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.
Many species of aphids live in galls, stimulating the plants they suck to form large, hollow chambers. These species tend to be cooperative and social, a more basic version of the supersocieties of ants and termites. They all have special ‘soldier’ individuals, who are tasked with defending the colony, repairing the gall and keeping it clean. Some do this at the cost of their own lives. Last year, another Japanese team discovered that the gall aphid Nipponaphis monzeni employs suicide-plasterers, individuals who repair holes in the gall by fatally leaching their own bodily fluids onto the hole.
Quadrartus yoshinomiyai is another gall-forming species whose soldiers have sticky fluids and suicidal tendencies. Its galls are formed on witch hazel by a single founding female, who initially seals herself away. This matriarch produces a line of wingless clone offspring, who then mate with one another to give rise to another generation of winged adults. All this takes around two years. Come mid-spring, the aphids cut an exit hole in the gall and the winged generation fly away to another host plant – the sawtooth oak.
The wingless middle generation are the ones who conduct suicide campaigns against attacking predators. By cutting into freshly collected galls, Uematsu found that these wingless adults tend to be clustered near the exit holes. When she set ladybirds upon these defenders, she found that more than half became stuck in an aphid pile-on, their jaws and legs gummed up with wax. Only 23% managed to infiltrate the gall. If the wingless adults were removed, only 14% of ladybirds were glued down, and 64% entered the gall.
Through April, as the winged aphids leave their homes, the wingless adults stay behind to defend the remaining colony. At the point when the gall opens, the wingless aphids go through menopause. They lose the ability to reproduce but they don’t die immediately – instead, they shift from sex to defence.
Virtually all aphids carry mature embryos if they’re still capable of breeding but when Uematsu dissected the defenders, less than 5% of them contained embryos. Instead, their abdomens were almost exclusively taken up by their defensive waxy liquid. Rather than producing embryos, they had devoted their energies towards producing wax. They still get benefits though, for their suicidal actions ensure that their young, who carry their genes, are more likely to survive.
This is subtly different to the situation in many other social insects, where some individuals never reproduce for the sake of helping their relatives who do. Nonetheless, there are other examples of insect menopause. The ant Pristomyrmex punctatus permanently shifts from reproduction to foraging as it ages. And in some paper wasps, individuals fight for dominance and the losers give up the right to reproduce and act as workers instead.
Reference: Current Biology http://dx.doi.org/10.1016/j.cub.2010.04.057
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