It’s not every day that you hear about spy missions that involve a lack of sex, but clearly parasitic wasps don’t pay much attention to Hollywood clichés.
These insects merge the thriller, science-fiction and horror genres, They lay their eggs inside other animals, turning them into slaves and living larders that are destined to be eaten inside-out by the developing grubs. To find their victims, they perform feats of espionage worthy of any secret agent, tapping into their mark’s communication lines, tailing them back to their homes and infiltrating their families.
Two species of parasitoid wasp – Trichogramma brassicae and Trichogramma evanescens – are particularly skilled at chemical espionage. They’ve learned to home in on sexual chemicals used by male cabbage white butterflies. After sex, a male coats the female with anti-aphrodisiac that turns off other suitors and protects the male’s sexual investment. These chemicals are signals from one male to another that say, “Buzz off, she’s taken.”
But the wasps can sense these chemicals. They feed on the nectar of the same plants that the cabbage white visit and when they do, the wasps jump her. They are tiny, smaller even than the butterfly’s eye (see the image below), and they hitch a ride to the site where she’ll lay her eggs. There, they lay their own eggs inside those of the butterfly. Amazingly, the wasps use the same trick for different species of cabbage white butterflies, which secrete very different anti-aphrodisiacs. They can even sense when the anti-aphrodisiacs are wafting among the general scent of a freshly mated female. It’s all part of a sophisticated “espionage-and-ride” strategy.
Viruses and bacteria often act as parasites, infecting a host, reproducing at its expense and causing disease and death. But not always – sometimes, their infections are positively beneficial and on rare occasions, they can actually defend their hosts from parasitism rather than playing the role themselves.
In the body of one species of aphid, a bacterium and a virus have formed a unlikely partnership to defend their host from a lethal wasp called Aphidius ervi. The wasp turns aphids into living larders for its larvae, laying eggs inside unfortunate animals that are eventually eaten from the inside out. But the pea aphid (Acyrthosiphon pisum) has a defence – some individuals are infected by guardian bacteria (Hamiltonella defensa) that save their host by somehow killing the developing wasp larvae.
H.defensa can be passed down from mother to daughter or even sexually transmitted. Infection rates go up dramatically when aphids are threatened by parasitic wasps. But not all strains are the same; some provide substantially more protection than others and Kerry Oliver from the University of Georgia has found out why.
H.defensa‘s is only defensive when it itself is infected by a virus – a bacteriophage called APSE (or “A.pisum secondary endosymbiont” in full). APSE produces toxins that are suspected to target the tissues of animals, such as those of invading wasp grubs. The phage infects the bacteria, which in turn infect the aphids – it’s this initial step that protects against the wasps.
There are so many fascinating stories about parasitic wasps that they have become a regular feature in this blog. Usually, their prey come off poorly in these tales, with caterpillars being reduced to little more than living, paralysed larders for macabre wasp grubs. But not always – some hosts don’t take the invasion of their bodies lying down. This post is an attempt to redress the balance between parasite and host, by telling the story of the caterpillar that fights back… with medicine.
One species of tiger moth, Grammia incorrupta, has a fuzzy caterpillar called the woolly bear. Like most other caterpillars, it’s exploited by several species of parasitoids including flies and wasps. If these body-snatchers lay their eggs inside a caterpillar, its menu changes and it develops a preference for a group of plant toxins called pyrrolizidine alkaloids (PA).
These have no nutritional value and they clearly come at a cost, for woolly bears that eat a PA-rich diet grow more slowly than their peers. And yet, infected caterpillars gulp down these poisons by the leaf-ful. They are the medicine that the caterpillar uses to kill its unwanted hitchhikers.
It’s a scene straight out of a horror film – you look around and see dead bodies everywhere. They haven’t just been killed either, they’ve been hollowed out from the inside-out leaving behind grotesque mummified shells. What would you do if you were confronted with such a macabre scene? Flee? Well, if you were an aphid, you’d probably just feel relieved and go about your business. Aphids, it seems, find security among the corpses of their peers.
Aphids, like almost all insects, are the targets of parasitic wasps that implant eggs inside their bodies. On hatching, the wasp grubs use the aphid as a living larder and eat their way out, leaving behind a mummified aphid-shaped husk.
These husks ought to be (quite literally) a dead give-away that parasites are afoot, valuable intel for any animal. But far from treating these bodies as a sign of danger, aphids actually see them as a reason to stick around. As Fievet says, “In human history, mummies had long been known to protect the dead; our study shows that in nature, mummies can also protect the living.”
This is the seventh of eight posts on evolutionary research to celebrate Darwin’s bicentennial. It combines many of my favourite topics – symbiosis, horizontal gene transfer, parasitic wasps and viruses.
Parasitic wasps make a living by snatching the bodies of other insects and using them as living incubators for their grubs. Some species target caterpillars, and subdue them with a biological weapon. They inject the victim with “virus-like particles” called polydnaviruses (PDVs), which weaken its immune system and leave the wasp grub to develop unopposed. Without the infection, the wasp egg would be surrounded by blood cells and killed.
The wasps’ partners in body-snatching are very different to all other viruses. Once they have infected other cells, they never use the opportunity to make more copies of themselves. They actually can’t. To complete their life cycles, viruses need to package their genetic material within a coat made of proteins. In most cases, the instructions for building these coats are encoded within the virus’s genome, but polydnaviruses lack these key instructions entirely. Without them, the virus is stuck within whatever cell it infects.
It’s such a weird set-up that some scientists have questioned whether the polydnaviruses actually count as viruses at all or whether they are “genetic secretions” from the wasps themselves. Where on earth are those missing coat genes?
Annie Bezier form Francois Rabelais University has found the answer and it’s an astonishing one. The viruses’ coat genes haven’t disappeared – they’ve just been relocated to the genomes of their wasp hosts.
In this way, the wasps and the viruses have formed an unbreakable alliance, where neither can survive without the other’s help. Without the virus, the next generation of wasps would be overwhelmed by the defences of their caterpillar larders. Without the wasp, the virus would never be able to reproduce. Some viruses may be able to live happily alongside their host with little ill effect; others may even be beneficial in some way. But this is the first example of a virus co-evolving with its host in a compulsory binding pact.
This is the second of eight posts on evolutionary research to celebrate Darwin’s bicentennial.
When new species arise, they can set off evolutionary chain reactions that cause even more new species to spring forth – fresh buds on the tree of life create conditions that encourage more budding on different branches.
Biologists have long suspected that these “cascades of speciation” exist but have struggled to test them. Enter Andrew Forbes from the University of Notre Dame – his team of has found a stunning new proof of the concept by studying a fruit fly called the apple maggot (Rhagoletis pomonella) and the parasitic wasps that use it as a host.
Contrary to its name, the apple maggot’s natural host is not apples – it’s hawthorn. The fly only developed a taste for apples about 150 years ago, when the fruit was first introduced to North America. This culinary switch has created two races of apple maggot – one that eats hawthorn and another that eats apples. Even though they are often found in the same place, the two races don’t mix and they don’t breed together. They are well on the road to becoming separate, genetically distinct species.
And so are their parasites. A wasp called Diachasma alloeum specialises in attacking apple maggots. It lays its eggs inside the fly larvae, and its grubs eat the victim from the inside out. Forbes found that the wasp has also started to form separate races that don’t crossbreed with one another, even though they have overlapping ranges. By adapting to new host plants, the flies inadvertently set up barriers that separated their respective parasites from one another. Now, the wasp, like its hosts, are also on the way to becoming separate species. It’s a fantastic example of diversity bringing itself about.