A good year for mind-controlling ladybird-munchers

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Dinocampus coccinellae: a fearsome foe of ladybirds. Photo: Wikimedia Commons

For my Masters Research Project (MRP) I am taking a look at righting and death-feigning among beetles, including a couple of ladybird species. I made the decision that I would try and work with natural populations wherever possible – the only problem with that, of course, is that nature can be the cruellest cohabiter.

Upon setting up a colony of seven-spot ladybird (Coccinella septempunctata), it quickly became apparent that I had a bit of an issue; namely finding quite a few specimens protectively guarding yellow cocoons. Doing such a good job of it, in fact, that it was almost impossible to remove them from both cocoon and plant. Dinocampus coccinellae had joined my project uninvited.

I was aware of possible parasitism of beetle juvenile stages, including those of ladybirds, but not the wasps with a penchant for taking on adults. Little did I know that this is the Braconid wasp D. Coccinellae’s stock-in-trade – and one that it’s building up such a reputation for its it’s started to become a minor mainstream news story. Indeed, once I was aware of it, I started to see ladybirds in their natural habitat sitting on cocoons seemingly everywhere, like some strangely beautiful horror movie in the undergrowth.

Coccinella septempunctata in aggressive cocoon-cuddling mode. Photo: Brad Foster (Twitter @bradwaspfoster)

The parasitoid was first given a full biological description in 1926 after it was first noted in Europe, and since then, boy has it settled in.

One facet of the appeal of insects is a feeling that the aliens are among us already – and indeed parasitoids really did inspire that scene in the film Alien. With D. coccinellae, it is isn’t just about depositing an egg inside the ladybird and leaving its young to, all being well (from the wasp’s perspective of course), burst out of its host’s body an adult. It goes in for full-on control and manipulation, creating ‘zombies’ from ladybird hosts that guard a wasp cocoon spun between its legs by larvae after they’ve had their feeding fun. In its sensory-controlled state, the ladybird twitches to deter predators as the larvae pupates and hangs in there for adulthood. They do a fine ‘bodyguard’ job too, with studies showing only 15% survival against predators with host protection, and 65% with.

This may well be a problem of particular concern to the seven-spot ladybird, as research has shown that while it’s wily wasp enemy can decimate its populations, successful parasitism and emergence rates of the wasp are significantly lower when it makes an ovipositional attack on the invasive Harlequin ladybird, Harmonia axyridis, while the two-spot ladybird, Adalia bipunctata, is an unsuitable host altogether. To make matters much worse for C. septempunctata, it has been demonstrated that its toxic defence chemicals actually act as a considerable attractant to D. coccinellae. Bloody hell, nature.

It seems that a perfect storm is gathering, where a wasp, a highly-successful invasive competitor, a fairly narrow (and early in the year) breeding period and other environmental stressors are combining to make life extremely difficult for one of the UK’s classic ladybird species. Will it withstand the onslaught? Too early to tell – but there are certainly reasons for concern.

As for my experiment, maintenance of colony health since the first sightings has proved a game of watching intently for signs of parasitism, removing rapidly, rinsing and repeating. But stare into D. coccinellae’s intensely beautiful shiny green eyes under a microscope and you can’t help feeling this is an animal you’re never going to quite have the measure of. Allow me this excessive sweep of anthropomorphism: its gaze really does follow you around.

While my run-in with this small wasp has provided much by way of irritation, once you’ve taken a step back from your emotional response, it’s hard not to simply admire a highly-specialised species succeeding in its niche. That is, however, likely to be extremely cold comfort for those looking to conserve the UK’s native ladybirds.

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What’s the point of wasps?

‘But seriously, what is the point of wasps?’ This is a question I often find myself being asked. Unlike their cute, somewhat fluffy cousins, the bees, it seems people have a much harder time accepting wasps. Indeed, they often find themselves on the wrong end of extreme prejudice with people willing to swat them without hesitation; there is even an ‘anti-wasp’ internet meme! This ‘speciesism’ should hardly be surprising given the emphasis placed on pollination services provided by the Apiformes. Wasps however, also play an important role in the functioning of our ecosystems; the health of which we rely upon for our very survival.

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Perhaps the single most important thing that wasps do for us is the provision of ecosystem services though pest control. Many species of social wasp are veracious generalist predators, with each nest capturing and removing many kilograms of arthropod prey from an ecosystem every year (Harris, 1996). Much of this removed biomass is that of species which would otherwise represent significant pests to our agricultural and forestry systems. Wasps can be so efficient at predating on arthropods that in some ecosystems where they have been introduced they actually represent a conservation concern by out-competing native insectivorous birds (Beggs, 2001).

Given this ability to help maintain the functioning of ecosystems, social wasps have occasionally been deliberately employed or encouraged as pest control agents. The introduction of nests has been used to provide successful biological control in production of cotton, tobacco, cabbage, coffee, fruit and timber (Spradbery, 1973). In fact, it is surprising how under-utilised they are considering their apparent ability to effectively control pests. Due to their generalist nature, in many ways social wasps are more highly suited for bio-control than some of some of the specialist species which are currently more widely used. For example, not only will they help maintain populations of multiple pest species below the levels that might affect yield, they are also able to maintain their own populations by utilising various other food sources. This means that their population is not tied to that of the pest species thus there is no ‘lag time’ between the initial outbreak and the time when there are sufficient numbers of predators to have a controlling effect. Furthermore, the social foraging behaviour of wasps causes them to return to sites with abundant food resources, meaning they will concentrate disproportionately at sites with highest pest densities, unlike other biological control agents which tend to distribute themselves more evenly (Richter, 2000). This will allow for more efficient control as the most damage occurs at the sites of greatest pest densities, which will be targeted first by the wasps.

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Contrary to what some people might believe, it is not just bees which pollinate! In fact, any insect which visits flowers has the potential to act as a pollinator. This includes beetles, butterflies, moths, flies and yes, even wasps. In fact, some plants, such as the Chiloglottis orchids, can only be pollinated by a single specific species of wasp (Peakall, 1990). Wasps normally pollinate during their search for carbohydrate rich nectar, but will occasionally frequent flowers during their search for prey. Sometimes they are even tricked into visiting flowers in reaction to volatiles released by the plant! Some species have evolved the ability to produce chemicals to attract male wasps by mimicking female sex pheromones (Schiestl et al., 2003) or by releasing damage signal volatiles to mimic pest damage to attract hunting wasps (Brodmann et al., 2008).

Wasps also have much to teach us. It was by watching wasps create their nests by mulching wood that Cai Lun, an official of the Chinese court of the Han Dynasty, first developed the idea of paper around 105AD. This now ubiquitous technology underlies much of the functioning of our society and owes its inspiration to the remarkable wasps. Even today we are learning much about social evolution by examining the range of socialities exhibited by wasps along with the underlying genomics.

By examination of the services that an organism provides us, it may become easier to justify why we should respect and safeguard them. This carries with it however, the danger that we should only value an organism by what it contributes to ourselves. This narcissistic view is dangerous as in reality human-kind knows very little about the complex world we inhabit. Surely species have a right to exist that is not solely determined by their detectable utility to one other particular species? Even if wasps did not provide us with all of these fantastic services free of charge, I would argue that they are beautiful creatures in their own way. Each individual organism we see around us represents the culmination of millions of years of evolution. Surely it should be a pleasure to share the planet with these creatures. This, I would argue, is ‘the point of wasps’.

By Liam Crowley.

References:

Ballou, H.A., 1913. Report on the prevalence of some pests and diseases in the West Indies during 1912. Barbados, West Indies, Bull. 13, pp.333-357.

Beggs, J., 2001. The ecological consequences of social wasps (Vespula spp.) invading an ecosystem that has an abundant carbohydrate resource. Biological Conservation, 99(1), pp.17-28.

Brodmann, J., Twele, R., Francke, W., Hölzler, G., Zhang, Q.H. and Ayasse, M., 2008. Orchids mimic green-leaf volatiles to attract prey-hunting wasps for pollination. Current Biology, 18(10), pp.740-744.

Harris, R.J., 1996. Frequency of overwintered Vespula germanica (Hymenoptera: Vespidae) colonies in scrubland‐pasture habitat and their impact on prey. New Zealand journal of zoology, 23(1), pp.11-17.

Peakall, R., 1990. Responses of male Zaspilothynnus trilobatus Turner wasps to females and the sexually deceptive orchid it pollinates. Functional Ecology, pp.159-167.

Richter, M.R., 2000. Social wasp (Hymenoptera: Vespidae) foraging behaviour. Annual review of entomology, 45(1), p.142.

Schiestl, F.P., Peakall, R., Mant, J.G., Ibarra, F., Schulz, C., Franke, S. and Francke, W., 2003. The chemistry of sexual deception in an orchid-wasp pollination system. Science, 302(5644), pp.437-438.

Spradbery, J.P., 1973. Wasps. An account of the biology and natural history of social and solitary wasps, with particular reference to those of the British Isles. London: Sidgwick and Jackson Limited, pp.282-283.