Not sure I am settled on the title of this blog post as I often baulk at calling some less aesthetically (or otherwise) -pleasing nature "horrible" or "evil" or "nasty" as many do.
Hey ho. Let's proceed anyway.
OK then. As Max Bygraves might have once said:
"I wanna tell you a storrrrry".
About a horrible fascinating biological relationship, or perhaps, a set of biological relationships to be exact.
Grab a glass of your favourite poison and I'll begin.
You'll recognise the below (I'm sure) as a few Large (or "Cabbage") White butterfly (Pieris brassicae) caterpillars - on what is nothing more than a Hedge Garlic plant (Alliaria petiolata). Normally, it's the Orange Tip butterfly that lay its eggs on this plant (one egg per plant) but in this case, the Large Cabbage White beat the Orange Tip to it, clearly. But other than that, nothing out-of-the-ordinary here. Well... nothing too strange, other than the caterpillars don't seem to be eating the leaves of the hedge garlic - and are a little... err.... exposed at the top tips of the plant, you might say (and if you would say that - you'd be dead right A).
Fast forward a few days - and the below has happened.
The caterpillars have all died (or are dying) and up to eighty tiny yellow (wasp) larvae have crawled out of their bodies and formed cocoons underneath the dying caterpillar. The ants, by the way, are almost certainly there to feed off the dying caterpillars.
So what IS the story here?
Let's start from the beginning. And in order:
The Large Cabbage White butterfly laid a load of eggs on the underside of the leaves of the plant. In this case hedge garlic, but more-often-than-not cabbages or nasturtiums etc.
The adult butterfly buggered off (no parental-care in butterflies) and a few days later her eggs hatched.
As SOON as they hatched, the tiny caterpillars began eating the leaves of the plant that their eggs were glued to.
As SOON as this happened, the plant sent out a distress signal in the form of a powerful (to insects anyway) pheromone or more accurately, a "kairomone". B. Now. The plant will detect which insect species is attacking it by chemically-analysing the insects' saliva. If it detects (for example) Cabbage White butterfly saliva, the kairomone is sent out immediately. But if it detects (for example) Cabbage moth (Mamestra brassicae) it tends NOT to send out the kairomone.
Now, in our case, the hedge garlic detected a Large Cabbage White (Pieris brassicae) attack, so yeah - it sent out the distress flare in the form of a kairomone - and this distress flare attracted not one but two parasitic wasps.
The first parasitic wasp it attracted was a tiny endoparasitic wasp called the glomerata wasp or Cotesia glomerata. The second wasp it attracted was very probably an even tinier ichneumon wasp going by the catchy name Lysibia nana - a hyper-parasite C.
OK. We're getting there now. The first wasp that this plant sent out its distress flare to, the glomerata wasp, flew in and laid between 16 and 52 tiny eggs INSIDE the body of the developing caterpillars, using its needle (or hair)-like ovipositor (literally - "egg placer"). These eggs are all covered with a virus which disables the caterpillar's rudimentary immune system.
These tiny endoparasitic glomerata wasp larvae would have grown from microscopic-sized things to wasp larvae about the size of a small grain of rice INSIDE the body of the unfortunate caterpillar - by eating the caterpillar from the inside but... and this is important... keeping the caterpillar alive by avoiding eating its vital organs. All the time, the unfortunate caterpillar would be rushing around eating as much leaf matter (perhaps over one and a half times the quantity of leaves it might normally eat D) as it could to sustain both it and the developing parasites within it.
Right then. IF the hedge garlic's kairomone distress signal DOES attract C ,the hyper-parasitic wasp, Lysibia nana, as well as the slightly larger but basic endoparasitic glomerata wasp, this is where the whole story goes bananas. Lysibia bananas in fact. These wonderful wee wasps will fly in and they will chemically-analyse the caterpillar's saliva in the same way the hedge garlic plant could. The nana wasps can detect if the caterpillars are "infected" with glomerata wasps because the glomerata wasps alter the smell of the caterpillar's saliva. To reiterate then, the nana wasps, like the glomerata wasps, have been attracted in by the plant's kairomone distress signal, but unlike the glomerata wasps, the nana wasps ALSO then taste the caterpillar's saliva and only continue their attack IF they detect the presence of the glomerata wasps in the caterpillar's saliva.
If the nana wasp arrives on the scene and detects glomerata wasp larvae inside the caterpillar, it lays its own eggs INSIDE the developing glomerata larvae, INSIDE the poor old caterpillar, using its supersensitive, even more hair-like, ovipositor - and THESE are the parasites that will eventually win this particular battle of chemical warfare - the hyper-parasitic nana wasps that is, rather than the endoparasitic glomerata wasps.
At a point where the wasp larvae (probably just the glomerata wasps in our case) need to leave the caterpillar's body to pupate into adult wasps, approximately two weeks after being "injected" into the caterpillar's body, the wasp larvae, now the size of a small grain of rice chemically alter the caterpillar's behaviour and force it to climb as high (A) as it can up the plant that it has been eating. The caterpillar, now little more than a hollowed-out zombie, obeys, stops eating and climbs.
When the caterpillar reaches the top of its plant - up to 52 wasp larvae chew their way out of the caterpillar's side. Like something out of the "Alien" film. These wasp larvae start to spin silken yellow cocoons around themselves, right under the (very much alive still) unfortunate caterpillar, which completely "zombified" now, spins a further layer of protective cocoon, using its own silk, around the wasp cocoons. With regards to this - the wasp larvae have chemically-forced the butterfly larva (caterpillar) to exhibit parental-care effectively. Behaviour that is indeed completely alien to a Large Cabbage White butterfly, whether it is an imago (adult) or indeed a larva (caterpillar).
The caterpillar hasn't fed for a while and other than a few vital organs hanging from the inside of its body, is pretty-well a shell of a caterpillar now, after it has helped form a protective cocoon around the pupating wasps. That and the fact that its side has been ripped open by up to 52 emerging parasites, might be the end of the unfortunate caterpillar. And to be fair, often that will be that - the caterpillar will simply die. That said, in many cases, the caterpillar doesn't die immediately. Not for a good while. Not, indeed until a week to ten days have passed and the wasps have all "eclosed" (that is pupated and become adult wasps). During that week, the zombified caterpillar sits still to guard over the developing glomerata wasp pupae and attacks anything that comes near them by violently thrashing about and throwing attackers off the cocoon mass.
But why and how would it do this? Why on earth would a caterpillar that has been opened up by up to 52 parasites and have them burst out of its body STILL sit there all meek and compliant, help its grisly murderers to form a protective cocoon and then aggressively fight attackers of these wasp larvae, the larvae after all that have basically all-but-killed the caterpillar? Surely after the endoparasitic glomerata wasp larvae have burst out of the caterpillar, it can't still be chemically "zombified" by them any more? No... probably not. But recent research suggests that a couple of wasp larvae STAY BEHIND, INSIDE the wasp - to continue to chemically-alter its behaviour, so it does help form a protective layer around the developing wasp pupae and does fight off potential attackers. So you have the situation here where one or two wasp larvae stay behind and take one for the team, so to speak. They will never pupate. These (one or) two wasp larvae continue to exist inside the dying caterpillar simply to ensure the rest of the brood which have left the caterpillar, have the best chance to make it to adulthood. These two wasp larvae will soon die with their caterpillar host.
Now, eventually, after about a week or so, if the caterpillar has managed to survive that long (ours haven't by the way) and has managed to protect the developing young from any would-be attackers, the glomerata (or Cnana) wasps "eclose" (emerge from their pupae), males first (as tends to be the way) then the females, which are pretty-well immediately set-upon by the waiting males - and therefore the life cycle continues.
Of course it might not be the endoparasitic glomerata wasps that emerge from the pupae to complete their life cycle. It could instead be the hyperparasitic nana wasps. That said, it could even be a smaller, meta-parasitic wasp which parasitized the hyper-parasitic nana wasps. No. Nothing in nature is safe and nothing is a given.
Finally I should perhaps point out this story above is far from rare. It is a story played out all over the land. Indeed, something like around 70% of ALL Large Cabbage White butterfly caterpillars meet their ends before getting the chance to pupate into adult butterflies themselves because of the actions of their parasitic glomerata (and also perhaps nana) wasps. A fact to errr.... chew on, perhaps?
Notes to the above.
A. The caterpillars are chemically-brainwashed by the glomerata wasp larvae developing inside them to climb as high as they can when it's time for the wasp grubs to emerge from the caterpillar's body.
B. "Kairomone". A (chemical) call-to-arms, (if you like). As opposed to a "Cairo Moan". An (audible) call-to-prayers. (From an Egyptian mosque's minaret?).
D. Hmmm. Why would the hedge garlic plant send out a kairomone to attract a parasitic wasp to attack the caterpillars - only for the caterpillars then to have to eat 150% more (than they would normally) of the plant's leaves to sustain the caterpillar AND the developing wasp larvae inside it? Doesn't make sense does it? If you ask me, the plant should send out a far better kairomone, which attracts a gurt big insect which simply immediately eats the tiny wee caterpillars, rather than fudge around laying eggs in them but keeping them absolutely ravenous. But hey. I don't make the rules...
All that writing (above) a struggle?
Can't say I blame you.
Watch the below then .... then read my words above.