We are being invaded. I had not seen many harlequin ladybirds in my garden this year, but a few weeks ago I noticed that the larvae were climbing over the hedge from next door, in droves...
We are being invaded. I had not seen many harlequin ladybirds in my garden this year, but a few weeks ago I noticed that the larvae were climbing over the hedge from next door, in droves. This south-facing fence is covered with ivy and a Muehlenbeckia complexa that now threatens to engulf the fire-escape. It's a great place to watch insects sunning themselves on the leaves, but ne'er one ladybird larva have I seen until now.
On the other side of the fence, there is a fine wilderness of bramble and grass, and it is here that I think they have been living all summer. Like many insects, once feeding is complete they deliberately move away from the food source to find a suitable place to pupate. There are, perhaps, two different reasons for this. First, they need shelter and warmth to aid (and speed) the complex physiological transition from larva to adult. Second, they need to get out of the way of things that might eat them.
For the harlequin ladybird, a major predator is another harlequin. It is the voracious appetite of the large larva that has given the harlequin ladybird, Harmonia axyridis, its bad reputation. Not only does it eat aphids, it also eats other insects, including other ladybird larvae and also lacewing and hoverfly larvae feeding in the aphid colonies. And it is known to be cannibalistic.
The ladybird pupa is attached to the leaf by a silk pad at its tail end, on which it can articulate. I noticed one of them had suddenly twitched up into its vertical flip position. This is an escape and startle response to deter potential predators, and sure enough the potential predator having a sniff around was another harlequin larva. In fact cannibalism is quite widespread in nature, and other ladybirds do it too. I am reminded of an early ecological lesson:
What does a herbivore eat? Plants.
What does a carnivore eat? Anything it can get.
After the harlequin ladybird first arrived in Essex five years ago, there was a great media hoo-hah about its potential to wreak ecological havoc. As can be seen from the yearly distribution maps of the harlequin ladybird survey, it has spread rapidly and widely across much of England. Incidentally, a link from the site allows even the most casual of observers to contribute observations and records to the survey.
In North America, where this Asian species was deliberately released 20 years ago as a biocontrol agent, it seems to have out-competed native species, to their great detriment. So too did the European seven-spot. But in Japan, harlequin, seven- and two-spots coexist in ecological balance. It is too early to say exactly what effect the harlequin will have in the British Isles. The invasion over the fence from next door is very impressive, but there are still plenty of seven- and two-spots hereabouts.
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