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Harlequin ladybirds' success is down to enemy dodging

4 December 2013, by Tamera Jones

The spectacular spread of the invasive Harlequin ladybird across much of Britain is down to its ability to evade natural predators compared with the native seven-spot ladybird, according to scientists.

Dinocampus larva

Dinocampus larva.

UK researchers found that in the first ten years after the Harlequin's invasion, it was much less likely to succumb to natural enemies than native ladybirds.

The findings support an idea called the enemy release hypothesis. This suggests that invasive alien species such as the Harlequin do well compared with native species, because resident natural enemies don't tend to attack them. They also explain why the Harlequin is the most invasive alien ladybird on Earth.

The ladybird is native to Asia, but arrived in Britain around 2004 probably on produce or through transport networks. It has since spread at more than 100 kilometres a year and is now established across Europe, Africa and the Americas.

The speed at which alien species are establishing themselves in Britain is accelerating. They're a major threat to native ecosystems, upsetting the diversity of life. Their invasion has led to declines in many or our native ladybirds.

Scientists think part of the success of the Harlequin could be because it can live in a range of different habitats, reproduces quickly, eats a wide variety of prey, and can cope with lots of climates.

But they also wondered if the enemy release hypothesis could play a role in the insect's success. This means that predators, parasites and pathogens haven't got used to the new arrival and so don't tend to attack it.

7 spots mating

7 spots mating.

Several species of tiny parasitic wasps attack native ladybirds. One, Dinocampus coccinellae, lays an egg inside them, which subsequently hatches into a grub-like larva that devours the inside of the ladybird before emerging to pupate. It uses the ladybird as a bodyguard which twitches defensively above the small parasitic cocoon. The wasp then emerges, leaving the ladybird host to die.

There's now growing evidence that the same wasp attacks the Harlequin ladybird. But scientists didn't know how much this is happening, or if this is having any effect on native ladybirds, until now.

Richard Comont decided to investigate while studying for his PhD at NERC's Centre for Ecology & Hydrology. Comont, now at the Bumblebee Conservation Trust, and colleagues monitored Harlequins and native seven-spot ladybirds for natural enemies over four years. They focused on populations across southern England in areas first invaded by the species between 2004 and 2009.

At the same time, the researchers wondered if huge numbers of Harlequin could lead to more parasites affecting more native ladybirds.

Comont and his colleagues found that the wasp naturally attacks about one in ten seven-spot ladybirds - around 18 per cent. But the wasp attacks fewer than two in 100 Harlequins - only 2 per cent.

The researchers found no evidence that the close proximity of Harlequin ladybirds put the parasites off attacking the native ladybirds.

The study is published in the journal Insect Conservation and Diversity.

Comont said, 'It's really exciting to find that native parasitoid species are attacking the harlequin, but they're only doing so in really small numbers - the native 7-spot ladybird is almost 11 times more likely to be eaten than the harlequin. That really shows in part why the harlequin is increasing so quickly - there's nothing holding it back.'

Co-author Dr Helen Roy of the Centre for Ecology & Hydrology said, 'Our work clearly demonstrates how some alien invasive species such as the harlequin ladybird can escape predators and parasites allowing them to successfully and rapidly spread across a country such as the UK.'

'We need people to keep sending in records to the UK Ladybird Survey and we would be delighted to hear about any sightings of parasitized ladybirds. Every observation helps us to unravel the story of this alien invader and more broadly to our understanding of invasion biology.

Richard F. Comont, Bethan V. Purse, William Phillips, William E. Kunin, Matthew Hanson, Owen T. Lewis, Richard Harrington, Christopher R. Shortall, Gabriele Rondoni, Helen E. Roy, Escape from parasitism by the invasive alien ladybird, Harmonia axyridis, Insect Conservation and Diversity, published online: 2 DEC 2013, DOI: 10.1111/icad.12060

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