When the leftover food in my fridge turns into a microbiology experiment, I simply throw it out and get a fresh meal elsewhere. But not all animals have this luxury. Some only get a single meal and, if it rots, the animal starves and dies. Such is life for the larva of the European beewolf, whose parent provides it with a single paralysed bee to literally eat alive as it develops. The problem is that the larva and the bee are buried in an underground chamber surrounded by microbes that are as eager to eat the bee as the larva is. So how do beewolves keep their food clean? As elegantly shown by Erhard Strohm from the University of Regensburg in Germany and his colleagues from Mainz and Jena, they fumigate it.
If a paralysed bee is placed into a beewolf brood cell, it quickly turns into a fuzzy tennis ball covered with fungus. But this rarely happens if a beewolf properly does its job before abandoning its offspring. Strohm and his colleagues have shown over many years that beewolves use a series of antiseptic strategies to keep their larval resource fungus-free. First, they embalm the bee in a waxy resin that delays fungal growth. Second, they inoculate the brood cell with symbiotic bacteria that produce a potent antifungal drug. But these do not cover all eventualities, because the wax is only partially effective and the bacteria only protect older larvae in cocoons.
The hint that something more was at play came from the conspicuous odour of the brood cells themselves, notably cells that also contained a wasp egg. The team tested eggs and found that they secrete huge quantities of a pungent, and highly toxic, volatile gas called nitric oxide. Nitric oxide is used industrially to fumigate fruit against fungal contamination, so the team hypothesized that the eggs were using it in the same way to protect their bee prey. To test this, they placed bees with and without an egg into a brood cell and counted how many rotted. As predicted, the bees with individual eggs were protected from fungi. And, if they added a second bee – not in contact with the egg – this was decontaminated too, proving that protection was due to the volatile gas.
A nitric oxide fumigant is the perfect solution to the problem faced by these wasps. It fills all spaces in the brood cell where fungi hide and can dissolve into the water droplets on the surface of the bee, giving rise to nitrogen dioxide, yet another antimicrobial agent. It can also linger after a well-timed pulse of production, much like an unfortunate odour in a car with the windows up. Finally, its effects are general because nitric oxide can prevent the growth of a broad range of possible fungal contaminants.
But these advantages have to work in the context of the wasp's other defences – and it seems they do. When the team combined waxy embalming with egg fumigation of their unfortunate bee victims, they found that the combination worked better than either alone. And importantly, although for unknown reasons, nitric oxide does not inhibit the growth of the symbiotic bacteria that make up the beewolf's third line of defence. The authors speculate that the bacteria may be protected in the brood cell or may have evolved resistance. Another possibility is that nitric oxide may be less active against bacterial spores, but this remains to be tested.
Parents go to great lengths to protect their offspring, but kids eventually have to fight their own battles. This amazing system shows that the unborn can also get in on the action of self-defence. Even eggs want a clean meal.