Insect Chemical Defense Mechanisms
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As some of the smallest creatures on Earth, insects face constant threats from predators. To defend themselves, they have evolved an array of unique chemical defense mechanisms.
From the stinky sprays of stink bugs to the explosive reactions of Bombardier beetles, these defense mechanisms have helped insects to thrive for millions of years.
Join us as we explore some of the most incredible chemical defenses in the insect world and discover how bugs use these weapons to stay alive.
Type of Insect Chemical Defense Mechanisms
Insect chemical defense mechanisms can come in several forms, solids, liquids, or gasses. These chemicals can either be created by the insect itself or the components gathered from plants and other external sources. The ‘ingredients’ for these chemicals are produced in sacs or glands in various parts of the body and there are two main types.
Toxic Chemicals
In order to defend themselves against predators, some insects produce toxic chemicals which cause an unpleasant reaction to their attacker. The saddleback caterpillar is covered in irritating hairs which contain a chemical known to cause a painful, burning sensation when brushing against them.
There is also the blister beetle that produces a chemical known as cantharidin that can cause a variety of symptoms, including gastrointestinal issues and skin blisters.
Repellent Chemicals
One of the best ways to avoid becoming dinner for a bigger creature is to be as repulsive as possible. Take the stink bug, for example, which emits a stinky chemical from its thorax or abdomen that contains hydrocarbons which can send any predator packing.
The great thing about this type of chemical is that the insect is able to repel predators without causing any harm.
How do Insects Produce their Chemical Defense Mechanisms?
Some insects are able to produce their own toxins thanks to complex structures within their bodies, while others require a little help from the plants they consume. Either way, it’s a pretty amazing ability.
Synthesize their Own Toxins
The toxins from some insects are obtained intrinsically which means they are produced by the creature itself. This is the case with the bombardier beetle, which has chambers in its abdomen, allowing it to keep toxins separate until it needs to use them. These beetles store both hydrogen peroxide and hydroquinone, and they’re able to control when the two combine to cause a reaction which it does by sending them to a third chamber that contains a catalyst. The result is an explosion of chemicals enough to drive any attacker away.
From Food/Plant Sources
For insects that are unable to produce their own toxins, there is still an opportunity to create chemicals and they do this through extrinsically obtaining chemicals via the plants they eat and other food sources.
The insects acquire the chemicals from host plants and then store them in their body tissues or within glands until they need to use them. The monarch butterfly is one such species that behaves in this way when they are caterpillars. They will store the toxins from the milkweed on which they feed, making them unpalatable to predators.
Insects that Use Chemical Defense Mechanisms
The insect world is filled with a surprising number of toxic creatures. Let’s take a closer look at some of the most fascinating.
1. Suicidal Attack Ants (Colobopsis explodens)
The suicidal attack ant, otherwise known as the exploding ant, is a tropical species that is known for its ability to rupture itself in the name of defense!
These ants release a yellow goo and it’s this substance that differentiates them from other exploding ant species. In fact, it’s thought that they may only be found in Brunei on the island of Borneo.
When the ants are threatened by a potential predator, they flex a part of their abdomen, called the gaster, so hard that it bursts and releases a toxic liquid. This liquid is intended to either repel or kill their attacker.
However, it’s only the minor workers that have this ability. Other members of the colony, like the major workers, have other traits, such as big heads, which they use to plug entrances to the nest.
2. Bombardier Beetles (Family: Carabidae)
The bombardier beetle has a pretty intense method of chemical defense in the form of a hot liquid shooting out of the back of the bug.
There are hundreds of species of bombardier beetles, and they typically live among the leaf litter on every continent apart from Antarctica.
When they feel threatened, these critters use a cocktail of hydroquinone and hydrogen peroxide which they fire out of their rear ends. What’s impressive is that this liquid can reach temperatures of up to 212°F (100°C). However, the bugs themselves remain unharmed because the chemicals only mix when they need to. This controlled reaction allows the bugs to use this dangerous weapon without injuring themselves.
Interestingly, even if the beetle does happen to become a meal for a predator, it may still escape. There is evidence that shows by firing the liquid inside the stomach of the predator after being swallowed whole, the attacker may vomit the bugs back up!
3. Stink Bugs (Family: Pentatomidae)
Most people are familiar with the stink bug. I remember picking one up as a child and being horrified at the smell it left on my hands. In North America alone, there are more than 200 species of stink bug, and the brown marmorated stink bug is one that you may often come across after it was introduced from East Asia.
These bugs use chemicals called trans-2 decenal and trans-2 octenal which create an odor not all that dissimilar to coriander. This is an irritating toxin that can cause skin rashes and itching in humans as well as irritation to the eyes.
However, the chemicals are designed to repel predators and are stored in exocrine glands in the abdomen. When required, the bug can release the chemicals onto its skin which smells and tastes bad enough to repel an attacker.
4. Monarch Butterfly (Danaus plexippus)
The monarch butterfly is a beautiful species that is known for its stunning annual migration. But these creatures have something else that makes them special and that’s their ability to defend themselves.
As larvae, the monarch butterfly feeds on milkweed. In fact, conservationists are encouraging people to plant more milkweed because a lack of it poses a serious threat to the monarch. When they eat these plants, they ingest chemicals called cardenolides which are toxic to the vertebrates that predate the caterpillars.
Moreover, these toxins have a secondary effect which is thought to prevent parasitic infections. What’s more, it’s thought that storing these toxins is what gives the adult butterflies their coloration.
However, studies have shown that there could be a negative effect to sequestering the toxins in the form of oxidative damage which dampens the coloration of the adults. A loss of color means a loss of the signal to a predator to keep away.
5. Ladybugs (Family: Coccinellidae)
There are more than 6000 species of ladybugs in the world and they’re actually a type of beetle. Have you ever picked up a ladybug only to find that it secretes a dark liquid? This is called hemolymph and is one of the ladybug’s primary forms of defense. While its bright colors signal to predators to keep away, it needs another tactic to back this up.
The ladybug releases this toxin in a process known as reflexive bleeding, which typically happens when they’re on their backs. This ‘blood’ comes from their legs and contains smelly, bad-tasting chemicals that can cause numbness in the mouth. But this does come at a cost to the ladybug as reflexive bleeding has been shown to weaken the immune system.
6. Assassin Bugs (Family: Reduviidae)
With a name like assassin bug, it’s clear that these insects know how to look after themselves, and there are more than 7000 species around the world. They’re commonly known for their kiss of death, which involves using their proboscis to inject and kill prey before sucking out their insides; obvious assassins!
However, while they are effective predators, these bugs are also excellent at defending themselves. They are able to fire venom up to 12 inches (30 cm) which might not seem like a lot but when you’re only around 0.5 inches (1.3 cm) in length, that’s some pretty impressive firing!
The toxins released by assassin bugs contain protease, hyaluronidase, and phospholipase, which can cause pain, widening of the blood vessels, and swelling.
7. Blister Beetles (Family: Meloidae)
Blister beetles are most commonly recognized because of the danger they pose to horses. The toxin that they release, called cantharidin, is particularly dangerous to horses and can cause poisoning. Although this is rare.
However, the reason that these beetles release this chemical is to defend themselves, and they’ll usually only do it if they are squished or threatened. When the cantharidin comes into contact with human skin, it can cause irritation, including blisters which explains where these bugs get their name. But the chemicals aren’t effective to all predators as there are some bird species that have become immune to it. Moreover, when they eat the blister beetles, the cantharidin acts as protection against parasites.
Just like ladybugs, blister beetles release their toxins through reflexive bleeding but it’s only the males that can actually produce the chemical. In order for females to defend themselves, they must mate with a male who will pass the toxin onto her.
8. Three-Lined Potato Beetle (Lema daturaphila)
The three-lined potato beetle is an interesting-looking species that is rarely found in large numbers. While it does feed on potatoes and tomatoes, it is not considered to be a pest. But it does have a rather interesting defense mechanism.
Unlike a lot of animals on this list, the three-lined potato beetle isn’t able to store chemicals. Instead, it feeds on a host plant within the nightshade family, and the toxins from these plants are found in the beetle’s poop.
As larvae, three-lined potato beetles will cover themselves in a fecal shield that contains toxins potent enough to cause hallucinations. What’s more, with the stinky layer over their bodies. Most predators would rather look elsewhere for their next meal.
9. Swallowtail Caterpillars (Papilio polyxenes)
There are several species of swallowtail butterflies, and their caterpillars all have an interesting way of defending themselves. They will use tactics such as rearing up and waving their bodies around to make themselves appear threatening.
But these caterpillars also have glands, called osmeterium, located on the head which they use to shoot out foul-smelling chemicals to warn off potential predators. They take their toxins from the plants that they eat and will store them in a sac in the thorax until they are needed.
What’s interesting is that it’s thought that different species of swallowtail produce different smells and this can even change over the course of their lives. For example, the chemical limonene would give off a citrusy smell whereas something like aliphatic acid would cause a rotting dairy sort of odor.
10. Two-Striped Walkingstick (Anisomorpha buprestoides)
The odor produced by the two striped walking sticks is so bad that the insect has been nicknamed the musk mare and the devil rider. It has two glands in the thorax, which it uses to spray a milky substance when it feels threatened.
This substance is enough to blind an oncoming predator, which gives the devil rider a chance to make its escape.
The name of the chemical is anisomorphal which is a form of cyclopentanol monoterpene dialdehyde and was discovered by Thomas Eisner and Jerrold Meinweld. The chemical causes intense irritation and smells pretty bad too. You’ll often see these bugs in the late summer when they start breeding.
11. Red Wood Ant (Formica rufa)
Many ant species will use formic acid as a way of stupefying their prey but the red wood ant also uses this chemical as a method of defense. It will shoot the acid which is stored in a sac located in the abdomen. Red wood ants have so much formic acid that the sacs can weigh up to 20% of their body weight.
This is a type of boreal ant native to Eurasia, where they are found in huge colonies numbering up to half a million! When the colony is threatened, thousands of individuals will shoot acid into the air, but this isn’t always a bad thing.
While the acid will cause problems for predators and taste very sour, there are bird species that use the acid as a way of controlling parasites.
12. Vinegaroon (Mastigoproctus spp.)
The vinegaroon is a type of scorpion, sometimes called the tailed whip scorpion, that is found in North America.
These large scorpions use a gland in the tip of their whips, called a pygidium, to release a concoction of chemicals octanoid and acetic acids which is similar to the make-up of vinegar; no prizes for guessing where the creature gets its name! What’s more, the chemical cocktail also contains another type of acid; caprylic acid, which can destroy the exoskeleton of the predator it is sprayed on, essentially meaning death.
For humans, a spray from the vinegaroon wouldn’t be fatal, but it would cause skin irritation. What’s more, if it gets into the eyes, it can be incredibly painful until the effects wear off.
13. Wood Tiger Moth (Arctia plantaginis)
If you ever see a wood tiger moth approached by a predator, you may notice that it makes no attempt to escape. That seems pretty weird until you learn that these moths have the ability to make themselves taste so bad, no predator would even want to eat them.
Males are able to secrete chemicals from glands in the thorax and abdomen which each contain different chemicals. In studies, it was shown that the glands in the thorax produced chemicals that deterred birds, while those in the abdomen warded off ants.
Even more amazing is the location of the glands in relation to the predators they deter. For example, the glands located in the top of the thorax deterred birds that typically grab the moths by their heads, therefore, they would easily come into contact with these glands. On the other hand, the abdominal glands, aimed at ants which would attack lower down the body.
