Or, Evolution marches on.
There’s a species of parasitic wasp (Aphidius ervi) that lays its eggs in pea aphids (Acyrthosiphon pisum). This is very unfortunate for the aphids, which get consumed alive by the wasp larva inside them. However, some aphids have a defense. Those pea aphids that are infected with a particular bacteria (Hamiltonella defensa) that are in turn infected with a specific bacterial virus (Acyrthosiphon pisum secondary endosymbiont, or APSE) are largely protected from the wasp eggs within them. This is because both H. defensa and APSE make toxins that in concert can kill a wasp larva.
So, to recap, pea aphids that are infected with APSE-infected bacteria are protected from parasitic wasps.
The parasitic wasp, Aphidius ervi, attacks the pea aphid, Acyrthosiphon pisum.
Credit: Alex Wild.
At this point, you may be wondering if the wasps have found a way around this problem of the fortified aphid. Why, yes, they have. According to research by Kerry Oliver from the University of Georgia and her colleagues from Akita Prefectural University and the University of Arizona, wasps can compensate for the toxins by laying two or more eggs in infected aphids. Interestingly, a single wasp emerges from each aphid, regardless of the number of eggs deposited. More eggs seem dilute the toxins so that a single larva can survive.
You won’t be surprised to learn that female wasps preferentially lay more than one egg in infected aphids, but only one in uninfected aphids. How can they tell the difference? Aphids secrete alarm pheromones, and after all, what’s more alarming than the prospect having your innards digested by a foreign invader? Other aphids, upon detecting the pheromone, try to flee the area. There is some evidence that aphids infected with H. defensa secrete less of the alarm pheromone, and that wasps can detect the difference.