Searching for the scarlet, speckled ladies of the garden

That brief dream of summer found the acre becalmed, its billowing perimeter of fuchsia and hawthorn shutting out whatever breeze there was. Midges hung about in a lovesick trance, their little clouds glinting in ambush under the trees; too many sudden white butterflies danced around me in their own licentious whirl. While the insect world caught up on reproduction, I hurried about with prophylactic nets to cast across my cabbages.

In calmer moments I went looking for ladybirds. It’s years since I rounded up a small fistful of the common, scarlet, seven-spotted ones and released them in the greenhouse to rid my basil of aphids. They promptly flew out the door. This time proved no better, as a fairly patient search of nettles, thistles and brambles found neither adults nor their larvae, even with my other glasses; a 10-year-old would have done better.

In Cork this summer the local branch of the Irish Wildlife Trust is recruiting keener eyes for a ladybird survey (go to biology.ie), hoping to find most, if not all, of Ireland's 15 surviving species. One it is hoping not to find, at least in any great number, is the dreaded harlequin, Harmonia axyridis.

Even a few years ago the arrival of the harlequin from Britain was fairly inevitable. Now it has landed and is breeding in the wild. The first report – a larva on a pear tree – came from Co Carlow in the summer of 2011. By that autumn more were crawling on bushes in a garden in Cork city. All were the young of the harlequin's commonest adult form, H axyridis var succinea, shown in my drawing.

Ireland thus joins most of the world in being colonised by the biggest, hardiest and greediest ladybird around. Since 1988 it has invaded at least 38 countries, outcompeting local native species, often eating them and their larvae (and, indeed, each other) from Siberia to South America.

In the US, where hibernating swarms have caused panic in city buildings, triggering smoke alarms and creeping into computers, the harlequin is called the “multicoloured Asian lady beetle”, which is clumsier but more explicit. The insect is notoriously polymorphic, reaching adult beetledom in an astonishing variety of liveries. Some 200 patterns in different locations, partly caused by habitat temperature, have long fascinated evolutionary geneticists, but they make things difficult for identification in the field.

So far Irish sightings have been of the three commonest forms: the orange succinea, with zero to 21 black spots, as above; the black conspicua, with two large orange spots; and the black spectabilis, with four orange or red spots.

There is a great page at biodiversityireland.ie of all the Irish ladybird species, including these. Whatever its colour, the harlequin owes much of its international spread to the trade in potted plants reared in greenhouses and polytunnels using Harmonia as a "biocontrol" of aphids.

Natural predators in place of pesticides can seem most desirable: even gardeners have started doing it. But a different insect taken from nature to improve polytunnel economics has brought its own threat to Ireland’s native species, this time of our wild bumblebees.

It is two decades since Europe's horticulture industry started breeding the big bumblebee Bombus terrestris for captive pollination, and more than a million packaged hives are now exported worldwide each year. About 1,450 of them come to Ireland, where the bumblebees' work in the polytunnels of Leinster has been judged to increase the yield of perfectly shaped strawberries by more than 25 per cent and their pollinating visit per flower to be more than twice as effective as that of a honeybee.

Unfortunately, Ireland's native wild subspecies, Bombus terrestris audax (shared also with Britain), is not the same breed as the imported bumblebee, whose origins owe more to central Europe. The long adaptation of the Irish bee to conditions on this island is reflected in its genes. If the aliens do wander out of their plastic prisons, they can interbreed with the natives, compete with them for pollen and, perhaps, infect them with new diseases and parasites, as has already happened in Canada and Japan.

That they do escape in Ireland has been proved in a three-year scientific study for Teagasc and the Department of Agriculture. It found that 75 per cent of pollen collected by the polytunnel bees did not come from strawberries. And where they are let loose on field crops they "have effectively unhindered access to interact with native bees".

The report, completed in 2009, concluded: “The evidence for disease transmission, hybridisation and establishment of non- native bumblebees in Ireland is now compelling. The consequences of maintaining the status quo regarding licensing and lack of independent screening without further research is to endanger all native populations of pollinators and to leave open an avenue for future pests and pathogens to enter Ireland.”

Enjoy your perfectly shaped Irish summer strawberries.