load to target for the best success.
So, the wasps follow the scent of DMNT right to the source. When they find a caterpillar feeding on the corn plant, they inject it with eggs. The wasp larvae hatch and consume the caterpillar from the inside, neutralizing the threat and giving the corn a chance to recover. A perfect example of just one of the many mutually beneficial relationships that exist in nature between seemingly unlikely allies.
How Corn Defends Itself
Equally impressively, neighboring corn plants can detect these VOCs and prime their own defenses before they’re attacked. These “eavesdropping” plants produce anti-herbivore compounds of their own, such as proteinase inhibitors that interfere with insect digestion, or increased levels of defensive enzymes and toxins, which may be toxic to the insects or just irritate or repel them. By preparing in advance, they reduce the damage when a caterpillar finally shows up.
Final Thoughts
So the next time you drive past a cornfield, remember: it’s not just a quiet sea of green, it’s filled with silent alarms, chemical messengers, and strategic alliances. That simple crop of corn is in reality a biological battlefield where plants actively recruit their allies to outwit their predators, to the benefit of both. So, plants, it turns out, aren’t passive at all. They’re just using a language we are only beginning to understand.