The 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…