Saturday, 5 November 2011

The maladapted parasite

There is a corner in my local park which is always teeming with Harlequins. At this time of the year adults ready to hibernate and grown larvae about to pupate dot the railings and fenceposts. A couple of days ago I came across this individual dragging a strange large lump behind as it sluggishly walked on top of a railing. I took a photo and wondered if it was a parasite. Parasites in Harlequins are very interesting, as, this ladybird being an invasite species, they might be adapting to the newcomer and helping keeping this species in check. Richard Comont, from the Centre for Ecology and Hydrology in Wallingford, after checking the photo commented:
It looks like a Dinocampus which has got caught up in the ladybird's wings - I think you can just about see some segmentation at the bottom of the yellow lump. Dinocampus seem to be a bit disorientated in Harlequins - they seem less able to fully paralyse the adults on the way out - so possibly might sometimes emerge from the top of the abdomen, from between the tergites rather than the sternites, which could result in the emerging larva ending up in the wing like this.
Dinocampus, on emerging, usually parasitise their host ladybird, cutting the nerves connecting the legs to the nervous system, and potentially also the tendons of the leg abductor muscles which allow the ladybird to move its legs away from its body (so that when the parasitoid's cocoon is spun between the ladybird's legs it can only be clutched tighter). However, Harlequins with Dinocampus are often not fully paralysed, and instead stumble around slowly, almost drunkenly.
It appears that some native ladybird parasites are also attempting to parasitize the newcomer invasive Harlequin, which is encouraging. Evolutionary success for parasites in expanding their host range to include Harlequins however, will only come if the parasite is successful completing its life cycle in the new host. The offspring of the parasite must actually be able to become a successfully reproducing individual in the new host, and the disorientation of the Dinocampus in Harlequins, which could often result in their larvae being unable to spin its cocoon under the host - which is walking around dragging it - suggests that their success in surviving and actually becoming an adult parasite might still be far lower than in the native ladybirds. They still might have some way to go.

I am most grateful to Richard Comont for allowing me to reproduce his comments in this post and to Lori Lawson Handley for kindly forwarding my photo to some knowledgeable ladybird experts.

From an e-mail from Richard Comont (15/11/11):
I’ve found a couple of these ‘parasitised’ Harlequins today – and the ‘parasite’ was just a pool of reflex blood, which the wing had folded around into a kind of bag – no evidence of parasitism at all!
That's science for you!

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