Wednesday, 17 July 2013

Ladybird Smallpox

Harlequin Ladybirds Harmonia axyridis arrived some years ago in Europe, introduced to greenhouses to control aphids. They are native to central Asia. Inevitably they escaped into the wild, and ever since scientists have been observing and monitoring the effect on native ladybird species.

They are bigger and more aggressively voracious than any of the native species here, and it was feared that they would out-compete the natives easily. However, the situation seems to be more complicated than that.

Harlequin Ladybird larva.
Harlequin larva [IGP7240]
Photo courtesy of Tim at Aigronne Valley Wildlife.
New research from Germany shows that the Harlequins carry a parasite. They are not particularly bothered by this freeloader, which seems to be dormant when in a Harlequin, but becomes active when ingested by other species of ladybird, who are killed by it. The scientist who conducted the research likens the situation to the European settlement of the New World, when diseases that the colonists survived proved to be deadly to the indigenous population.

The parasite lives in the Harlequins' haemolymph ('blood'). Their blood also carries a toxin called harmonine, the function of which is to poison any creature that eats the Harlequins' eggs. Previously it was thought that this was what was killing the natives, but it turns out that 7-spot Ladybirds Coccinella septempunctata, at least, are immune to harmonine. Further examination revealed the real culprit -- a tiny single celled microsporidian parasite. Ladybird species commonly eat one another's eggs and larvae, so the parasite could be transfered very easily from one species to another. It could also be transfered by parasitic wasps, which lay their eggs in a variety of ladybird species.

A rather typical adult Harlequin Ladybird, although they can be difficult to identify as they exhibit many different pattern combinations. One sure way of identifying them is their brown legs. The native species of similar size all have black legs.
Harlequin Ladybird [IGP7258]
Photo courtesy of Tim from Aigronne Valley Wildlife.
However, other scientists note that the Harlequin's success is at least partly due to the fact it is more resistant to fungal diseases than the native species. They also note that one species of native ladybird has weathered the invasion without too much concern, whilst others are in severe decline. 7-spot Ladybirds rarely eat Harlequin eggs and this may be protecting their numbers in the wild. Certainly here in the Touraine the 7-spot is the dominant species. Harlequins are not seen that often (except apparently in the Aigronne Valley...) and numbers of 2-spots Adalia bipuntata remain good. I don't know what the situation with other species is.

The research has implications for bumble bee decline in North America, where habitat loss, climate change and pesticide use don't quite explain the sudden and extreme reduction in numbers of certain species. An introduced microsporidian is suspected here too.

See also Ed Yong's article in Nature for an overview of the research, and Tim's post on Aigronne Valley Wildlife for some history and great pictures of some of the different patterns the little blighters come in.
A la cuisine hier: Simon made another yellow cherry based Asian sauce, this time a hoisin like concoction. He can't wait to try it as a marinade on beef. I'd better keep the supply of cherries up to him as he's really on a roll with these sauces.


Tim said...

One thing I noticed in Leeds was that the sizes differed greatly, also...
they are not all great big things... some are quite small...
the colour of the legs is the best indicator...
that and the oval formation of four spots on the multi spotted ones.
Thanks for the mention.

lejardindelucie said...

Article très intéressant concernant cette résistance des Harmonia axyridis. Elles sont vraiment des tailles et des aspects très variés.
Elles sont présentes chez moi, mais aucune des autres espèces "autochtones " présentes ne semble diminuer.

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