Aphids' sticky suicide missions

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Aphids' sacrificial home defence

An aphid species has "soldiers" who often die in the process of repairing damage to galls, the homes that aphids force their plant hosts to grow.

The aphids crowd around a hole in the gall and squeeze out a fluid comprising two-thirds of their body size, using their legs to mix it and form a "scab".

Full recovery of the plant tissue was only possible with the presence of the aphids after the scab formed.

The work is published in the journal Proceedings of the Royal Society B.

Many of the soldier aphids, of the species Nipponaphis monzeni, die from the significant loss of body mass.

Many others get stuck in the viscous fluid and fail to escape. Like workers on the Great Wall of China, they simply become a physical part of the building work.

The aphids' self-sacrifice was first noted by Utako Kurosu of the Tokyo University of Agriculture in 2003, who called it "the most elaborate social behaviour so far known among aphids".

It's an interesting evolved set of behaviours and physiologies that are closely linked, that have co-evolved Peter Smithers, University of Plymouth

What remained unclear was the fate of the galls after N. monzeni's emergency repair mission.

As a living part of the plant and a food source for the growing aphids, the gall's survival is tied to that of the creatures, which mature and escape fully grown in the autumn.

Co-evolution?

Takema Fukatsu of the University of Tokyo and colleagues followed the progress of a number of damaged galls after the short-term fluid fix was performed.

The team found that the galls that were left unpatched were significantly more likely to die.

Moreover, the soldier aphids' efforts didn't stop at simply plugging the gap; they tended to cluster around the damaged area for weeks afterwards.

"After the hole is plugged by solidified body fluid, soldier nymphs manipulate the growth and regeneration of plant tissue nearby the breach in an intricate manner, which leads to complete sealing of the hole by plant tissue," Dr Fukatsu told BBC News.

Even 13 days after damage, soldiers(white) crowd around the healing hole

When aphids inside the gall were prevented from clustering around the damage after the initial fix, the regeneration did not occur.

Dr Fukatsu says the evolutionary path that has led to the repair behaviour is hard to unpick, because N. monzeni is the first species to be observed exhibiting it.

Research is currently underway to understand the mechanism by which the soldiers' body fluid forms the patches; Dr Fukatsu says that the fluid contains several molecules that are involved in the clotting when insects themselves get injured.

"Once the gall is actually growing with the aphids inside, you'd think it'd be in the plant's interest not to help them out," said Peter Smithers, an entomologist at the University of Plymouth.

"This is the aphids fooling the plant into doing something that it doesn't need or want to do. It's an interesting evolved set of behaviours and physiologies that are closely linked, that have co-evolved.

"But the aphids are calling the shots."