Buzz
A constant, subtle hum surrounds us, barely perceptible to the human ear. Yet, within this quiet hum, life-and-death conflicts unfold, and new alliances are formed. Our environment itself serves as a battleground, where plants engage in a relentless war with their long-time adversaries: herbivorous insects. This conflict has been raging for over 350 million years, with both sides continuously refining their strategies for survival.
10. Chemical Warfare
For insects like caterpillars or beetles, the leaves and other tender plant parts serve as their meal. But in the insect world, there’s no such thing as an easy feast. Over time, plants have found ways to defy their rooted nature by evolving an array of defenses against their herbivorous foes. These range from external armor (like thorns, spikes, or toxic hairs known as trichomes) to full-fledged chemical defenses.
These chemical responses are diverse and can be triggered individually or in concert. Typically, when an insect begins to feed, the plant releases chemicals that either kill the insect or disrupt its growth. For example, a plant might occasionally coat its leaves with bitter tannins, making it a less appealing meal for the bug, while also making it harder to digest.
But how do plants detect they’re under attack? There are several ways, but the one that allows them to respond the quickest involves ‘hearing’ the insects chewing. Incredibly, a recent study revealed that a small flower called Arabidopsis can sense the vibrations caused by a caterpillar munching on its leaves, prompting the plant to ramp up its defenses accordingly.
9. The Ally of My Enemy
When a plant can't fend off hungry invaders on its own, and doesn't have a private army of fierce ants like the acacia tree, it often calls for help. It releases volatile compounds into the air that act as a ‘lunch invitation’ for the herbivores' natural enemies. To sweeten the deal, the plant might offer some extra floral nectar. Among the ‘good guys’ that rush to the rescue are ladybugs, ichneumon wasps, and even insects with dramatic names like the assassin bug or the mealybug destroyer.
Nearly all plants can emit these distress signals, though some plants are more ‘vocal’ than others. This means they produce a higher concentration of volatile compounds. The louder their ‘cry for help,’ the healthier the plant tends to be.
These volatile compounds also act as a warning to nearby plants, urging them to enhance their own defenses. On top of this, plants often communicate impending dangers by using a fungi-based underground network to ‘share’ information. It’s nature’s own version of the Internet, but free from bugs and worms.
8. The Silken Shield
Weaver ants are renowned by farmers for their exceptional pest control abilities, but they aren’t the only arthropods capable of defending plants from harmful insects. Researchers at Miami University have discovered that spiders can also form partnerships with plants in their fierce struggle against voracious bugs. And the arachnid doesn’t need to work around the clock—simply weaving its web does the trick.
Spider silk alone is intimidating enough to make pests rethink their dining options at a plant. Scientists found that beetles were repelled by just the presence of the silk, reducing leaf damage by around 50 percent. After all, spiders are predators of beetles, and the sight of silk is a signal that a danger lurks nearby.
Interestingly, researchers also tested silk from silkworms and found it to be less effective in field experiments. These results are quite promising, offering potential for a more environmentally friendly approach to pest management.
7. An Unexpected Ally
In 2015, researchers from Florida State University conducted a multi-year study exploring the impact of ants and treehoppers on a plant species called rabbitbrush. The study uncovered a surprising network of interspecies partnerships in the meadows of Colorado. It's a battle for food and survival, full of unexpected twists.
The researchers found that as treehoppers carefully fed on the sap of the rabbitbrush, they risked becoming prey for predators like ladybugs. To defend themselves, treehoppers called on nearby ants for help. In return for protection, the ants were rewarded with a sweet liquid from the treehoppers.
But, in this harsh world, the ants had their own predators to worry about: larger threats like hungry bears. Over four years, the researchers observed that bears caused significant damage to ant nests, destroying up to 86 percent of them. With fewer ant bodyguards for the treehoppers, the result was healthier, happier plants.
6. Ingenious Defenses
Despite all the protective measures plants have, a caterpillar still has to eat, so it comes up with some clever tricks. A study by researchers from Penn State’s College of Agricultural Sciences revealed that to gain more time for munching on corn tissue, fall armyworm larvae release their feces (or frass) onto the leaves. This deception makes the plant believe it is under attack by fungal pathogens.
As a result, the corn plant changes its defense tactics to address the fake threat. Since it can't defend itself against both the insects and the pathogens simultaneously, the caterpillars get the chance to take a few big bites. On the positive side, this research might lead to the development of a safer, frass-based pesticide to help plants fend off pathogens.
In a similar fashion to plants, insects also warn each other of nearby dangers, using the plant itself as a communication hub. Underground insects can send signals to those above ground via chemicals transmitted through the plant’s leaves. They can even leave a 'voicemail' for the next generation, warning them about the suspicious nature of a particular plant.
5. Seizing the Weapon
In an interesting turn of events, some insects are using plants to shield themselves from their own predators. A prime example is the cinnabar moth, a striking black and crimson insect that stays near ragwort flowers in Europe and Asia. The ragwort, a highly toxic weed, can even pose a threat to large animals like horses and cattle.
A cinnabar caterpillar, just as brilliantly colored as its adult form, can devour a whole ragwort plant in a couple of days. (In fact, this insect was introduced to various parts of the world as a biological control for ragwort.) Much like the African variegated grasshopper, cinnabar moths have developed a unique enzyme that allows them to ingest and store toxins, which are meant to protect the plant from herbivores. These insect enzymes transform the plant toxins into chemical weapons.
This is also why creatures like the cinnabar moth and African variegated grasshopper have such bright and vivid colors. They’re essentially broadcasting their toxic nature, warning predators to keep their distance.
4. Battling On Several Fronts
Predators come in many forms, each demanding a different defense response. However, it’s common for multiple herbivorous insects to arrive at the same time to feast on the same plant.
This situation creates quite a dilemma. For example, when a corn plant is simultaneously attacked by aphids and caterpillars, it must activate distinct chemical defenses to combat each group of pests. The problem is, these defense compounds are derived from the same base molecule.
Research has shown that defending against one type of insect weakens the plant's ability to repel the second. A study by the Boyce Thompson Institute on a common corn variety revealed that after a plant was exposed to caterpillars, it became a more favorable environment for aphids, leading to an increase in aphid offspring on the corn.
However, the results weren’t the same for all corn varieties. Some actually showed greater resistance to aphids after a caterpillar infestation. These discoveries could help in developing more resilient crops in the future.
3. Shifting the Approach
Devising new defense strategies can be a draining task. Recognizing the futility of these efforts, the milkweed decided to change its focus from developing new ways to fight to simply finding a way to coexist with its predators.
At first, the milkweed attempted to defend itself by growing hairs on its leaves, secreting a poison called cardenolide in its tissues, and even filling its tubes with toxic latex. However, the determined caterpillars always managed to find ways around these defenses. They trimmed the leaves, became immune to the cardenolides, and learned to dodge the latex. Additionally, monarch butterfly caterpillars store toxins from the plants they consume, using them to defend themselves against their own predators.
It seemed that no matter how the milkweed tried to adapt, the monarch butterfly was always a step ahead. With its efforts failing, the milkweed decided to change its approach. Researchers from Cornell have observed that the plant is now focusing on recovering more rapidly from the damage caused by its predators.
Well, if you can't beat them...
2. The Origins of Mustard
Scientists at the University of Missouri have discovered that without caterpillars, mustard wouldn't even exist. The same can be said for condiments like wasabi and horseradish. It turns out that mustard oils are the result of a prolonged battle that has lasted tens of millions of years, a fierce conflict between butterfly larvae and plants in the Brassicales order.
The pungent, unique flavor of mustard and wasabi comes from glucosinolates—compounds that are lethal to most insects. These compounds began to evolve during the late Cretaceous period as part of an evolutionary “arms race” between plants and the creatures that consume them.
For countless years, plants and insects have been locked in a battle of chemical one-upmanship. Plants developed new defenses to protect themselves from being consumed, but insects adapted to neutralize these defenses, pushing the plants to evolve even stronger weapons. The result? Over 120 types of glucosinolates, which contributed to a greater diversity of both plants and insects, and, of course, tastier hot dogs.
1. The Battle in the Garden
Though it may appear to be a lovely flower, the serpentine columbine is actually a deadly trap. Countless tiny insects meet their doom on its stem, caught in the plant’s sticky hairs.
These trapped insects serve as a feast for predators, who quickly arrive to devour them. In the process, these predators also consume the caterpillars and moth eggs ensnared on the plant. Since caterpillars are a major threat to the sticky columbine, the arrival of spiders for a meal is a great benefit to the plant. Luckily for the arachnids, the columbine's adhesive surface doesn't affect them, making it a win-win for all... except, of course, for the caterpillars.
In a study conducted by researchers at the University of California–Davis, it was discovered that the columbine’s behavior is influenced by its surroundings. After the researchers cleared away the dead insects from the flowers, fewer predators came around, allowing caterpillars to feed without interruption. The researchers also found that the columbine releases chemical signals that attract unsuspecting insects that happen to fly nearby—almost like a siren's call.
