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The Invasion of the multicoloured Asian ladybird

9/10/12

Once used in the integrated aphid control, the multicoloured Asian ladybird – Harmonia axyridis – is today one of the most abundant species in our regions, even entering the very heart of our homes. But the scientific clever riposte is little by little being put together. Gembloux Agro-Bio Tech has just taken a decisive step in the identification of…the hydrocarbons which play a role in the formation of the animal’s Winter aggregations.

Who could have predicted such a scenario? The scientists maybe, had they been consulted. European horticulturists without a doubt, if they had taken a little more interest in the problems experienced in the United States several years previously. When the Belgian, French, Dutch, etc. horticulturists introduced the multicoloured Asian ladybird into their crops over the 1990s it was thought to be blessed with every virtue. Renowned for its voracity, Harmonia axyridis makes short work of theswarms of aphids that invade the greenhouses. Easy to breed, it does not cost much and has an undeniable advantage over insecticides: no toxicity for handler! Like the professionals before them, ordinary citizens also bought it, delighted to use this perfect auxiliary of the ‘integrated pest control’ against plant parasites  in their vegetable plots and gardens. Thus, preceded by its reputation as one of God’s good creatures (in French, the ladybird is called ‘bête à bon Dieu’ as well as ‘coccinelle’), the multicoloured Asian ladybird – it can indeed be yellow, red, orange or black – became, unknowingly, the symbolic standard bearer of all these ‘useful’ insects, loyal helpers for human beings, which it was necessary to protect at all costs.

Harmonia_axyridis

Invasive species

From 2001, people started to become disenchanted. Whilst the insect was thought unable to survive the rough Winters, it started to be noticed here and there in Flanders, out in the wild nature. Then in Brussels. Next in Wallonia. In hardly ten years, the ladybird spread extraordinarily quickly and ended up invading the whole country, a process which has today earned it its status as an invasive species.

A problem? Yes, and in three ways. First of all, the animal is an ‘intraguild’ predator. This means that, far from settling for devouring aphids (Hemiptera: Aphidoidea) and other small soft bodied insects, the multicoloured Asian ladybird does not hesitate to take on aphidophagous predators. It enters into direct competition with other insect species, including indigenous ladybirds, of which it devours the larvae. In this respect, the most recent scientific reports published in Belgium seem to indicate a decline in the two spotted and the ten spotted ladybirds, species which our  ecosystems have good need of.

A second issue: the animal has the annoying habit of feeding on ripe fruits. Wilting is not caused by the insect itself (feeding usually occurs after another factor has damaged them), but it seems particularly interested in very ripe grapes,  aggregating on them at the end of the season. Because it is difficult to extract the ladybirds during the manufacturing process, they are found inside the wines, whose taste is then affected. Given the craze to create vineyards in Belgium, that is something which could give our (future) viticulturists, a hard time, as it has for their French colleagues.

Finally, a third problem: when Autumn arrives, Harmonia axyridis shows an aggregating behaviour to protect itself from the fall in temperatures. Natural shelters not appearing to be suitable sites for their gathering, they thus settle within building interiors, mostly located in the countryside, and more especially in proximity to the wooded areas it has frequented during ummer. To seal crevices would be useless: it is able to slip into small cracks within frames or walls. In their dozens or hundreds (even aggregations of thousands of individuals have already been observed!), the multicoloured Asian ladybirds then tranquilly overwinter in the folds of curtains or behind walls, their metabolism slowing down throughout the rough season.

If the Winter company of some of these insects may be pleasant, it fails to be when hundreds of animals invade the living or sleeping spaces, staining the supports where they are gathered with an obnoxious yellow liquid when they are disturbed. The substances it contains can sometimes induce allergic reactions: asthma, rhinitis, conjunctivitis, etc. The presence of allergens in the hemolymph has been demonstrated. Quite recent American studies have showed cross sensitivity allergies between the multicoloured Asian ladybird and cockroaches.

Shapes-pronotums

Trapping techniques

For all these reasons, their populations have to be controlled, but we should have no illusions: their presence has become inevitable and it is now probably impossible to completely eliminate this invasive species. At the Functional and Evolutionary Entomology of Gembloux Agro-Bio Tech (University of Liege), researchers have been working for a long time on this subject in order to improve the understanding of Harmonia axyridis behaviours. One of their main objectives consists in developing trapping techniques that respect both the ecosystems frequented by animals (other insect species, and in particular other ladybird species, cannot possibly be threatened,) and user safety. An important step has just been taken by thework of Delphine Durieux, a researcher at Professor Eric Haubruge’s Entomology Department, carried out jointly with the Analysis, Quality and Risk Unit (Laboratory of Analytical Chemistry). This research has been devoted to the roles of hydrocarbon chains in the formation of ladybird aggregation in houses. ‘We know, through other research work, that, in Autumn, Harmonia axyridis is attracted by prominent elements in the landscape, standing out clearly on the horizon: mountains, hills, imposing buildings, isolated houses, etc.’ explains the young researcher. ‘We still have to discover what happens after the migratory flight toward buildings: how and why do multicoloured Asian ladybirds form these aggregations? How is the cohesion of their gathering ensured throughout the bad season?’

gathering-ladybirdsThe researcher looked into the non-volatile molecules present on the insect cuticle. These molecules – which could play the role of pheromones – are deposited passively on the substrate by simple contact with legs or the abdomen. To be detected by insects these molecules have to be tasted, probably through the mouthparts (but that still needs to be proved). After having been removed from privately owned buildings, some twenty aggregations and their substrate (frames, walls, etc.) have partly given up their secrets. ‘Through gas chromatography, we have identified two types of marking, one deposited in the surroundings of aggregation sites, the other specific to gathering sites themselves. In the first case, the marking leads the animals toward the aggregation site. In the second one, the deposited cues  ensure group cohesion throughout the winter period. In both cases, the same hydrocarbons are involved, but chemical analyses have revealed differences in the profile: the proportion of unsaturated hydrocarbons is higher in the blend collected around the gathering area.

The study, carried out jointly by the Unit of Entomology and the Laboratory of Analytical Chemistry, has also highlighted the fact that a portion of the hydrocarbon blends was still present on the aggregation substrate even after one year. ‘Their concentration can certainly evolve,’ explains Delphine Durieux. ‘But the markings contain saturated molecules, whose stability explains why they are still present – and tasted – the following Autumn, enabling the orientation at short distance of the first overwintering ladybirds. The latter would refresh and reinforce these cues during their comings and goings around the aggregation site. Only the presence of these substances, deposited the year before is not sufficient to lead once again to the aggregation of multicoloured Asian ladybirds on these sites. Indeed, if the environment of the previously selected site has been modified and if those sites are no longer suitable for aggregation, ladybirds would no longer establish their gathering in these places and choose more appropriate sites. This fact suggests the particular importance of visual elements in the aggregation process of this species.’

Does that mean that the design of multicoloured Asian ladybird traps can begin? Not yet. ‘The role of unsaturated hydrocarbons must still be confirmed and understood in depth,’ explains the researcher. ‘We need to identify precisely the molecules responsible for the behaviour we have observed. The potential role of volatile compounds has also to be investigated. Indeed, associating such substances to the traps is essential to allow remote attraction of insects. Numerous experiments are still necessary.’

Householders faced with this type of invasion thus still have to wait for a while. However to get rid of them, they can meanwhile vacuum the aggregations and place them in the freezer for a sufficiently long time. In fact, released alive outdoors, the ladybirds have only one reflex: to return as soon as possible to their overwintering site. The best filling-up of cracks does not totally prevent ladybirds from coming in. The only disadvantage of this somewhat drastic method is the risk of destroying, in the hoovered and ‘frozen’ lot, two spotted ladybirds or other indigenous species which regularly gather with multicoloured Asian ladybirds during winter. The same insects which, in the warm season, devour their larvae…


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