Suicide aphids have been discovered lurking amid the cabbages ready to explode chemical bombs against an onslaught of ladybirds.

Researchers were astonished to find that cabbage aphids sacrifice their lives to protect other members of the colony when a predator attacks. The aphids detonate mustard-oil bombs to scare off ladybirds and any other any predators. The oil kills, disables or puts off the predators but the defender of the colony usually dies too.

Aphids are well known for recruiting ants to protect them, in return for a sugary liquid honeydew, but it is the first time that they have been known to resort to suicide bombing.

The deadly cocktail used in the bombs is a combination of a chemical taken from cabbages and an enzyme called myrosinase, which the aphids produce themselves. Glucosinolates are extracted from the cabbages and stored in the aphids’ blood while the enzyme is kept in the muscles of their head and thorax. When a ladybird or other predatory bug attacks an aphid the enzyme and the glucosinolates are mixed together causing a miniature explosion that sprays mustard oil at the attacker’s head.

“This is a great example of the way in which a species provides an ingenious method of protecting itself, whatever the circumstances,” said Glen Powell, of Imperial College, London, who was one of the scientists studying the aphids.

The joint British and Norwegian research team found that when there was a ready supply of glucosinolates in their diet the aphids enjoyed great success in warding off attacks, whereas those without the chemical bombs were at the mercy of predators.

Only the wingless cabbage aphids, Brevicoryne brassicae, store the glucosinolates. Once wings begin to grow they start getting rid of the chemical.

Dr Powell said: “Our study seems to show that aphids that develop wings cease to store this chemical as they mature, as they don’t need the mustard-oil bomb to defend themselves when they can just fly away.

“In the wild, aphids live in clonal colonies, with often many hundreds of individuals crowded together on a plant, and using this poisonous mustard-oil defence provides wingless individuals with a powerful means of dispelling a predator which poses a risk to the entire colony.

“Unfortunately the nature of the mechanism – with the chemical stored in the insect’s blood and the catalyst in its muscles – means that in most cases the individual aphid seeing off the ladybird predator dies in the process of protecting the colony.”

In their report, published in the journal Proceedings of the Royal Society, the researchers said that it was the first time that aphids had been shown to have resorted to chemical warfare. They suggested that because so many of the members of a colony were clones of each other the sacrifice of one had little impact on the genetic make-up of the group.

They concluded: “Aphid colonies risk attack by a wide range of natural enemies. The cabbage aphid’s exploitation of glucosinolates therefore appears to confer defensive advantages to the herbivore. The cabbage aphid has evolved a highly specialised and unique form of defence.”

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