All neatly lined up!

After two hours on average, the result – reproduced around fifty times – was clearly visible. In the first series of tubes (with a live barley root at one end), 75% of the wireworms went to it, 15% went to the empty end of the tube, and 10% remained in the middle of the tube. This demonstrates that wireworms are attracted by the ‘fragrance’ of barley roots, without any visual contact. In the second series of tubes (with the four aldehydes inserted at one end), the attraction was slightly less obvious, with 66% of the wireworms going towards the bait. ‘This result is just as meaningful as with the roots,’ says François Verheggen. ‘It clearly demonstrates that aldehydes can attract wireworms. The reason why the percentage of wireworms attracted is slightly lower than the 75% that were recorded in the first experiment is likely the fact that the roots release CO₂, which contributes to spreading odours in the test tube. This observation could help us develop a product with maximum attractive power.’ 

What about the three different baits? ‘The wireworms responded much more strongly to the two lower dosages: 10 micrograms and 1 milligram. We even observed that the wireworms tended to be driven away as the dosage gets closer to 100 milligrams. This was not really surprising, as we had previously observed the same phenomenon with aphids and some species of butterfly. If more than a certain quantity of attractive product is released, it is no longer advantageous for insects to go towards the food source. This is because despite the molecule being an attractive signal, its high concentration means that many competitors are also already there. Given the time necessary to reach the source, all the food might be gone by the time they get there.’ The researcher does acknowledge that this is only a hypothesis for the time being: what is true of adult plant-eating insects might not be true of larvae, especially ones from different orders such as Hemiptera and Lepidoptera. 

A wealth of possibilities

As soon as the results in Fanny Barsics' thesis were first published, just six months ago, the functional and evolutionary entomology laboratory in Gembloux was contacted several times by various Belgian and French players in the agricultural sector who were interested in a potential new way to protect crops from wireworms. Among these, a French cooperative with 22,000 members, active in biological pest protection and willing to make fields available to the Gembloux team for experimental purposes. While trying to put a damper on the eagerness of the interested parties, François Verheggen is also extremely optimistic about the developments that can be expected in the coming months and years. ‘We are certainly not at a stage where we can produce wireworm traps at an industrial scale. Our main objective at this point is to develop monitoring systems that can predict invasions. An essential part of integrated pest management is detecting the signs of an infestation as early as possible. This means we could envision permanent monitoring stations set up in fields, where they would release aldehydes in the ground and sound an alarm as soon as the very first wireworms appear. By intervening at that stage, it is generally possible to use only a minimal amount of natural or chemical pesticides. After all, such early warning systems have already been successfully used to eliminate several insect species with only natural products (pheromones), for instance in apple and pear orchards.’  

Still, before these types of integrated systems can be developed, various odour mixes will need to be tested and optimised, for instance by adding CO₂. These ‘cocktails’ will also have to be tested on several species of wireworm: while the Agriotes sordidus – the species studied here – is one of the top three crop-damaging wireworms, it is just one among dozens of species living in Europe. Researchers have been focusing on this one because its reproductive cycle is shorter than other species’ (still two to three years!), which makes it easier to experiment on. ‘One of the possible eventual developments is the use of odour-emitting stations placed in the ground, together with fungus or nematodes known to be entomopathogenic or entomophagous: this combination relies on two mutually-reinforcing biological protections.’ Starting this October, the entomology laboratory will continue its work in this field, as a part of a European consortium (C-IPM, Coordinated Integrated Pest Management in Europe) specifically dedicated to wireworm studies and consisting in 14 research institutions from the European Union. Its focus is developing models predicting click beetle invasions for various crops (barley, but also maize), both in geographical and temporal terms.