Wanted: nanny. Must be willing to be eaten alive.

Walking the dog the other day, I chanced upon something interesting going on in the path verge. At first, it was the black clusters covering the plant that drew my eye- on closer inspection, aphids. They were not alone- stepping between them were two spindly, fly-like insects that were several times the aphids’ size, but still very small. The aphids seemed agitated by their presence, flicking up their abdomens when one passed close, and it soon became clear why. Every now and then, one of the larger insects would curl its abdomen underneath its body, and dab the tip against one of the aphids: it was laying eggs on them. They were female wasps- not the big, sugar-guzzling yellow and black stingers that have holidaymakers in a state of constant paranoia at this time of year, but members of several related groups that are collectively known as parasitoid wasps. 

You get the idea.

This term covers a broad but very specialised range of insects that provide generously for their larvae… by giving them a live meal. Victims are usually other insects, and they’re not treated with much respect: the eggs deposited on them by the adult female hatch into ravenous young, which devour the host at just the right rate to keep it ticking over until they’re finished with it. There are all sorts of grim but fascinating ways in which the larvae of different species get the most out of their host, with examples of brainwashing, mutation and virus warfare, but the main distinction between a parasitic and a parasitoid lifestyle is that in the latter, the host usually winds up dead. 

It’s worth remembering that as grisly and intriguing as the larval lifestyle is, they couldn’t do it without their mum’s hard work. How does a tiny female wasp track down a suitable host that will take care of her babies until they are grown? (Now there’s a good plot twist for Mary Poppins…) How did my two wasps know that the aphids were there, both ending up on a single plant amongst thousands of others in the area? I won’t keep you in suspense- they used their brains.

Now, insects aren’t exactly famous for being intelligent. Nobody has ever uttered the words “ as wise as a mosquito” (as a compliment, anyway), and anyone who’s seen a bee repeatedly ramming itself against a pane of glass won’t have much respect for their intellect. To be honest, plentiful brain power and learning ability just aren't necessary for most insects: they get through their short lifespans just fine on innate behavioural responses to environmental cues, investing their energy in reproduction rather than IQ. But for some insects, the environment can be quite unstable, and so a rigid response to a certain stimulus might vary quite widely in its effectiveness. 

Parasitoid wasps have just this problem. Different species have a range of hosts that their larvae can survive on: some specialise on just one host, others have hundreds of options. Many host species are herbivorous, and so the volatile distress signals released by damaged plants can be a useful cue for the wasps to follow to find their quarry. Unfortunately, many hosts also have a varied diet, and so always following the same plant species’ signal might yield disappointing results. With the abundances of different hosts and their distributions changing within seasons and between different years, wasps are faced with a dilemma. An innate response to a cue that is successful for one generation may be totally useless to the next generation, depending on where the best food supply is: the changes are much too fast and unpredictable for evolution to synchronise alterations in wasp behaviour with the environment. So the parasitoids have a trick up their sleeves, allowing them to fine-tune their behaviour to suit the current conditions- they LEARN.

In general, the system works like this. Wasps are born with a set of scents that they are programmed to respond to, by following them to their source. If a female wasp finds a suitable host at the end of a certain trail, she remembers her success, and becomes more likely to follow similar scents in future. This process- called associative learning- allows some adaptation to the present environment, but it’s risky: what if her success was only down to chance? It’s a bit like if you once saw a hospital next to a church, and decided that the best way to find a hospital elsewhere in future was to look out for a church spire. You'd be in trouble, basically. So parasitoids have tweaked this basic learning response, and we can get a good idea of how and why by doing comparative studies on closely-related wasp species that face different ecological challenges…

Cotesia glomerata and its close relative, Cotesia rubecala, spend their childhoods inside juicy caterpillars: largely within two species that may be familiar to anyone with a garden or allotment. C.glomerata favours the caterpillars of the Large White: those big, funky-looking grey, black and yellow things that selflessly eat cabbages so that human children don’t have to. C. rubecala has a taste for Small Whites, which are (shockingly) smaller and a more straightforward green colour, but similarly enthusiastic about their five-a-day. Chances are, if you search enough domesticated brassica leaves you’re bound to find at least one of these species crawling around- but in a wild setting, the search is not so straightforward.

See, the lifestyles of the caterpillars are actually quite different, and this has influenced the searching behaviour of their parasitoids. Large White caterpillars have a fairly predictable distribution: they have a small range of plant species that they like to eat, and tend to hang out in big gangs. In fact, a female C.glomerata coming across a bunch of these has hit the reproduction jackpot: she may be able to deposit most of her lifetime supply of eggs on a single colony, and so needs to make relatively few successful forays to fulfil her mission. This means that even if she commits a fairly unreliable scent cue to memory, it’s unlikely to be disastrous, as she’s already secured the future of many of her offspring. So she can afford to be a little careless: usually, only one successful encounter is needed for the guiding scent to be encrypted in the wasp’s long-term memory. This ensures that if she comes across a similar scent again- no matter how far in the future- she’ll be in with a chance of boosting her reproductive success even further.

Small White caterpillars, on the other hand, take a bit more work to track down. Mother butterflies lay just one egg per plant: as our second wasp species C.rubecala also lays one egg per host, it must make far more hunts than its cousin to maximise its reproduction. Even more frustratingly, Small Whites are much less picky about the plants that they lay on, so there are a whole range of potential cues that a wasp must look out for. Committing one to memory too quickly could be disastrous for a female: a single successful search is unlikely to provide reliable evidence for where more caterpillars might be found, and so she could end up wasting her time and energy looking on the wrong foodplants. Furthermore, committing things to long-term memory is actually quite costly for insects.  Unlike shorter-term memory, it requires new proteins to be set up in the neural system, which takes energy. Over the course of a lifetime, brainy can be a bad thing: some experiments show fruit flies that have been working their long-term memories are less resilient to environmental challenges like dessication, and are thus more likely to die early. So to avoid these costs to its health, and sidestep ruining its reproductive success, C. rubecala is much more cautious about forming long-term memories. Until a single cue has resulted in about three successful searches, it will not encrypt it into its long-term memory, storing it using less resilient- but much cheaper- options until then.

Not all wasps use smells to hunt down their hosts: Hyposter horticola has a very different strategy that uses visual cues, but plenty of brain power, too. This species also likes caterpillars, but there’s just one problem: the wasp needs to lay its eggs when the caterpillar is fully developed, but has not yet hatched from the egg. This is a tiny window- just a few hours- and so a wasp that searches randomly with scent cues is unlikely to have much luck. Instead, the parasitoids operate in quite a sinister way to make sure they get their timing right. When the female finds a clutch of eggs, she’ll memorise its location using visual landmarks. She will then frequently visit her hosts-to-be over the next few days to check up on how they’re coming along- presumably standing over them rubbing her hands together and cackling lullabies to the embryonic caterpillars. These regular visits also help to refresh her short-term memory about the location of the eggs, allowing her to monitor several clutches at once. 

It seems a little labour-intensive: why not stick a scent marker on the eggs and follow it back later, freeing up more time to hunt down extra hosts? Unfortunately, butterfly eggs are a much-sought resource in the meadows where Hyposter makes a living, and females are constantly competing to infect a limited number of clutches. Scent-marking the prize would be like erecting a neon sign inviting everyone to join in the macabre baby shower- much better to use natural clues, and keep them to yourself! This system is so effective, and competition so rife, that in many meadows every single egg cluster is parasitized. Luckily for the caterpillars (and in the long term, the wasps), only a third of the eggs in each clutch tend to be infected, though it is not certain why. These tight numbers mean that in any year, the local wasp population is usually exactly one third of that of the butterfly- creating some odd-looking population graphs that look nothing like the classic model of predator-prey fluctuations. Finally, other female wasps get put off laying in the same clutch by a scent mark left by the “winner”- so sibling caterpillars always have sibling parasites inside them. How cute!

As we can see, learning is a costly tool to maintain (kids- add this one to the homework excuses phrasebook). But in the right situations, the benefits of learning can outweigh the cost of investment: whether it’s outwitting predators, prey or competitors, keeping up with ecological flux or interacting with other members of a social group, complex, changeable conditions often require more than simple hardwired reactions from animals if they are to succeed. Now, if an insect the size of a pinhead can use its brain to succeed… where am I going wrong?