When Shakespeare had King Henry V say: “Once more unto the breach, dear friends, once more; Or close the wall up with our English dead,” he probably wasn’t thinking literally. However, for some aphids, English or otherwise, this is exactly what they do. By shooting fluids from their body, often at the cost of their own lives, they repair breaches in the colony’s walls as a defense against invading insects. By finding the biochemical basis of this approach, scientists have shed light on its evolution.
The willingness of social creatures to surrender their own lives for the common good fascinates zoologists. Honeybees, among others, refute crude interpretations of evolution, where selfishness is paramount and care for others a maladaptation. Although the evolutionary benefits of this approach are now well understood, how it emerged is more mysterious.
Dr Mayako Katsukake of Japan’s National Institute of Advanced Science and Technology investigated one example of such behavior, known as “suicide bombing”, in the social aphid Nipponaphis monzeni. Hundreds of thousands of these insects live inside galls on witch-hazel trees, gaining protection against the weather and predators.
When butterfly and moth larvae succeed in breaking the walls of the gall, “soldier nymphs erupt to discharge a large amount of body fluid, mix the secretion with their legs, and skilfully plaster it over the plant injury,” Katsukake writes in Proceedings of the National Academy of Sciences. Even if the soldiers do not die from the damage to their bodies in the process, they can be smothered by other’s juices or end up on the wrong side of the protective wall.
Katsukake and co-authors found the nymphs have body cavities full of chemicals that contain lipids, which clot when released from the body. Proteins and the amino acid tyrosine, stored in separate parts of the aphid’s body, react together when combined to form macromolecules that reinforce the clot.
Perhaps we may one day find a way to harness aphids’ methods to build better sealants, but in the meantime Katsukake is more interested in the past. The molecules used to seal the nest are used by individual aphids to provide immunity against disease or to heal injuries, although soldier aphids produce far more of them in readiness for these attacks.
Biologists have described the methods used by animals to protect the herd as “social immunity”, but they had not anticipated the co-option of individual immunity to the wider community’s defense. Katsukake’s work shows the two forms of immunity are deeply connected in aphids and possibly in other social insects.
Aphids are certainly experts in chemical warfare, with a different species using it to fool ants into taking them into their nests, where they suck the internal fluids from the young.
