Anyone with a garden will dread the invasion of aphids - tiny insects that suck the juices out of plants - just as they will rejoice in equal measure at the arrival of aphid-eating ladybirds. Ladybirds are neither birds nor especially ladylike, but colourful (and, it has to be said, rather loveable) little beetles. But aphids have ways of avoiding being munched by this colepteran nemesis, as A. F. G. Dixon of the University of East Anglia, Norwich, UK, and B. K. Agarwala of Tripura University, India, discuss in the 7 August issue of Proceedings of the Royal Society of London.
The researchers show how the pea aphid Acyrthosiphon pisum responds to the presence of tracks left by the two-spot ladybird Adalia bipunctata by producing winged offspring that can fly themselves out of danger. When ladybirds aren’t around, aphids will produce wingless offspring.
Other aphids - the vetch aphid Megoura viciae and the black bean aphid Aphis fabiae fabiae - did not respond to the presence of ladybirds in this way. This may be because the vetch aphid is poisonous to ladybirds; and the black bean aphid doesn’t taste very nice, and is in any case often protected by ants. The pea aphid, though, is a delicious ladybird snack without a bodyguard, and must adopt some other way of getting out of trouble.
Many creatures are known that respond to the presence of predators by radically changing their forms or behaviour. This kind of response is believed to be mediated by substances secreted by the predators. For example, the offspring of the microscopic rotifer Keratella cochlearis, and those of the water flea Daphnia pulex, develop impressive spines and crests when predators threaten their parents. Other animals respond by shortening the time they need to develop to maturity. One species of microscopic protozoan that features on the menu of the larva of the yellow-fever mosquito responds to its attacker by turning against it.
Many animals, from algae to fishes, show this behaviour - and they are, almost without exception, aquatic. This makes sense, given that water provides the ideal medium for predator-produced substances to disperse. The ladybird-induced dispersal response in aphids, as described by Dixon and Agarwala, is one of the very few known cases of the phenomenon in terrestrial organisms.
Such a drastic response to predators can only really evolve in creatures whose generation time is much shorter than that of the predator. Aphids are parthenogenetic - that is, females can spontaneously produce large numbers of (female) offspring without being fertilized by males. Three generations of pea aphid can live and die in the time it takes for a single hungry ladybird to mature, giving the aphids plenty of time to plan their escape.