A very hungry caterpillar munches on a cabbage leaf and sets off an alarm. The plant releases chemicals into the air, signalling that it is under attack. This alarm is intercepted by a wasp, which stings the caterpillar and implants it with eggs. When they hatch, the larval wasps devour their host from the inside, eventually bursting out to spin cocoons and transform into adults. The cabbage (and those around it) are saved, and the wasp—known as a parasitoid because of its fatal body-snatching habits—raises the next generation.
But that’s not the whole story.
Some parasitic wasps are “hyperparasitoids”—they target other parasitoid wasps. And they also track the cabbage’s alarm chemicals, so they can find infected caterpillars. When they do, they lay their eggs on any wasp grubs or pupae that they find. Their young devour the young of the other would-be parasites, in a tiered stack of body-snatching. It’s like a cross between the films Alien and Inception.
In Australia, a pair of superb fairy-wrens return to their nest with food for their newborn chick. As they arrive, the chick makes its begging call. It’s hard to see in the darkness of the domed nest, but the parents know that something isn’t right. Whatever’s in their nest, it’s not their chick. It doesn’t’ know the secret password. They abandon it, flying off to start a new nest and a new family somewhere else.
It was a good call. The bird in their nest was a Horsfield’s bronze-cuckoo. These birds are “brood parasites” – they lay their eggs in those of other birds, passing on their parenting duties to some unwitting surrogates. The bronze-cuckoo egg looks very much like a fairy-wren egg, although it tends to hatch earlier. The cuckoo chick then ejects its foster siblings from the nest, so it can monopolise its foster parents’ attention.
But fairy-wrens have a way of telling their chicks apart from cuckoos. Diane Colombelli-Negrel from Flinders University in Australia has shown that mothers sing a special tune to their eggs before they’ve hatched. This “incubation call” contains a special note that acts like a familial password. The embryonic chicks learn it, and when they hatch, they incorporate it into their begging calls. Horsfield’s bronze-cuckoos lay their eggs too late in the breeding cycle for their chicks to pick up the same notes. They can’t learn the password in time, and their identities can be rumbled.
Illustration by Inna-Marie Strazhnik
Some flies, known as phorids, specialise in decapitating ants in a gruesome way. They lay their eggs inside their victims. When the maggots hatch, they move towards the ant’s head, where they gorge upon the brain and other tissues. The ant stumbles about in a literally mindless stupor until the connection between its head and body is dissolved by a enzyme released from the maggot. The head falls off and the adult flies burst out.
There are hundreds of species of phorid flies, each one targeting its own preferred ants. But some ants are naturally defended against these parasites because they’re incredibly small. Most phorids are a few millimetres long. If an ant is the same size, its head wouldn’t be roomy enough for a developing fly. Thailand, for example, is home to an acrobat ant (Crematogaster rogenhoferi) which can be just 2 millimetres long. Surely these workers are safe from decapitating parasites?
No, they’re not. Brian Brown from the Natural History Museum of Los Angeles County has just discovered a Thai phorid that’s just 0.4 millimetres in length. It’s the world’s tiniest fly, small enough to sit comfortably on the eye of a common housefly. It’s easily small enough to fit inside the head of even the smallest acrobat ant. It just goes to show that there is no way of truly escaping from parasites. If you evolve a miniscule body, they will shrink even further in pursuit.
In a French meadow, a creature that specialises in corrupting the bodies of other animals is getting a taste of its own medicine.
Leptopilina boulardi is a wasp that lays its eggs in fly maggots. When the wasp grub hatches, it devours its host form the inside out, eventually bursting out of its dead husk. A maggot can only support a single grub, and if two eggs end up in the same host, the grubs will compete with one another until only one survives. As such, the wasps ensure that they implant each target with just one egg. And if they find a maggot that has already been parasitized by another L.boulardi, they usually stay away.
Usually, but not always.
L.boulardi is sometimes infected by a virus called LbFV, which stands for L.boulardi filamentous virus. And just as the wasp takes over the body of its maggot target, so the virus commandeers the body of the wasp. It changes her behaviour so that she no longer cares if a maggot is already occupied. She will implant her eggs, even if her target has an existing tenant. After infected wasps are finished, a poor maggot might have up to eleven eggs inside it.
A black bean aphid is about to have a rough day. It has been targeted by a parasitic wasp, which lays several eggs inside its body. When the eggs hatch, the wasp grubs will try to eat the aphid from the inside out. If they succeed, the aphid will die, and the young wasps will burst from its corpse to find aphids of their own.
But the aphid isn’t necessarily doomed. There’s a chance that it will resist the attempt to usurp its body. If it does, the wasps will have done it a favour. When the mother wasp implanted its eggs, it also infected the aphid with bacteria that protect against parasitic wasps. It inadvertently vaccinated the aphid against its own kind.
Some people drink alcohol to drown their sorrows. So does the fruit fly Drosophila melanogaster, but its sorrows aren’t teary rejections or lost jobs. It drinks to kill wasps that have hatched inside its body, and would otherwise eat it alive. It uses alcohol as a cure for body-snatchers.
D.melanogaster lives in a boozy world. It eats yeasts that grow on rotting fruit, which can contain up to 6 per cent alcohol. Being constantly drunk isn’t a good idea for a wild animal, and the flies have evolved a certain degree of resistance to alcohol. But Neil Milan from Emory University has found that alcohol isn’t just something that the insect tolerates. It’s also fly medicine.
“God save thee, ocean sunfish
From the fiends that plague thee thus
Why look’st thou so? With thy large shoals,
Thou fed the albatross.”
- Samuel Taylor Coleridge, sort of.
Albatrosses are superb long-distance fliers that can scour vast tracts of ocean in search of food. But sometimes, food comes to them. In July 2010, Tazuko Abe from Hokkaido University found albatrosses cleaning a school of ocean sunfish, basking at the surface of the western Pacific Ocean.
The ocean sunfish is a truly bizarre animal. It looks like someone cut the head off a much bigger fish and strapped fins to it. It’s the largest of the bony fish*. The biggest one ever found was 2.7 metres in length and weighed 2.3 tonnes. The youngsters, of course, are much smaller. The ones that Abe saw on his research cruise were just 40 centimetres long. There were at least 57 of them, each turned on its side so its broad flank faced the water surface.
The basking shoals were attending a sort of sunfish spa. The fish were infested with parasites. Pennella, a long scarlet relative of shrimp and crabs, was embedded headfirst in the flesh beneath their fins, busily sucking their blood. But not for long – black-footed and Laysan albatrosses were attracted to the shoal and picked the Pennella off their bodies. In some cases, the sunfish seems to be courting the birds, following them around and swimming sideways next to them.
Ocean sunfish live throughout the oceans but they often spend time at the surface before diving to the depths. Some scientists think that they’re absorbing heat from the sun, but it’s possible that they could also be looking for a spot of personal hygiene.
These fish can play host to at least 50 species of parasites, and they often have considerable numbers on their large bodies. Many ocean animals rely on cleaner fish or cleaner shrimp to rid them of parasites. It’s possible that albatrosses might fulfil the same role for ocean sunfish.
Of course, the association might have been a one-off. However, there are other reports of seabirds such as shearwaters and albatrosses flocking around schools of basking sunfish. This instance stands out only because Abe has photographic evidence that they were actually parasites. As he rightly points out, such events would be difficult to spot among the vastness of the open ocean.
* Fish have skeletons that are either made of cartilage, as in sharks and rays, or bone, as in all the others.
Reference: Abe, Sekiguchi, Onishi, Muramatsu & Kamito. 2011. Observations on a school of ocean sunfish and evidence for a symbiotic cleaning association with albatrosses. Marine Biology http://dx.doi.org/10.1007/s00227-011-1873-6
Throughout North America, honeybees are abandoning their hives. The workers are often found dead, some distance away. Meanwhile, the hives are like honeycombed Marie Celestes, with honey and pollen left uneaten, and larvae still trapped in their chambers.
There are many possible causes of this “colony collapse disorder” (CCD). These include various viruses, a single-celled parasite called Nosema apis, a dramatically named mite called Varroa destructor, exposure to pesticides, or a combination of all of the above. Any or all of these factors could explain why the bees die, but why do the workers abandon the hive?
Andrew Core from San Francisco State University has a possible answer, and a new suspect for CCD. He has shown that a parasitic fly, usually known for attacking bumblebees, also targets honeybees. The fly, Apocephalus borealis, lays up to a dozen eggs in bee workers. Its grubs eventually eat the bees from the inside-out. And the infected workers, for whatever reason, abandon their hives to die.
A stickleback is heading for a warm bath. While its peers prefer to swim in lukewarm water at around 16 degrees Celsius, this individual likes it hotter. That’s not because of a personal preference – instead, it is being steered by a parasite. A tapeworm has lodged in its guts, and it needs warmer temperatures to grow as large as possible. The stickleback becomes little more than a living car that drives the worm to the heated pools that it prefers. Read More
Such is the life of the Australian plague locust, a common pest that is targeted by the black digger wasp. The wasp is a parasite that creates living larders for her grubs. She stocks them with the bodies of paralysed insects. Last December, the locusts formed dense plagues in southeastern Australia just as the wasps were starting to collect fresh meat for their young. And Darrell Kemp from Macquarie University was watching as the two species collided.