Ladybirds: good for pest control, or pests themselves? As our Who’s the Pest? season draws to a close, Emma Rhule takes a closer look at our relationship with these tiny flying beetles.
Bright, colourful and hearty eaters of aphids (the sticky little flies that infest everything from cabbages to roses), ladybirds have long been the gardener’s friend. Gardeners and farmers will go to great lengths to encourage more ladybirds onto their patch. One way you can entice ladybirds into your garden is to plant wildflowers – they are particularly fond of marigold and nettles. And, if all else fails, the eggs and larvae can be bought online.
There is a long history of introducing exotic ladybirds to help control agricultural pests. The Vedalia ladybird, Rodolia cardinalis, which was originally from Australia, is credited with saving the Californian citrus industry in the late 1880s. It was the start of the biological control industry that controls pest species using other organisms.
Following in this tradition, the harlequin ladybird, Harmonia axyridis, was released across the USA and Europe to help combat aphid infestations. Originally from Asia, the voracious appetite of this beetle makes it an extremely successful control agent. A single adult can eat more than 200 aphids a day, and a developing larva can consume upwards of 1000 aphids. Their use allows farmers to reduce the amount of insecticides they need on their crops which should, in turn, allow other beneficial insects to flourish.
The very hungry ladybird
Unfortunately, the harlequin’s insatiable appetite is not limited to aphids: the larvae are particularly indiscriminate. They will eat almost anything they encounter – including other aphid eaters such as hoverflies and lacewings, as well as caterpillars. They will also eat the eggs and larvae of other ladybirds, and their cannibalistic tendencies even extend to their own siblings. To make matters worse, there is little that eats the harlequin. Like our native ladybirds, their bright colours advertise the toxins present in their blood. These chemicals make ladybirds taste foul and can be deadly to imprudent would-be predators.
In 2004, the first British harlequin was spotted in the car park of a country pub in Essex called The White Lion. Never intentionally introduced, researchers suspect the early arrivals that reached the UK were a combination of hardy pioneers flying across the Channel and sneaky stowaways in imported goods. Since then, they have spread across the UK and been reported as far north as the Shetland Islands.
Since arriving, the harlequin’s wide-ranging diet has had serious implications for British ladybirds. The once familiar two-spot ladybird has been particularly hard hit, losing out to the larger, hungrier competitor with which it now shares its habitat. The wider ecosystem is affected, too – particularly the predators, parasites and pathogens that rely on these native species for their own existence.
And humans do not escape unscathed. Over the past few years, come October and November, you may have seen harlequins in your house. In the winter, our native species mostly hibernate outside in dead leaves or in the crevices of tree trunks, sheltered from the cold and rain. In their native Asia, harlequins normally use rocks and cliffs. In the relatively flat UK, houses must seem like a good alternative. Once the central heating comes on, though, the beetles warm up and start flying around. They may be hungry and take a nibble. A little nip is usually not a problem, but some people can experience an allergic reaction. If they are feeling threatened, the natural response of all ladybirds is to leak a small amount of foul-smelling yellow blood from their knees, which can stain curtains and sofas.
So what can we do? It would be impossible to capture and kill all the harlequin ladybirds in the country. In fact, the Harlequin Ladybird Survey, which has been monitoring the spread of the ladybird since its arrival, strongly discourages this because there is a real risk of misidentification. The harlequin was the focus of my PhD research: one of my jobs involved opening letters containing live ladybirds, sent to us by members of the public for identification. These days, photos can be uploaded with records of sightings allowing researchers to verify exactly which species has been found, saving us from climbing onto tables to recapture escapee beetles crawling across the ceiling.
When the harlequin was taken from its native environment, researchers made sure that any released into the wild were free of parasites. As such, many of the natural checks and balances that would have kept the numbers of individuals under control were removed. Nothing in the new environment would have been immediately able to fill the gap, allowing populations to flourish. One hope is that the parasites and pathogens that exploit our native ladybirds will start to adapt to the newcomer. Nearly ten years on, there is some evidence that this is starting to happen.
But how long can we wait when the problems are already apparent? My research looked at whether a sexually transmitted parasite found on European ladybirds, including the harlequin, could help. The blood-sucking parasite can be thought of as a form of ladybird birth control. When females are infected with adult parasites, they still lay eggs, but these do not hatch – the ladybird is effectively sterile. If the parasites die, within a couple of days any eggs laid will hatch. Of course, I am aware of the irony of my work – could we, and should we, introduce yet another species to help control an exotic beetle that was itself introduced to fight an insect pest? Could the parasite become the next problem in town?
After five years, I came to the conclusion that we still didn’t know enough to justify taking that risk. Increasingly aware of what can go wrong when we tinker with ecosystems, we are much more cautious than we used to be, and rightly so. From grey squirrels to cane toads, knotweed to ladybirds, there are lots of examples of the unintended side-effects of our actions. So, who is really the pest? Those species that arrive in a new place and wreak havoc, or the species that put them there?
Emma Rhule is a graduate trainee at the Wellcome Trust. Play our Who’s the Pest game online.