A ladybird larva is on the prowl on a witch hazel plant. The youngster is a voracious predator and it’s hunting for aphids. It seems to have found a bountiful feast – a swollen structure called a gall that houses an entire aphid colony. With so many meals in one place, the colony seems easy prey, but it has staunch defenders.
As the ladybird approaches, aphids pour out of the gall and grab the predator by their jaws and legs. It’s a suicide defence. The aphids secrete massive amounts of waxy liquid from their bodies, which quickly solidifies and glues the ladybird to the plant. Unable to walk or bite, the ladybird dies and the aphids go with it. In the video below, you can see what happens when one of these aphids is prodded with a needle.
There is more to these suicidal protectors that meets the eye. Keigo Uematsu and University of Tokyo found that all of them are ‘menopausal’. They are the parents of the other aphids in the gall but their reproductive days are long behind them. With no further opportunities to raise the next generation, their final role is to defend their offspring, with their lives if necessary.
Viruses and bacteria often act as parasites, infecting a host, reproducing at its expense and causing disease and death. But not always – sometimes, their infections are positively beneficial and on rare occasions, they can actually defend their hosts from parasitism rather than playing the role themselves.
In the body of one species of aphid, a bacterium and a virus have formed a unlikely partnership to defend their host from a lethal wasp called Aphidius ervi. The wasp turns aphids into living larders for its larvae, laying eggs inside unfortunate animals that are eventually eaten from the inside out. But the pea aphid (Acyrthosiphon pisum) has a defence – some individuals are infected by guardian bacteria (Hamiltonella defensa) that save their host by somehow killing the developing wasp larvae.
H.defensa can be passed down from mother to daughter or even sexually transmitted. Infection rates go up dramatically when aphids are threatened by parasitic wasps. But not all strains are the same; some provide substantially more protection than others and Kerry Oliver from the University of Georgia has found out why.
H.defensa‘s is only defensive when it itself is infected by a virus – a bacteriophage called APSE (or “A.pisum secondary endosymbiont” in full). APSE produces toxins that are suspected to target the tissues of animals, such as those of invading wasp grubs. The phage infects the bacteria, which in turn infect the aphids – it’s this initial step that protects against the wasps.
Imagine that a massive hole appeared in a wall of your house, and you’d decided to fix it yourself. You head over to a DIY store and load up on plaster, tools and paint and look forward to many hard and tedious hours of work. If that seems like a chore, you might get some perspective by considering the plight of the gall aphid Nipponaphis monzeni. When holes appear in their homes, some unlucky individuals are tasked with repairing the damage using their own bodily fluids. They sacrifice themselves for the sake of some DIY.
Some species of aphids are heading towards the incredibly cooperative lifestyles of social insects like ants, bees and termites. They live in large hollow growths called galls, that sprout from the very plants whose sap they suck. The galls provide them with protection from predators, shelter from the elements and constant food. They are a precious resource indeed, and every colony of social aphids have a special caste of sterile individuals whose job it is to defend the gall and attack intruders. These are the soldiers.
But in a few species, such as N.monzeni, soldiers have a truly bizarre part-time job- they are suicide-plasterers. When their homes are breached, the soldiers leach their own bodily fluids onto the wounded area, mix it with their legs and plaster it over the hole. The liquids soon harden and within an hour, the gap has been plugged at the cost of the soldiers’ lives.
The aphids’ gall-repairing antics are remarkably similar to what happens when animals develop cuts and wounds. The fluid around the area clots and hardens to form a scab. This provides a temporary seal, that gives the surrounding cells enough time to grow, divide and restore the broken tissues. The exact same thing happens to the gall – the only difference is that its clots and scabs are provided not by the plant, but by the aphids it houses.
It’s a scene straight out of a horror film – you look around and see dead bodies everywhere. They haven’t just been killed either, they’ve been hollowed out from the inside-out leaving behind grotesque mummified shells. What would you do if you were confronted with such a macabre scene? Flee? Well, if you were an aphid, you’d probably just feel relieved and go about your business. Aphids, it seems, find security among the corpses of their peers.
Aphids, like almost all insects, are the targets of parasitic wasps that implant eggs inside their bodies. On hatching, the wasp grubs use the aphid as a living larder and eat their way out, leaving behind a mummified aphid-shaped husk.
These husks ought to be (quite literally) a dead give-away that parasites are afoot, valuable intel for any animal. But far from treating these bodies as a sign of danger, aphids actually see them as a reason to stick around. As Fievet says, “In human history, mummies had long been known to protect the dead; our study shows that in nature, mummies can also protect the living.”