Ladybird invaders not taking over the world!
Harlequin ladybird update
By Dave Bolton, RAMM Curator of Natural History
Click on any image for a larger view.
Harlequins are known as 'the world's most invasive ladybird'. Dave Bolton and his team at RAMM are keeping an eye on them. The good news is that in 2008 numbers are down, so it looks like they are not taking over after all. Dave explains:
Last year, 2007, was the first year that we had seen the harlequin ladybird in Exeter. Their appearance in large numbers seemingly presaged the predicted 'invasion' which had had been forecast to have a large impact upon the native species. This year we have been trying to keep an eye on developments in the local populations, although my personal observations have been centred around the Museum and the city centre and around Crediton.
Marsh Barton for first sighting in 2008
The first sightings made this year were spotted by Mark Pajak, the new Assistant Curator of Natural History who replaced Jess who wrote last year's report. Mark ,like Jess, has come from Bristol where the ladybird has had a couple of years head start and staff at Bristol museum are used to dealing with harlequin ladybird enquiries. His records, shortly after his arrival in the first week of July, were from a sycamore tree next to the Museum's new store, the ARK, on Marsh Barton. Here there were a few nearly mature larvae but on further visits we have not found further specimens.
Scale insects no longer evident
The locality of our first 2007 sightings, a sycamore tree in the corner of the RAMM car-park, is no longer accessible, but I saw no further harlequins until late August when a moderate number of larvae, pupae and newly emerged adults were present on lime trees along Queen Street. These trees in former years have been covered in scale insects that are no longer evident, but clearly there are some prey on the trees.
Poor showing west of the Exe this year
West of the Exe, a visit to last year's hot-spot was not rewarded with the expected numerous sightings. I saw only one harlequin, a larva, on a sycamore, but there were a few pine ladybirds, Exochomus quadripustulatus, as adults and pupae. Apparently the pine ladybird is feeding upon the horse chestnut scale insect, Pulvinaria regalis, as is the harlequin, so the near absence of the latter species suggests that a factor other than food availability has been responsible for the poor showing this year.
Horse chestnut scale
This view is supported by the situation in Crediton where the harlequin was very abundant late last year, feeding upon the horse chestnut scale which was reaching seeming plague proportions on a group of sycamores, but has not been seen this year.
Horse chestnut scale is itself a relative newcomer on the British scene, but having no natural predators here has experienced an explosive expansion in range, similar in many respects to that of the harlequin itself.
What do these changes mean?
So how can these changes be interpreted? With a potential for several generations in a year, with adults emerging as late as November, there is scope for rapid increases in numbers if the food supply is not a limiting factor. Feeding upon a newly arrived scale insect without natural predators may have allowed for a very rapid population expansion. I have read that the aphid populations in the London area this year have been high, with fair numbers of harlequins, but I have not experienced any problems with aphids in my own garden. The broad beans were totally free of blackfly and a willow, which in some years is so badly infested with the giant willow aphid that it becomes a seething mass of beligerant and drunken wasps has, this year been spared the aphids but has suffered sawflies instead.
Mild and damp winter
I suspect that in the Exeter area the mild and damp winter has led to poor success in overwintering. For an insect that might be better adapted to a continental climate the oceanic effect in the south-west might have brought about bacterial and fungal infections amongst the hibernating adults or they may not have found it cool enough to hibernate properly.
Have you seen harlequins? Please let us know...
I would be most interested in finding out what other peoples' experiences of the harlequin or any native ladybird have been. Since so much press coverage has been negative it would be very interesting to monitor the harlequin's progress to see how far it lives up to its title as 'the world's most invasive ladybird'.
To contact Dave, the Curator of Natural History please e-mail dave.bolton@exeter.gov.uk or telephone RAMM reception on 01392 665858