After having been eradicated from the countryside using first-generation pesticides, wireworms are slowly but surely returning to our fields. Despite the amusing acrobatics of the adults, the larvae are a menace to a wide variety of crops. In order to eliminate them in a more environmentally-friendly way, researchers from Gembloux Agro-Bio Tech (university of Liège) have managed to identify the specific molecules that attract the larvae to the roots of barley crops (1). These findings could be used in various systems for early pest monitoring and capture, resulting in significant progress in integrated control of wireworms.

Even though few know its scientific name, many vegetable gardeners are well aware of the click beetle (family Elateridae). This beetle, less than a centimetre long (of which 7,000 to 9,000 species exist worldwide), is known for its ability to right itself when turned on its back by jumping dozens of centimetres in the air with a loud ‘click’. Generations of children have been amazed by the insect's jumping ability, which is made possible by a small organ – the prosternal spine – that the click beetle wedges into a groove in its mesosternum and whose release produces the click that enables the insect to jump. What amateur gardeners often do not know is that click beetle larvae, also called wireworms because their thin body shape, spend most of pre-adult life (i.e. three to five years!) in the ground, at depths in excess of two metres! Depending on the season and the climate, the plant-eating larvae move towards or away from the surface by borrowing through the rhizosphere, feeding on the roots of certain plants which make up most of their diet.

In certain areas, wireworms can number in the millions per hectare, with obvious consequences on crops: the larvae's attacks cause the plants to quickly dry out, as their roots are consumed and can no longer provide the plants the water they need to grow. For over three decades, organophosphate and organochlorine crop protection products have easily defeated click beetles and wireworms, as well as many other crop pests. However, now that most of these synthetic pesticides have been banned for obvious environmental and health reasons, the insect is once again rearing its head: for the past 15 years, yellow dried-out ‘spots’ have been appearing here and there on crops in Belgium, although not in as dramatic a fashion as in other countries (France, Austria, etc.) – for the moment, that is. François Verheggen, assistant professor at Gembloux Agro-Bio Tech's Functional and evolutionary entomology unit, acknowledges that ‘the return of wireworms is a well-known problem in the farming community and that the insect is one that must be closely monitored all over Europe in order to prevent significant crop damage.’

A more respectful protection

As we cannot go back to using synthetic products that eliminate insects indiscriminately, we must come up with alternative solutions that are more targeted, less harsh and more advanced. Integrated pest management, which involves using fewer chemicals (or even none) through a combination of various compatible and effective techniques, is clearly one of the most promising areas of development. Scientific research and industrial development have already produced results for the integrated management of click beetles. The main pheromones involved in their reproduction are known, and these molecules have been used for the past 15 years to attract and trap the insects, which can then be destroyed. Still, the profile of these pheromones still needs to be refined, including depending on the region in which they are used. Due to their high cost, they are often only used for population monitoring and not to trap click beetles on a large scale. ‘Furthermore, even when they are used, pheromone traps generally capture only a small percentage of the adult click beetles living in the crops,’ says François Verheggen. ‘These highly mobile insects can fly hundreds of metres, while pheromone traps can only attract them within a radius of a few dozen metres at most. This means that in practice, infestations are not truly contained.’

(1) Foraging wireworms are attracted to root-produced volatile aldehydes, Barsics, Fanny et al. Journal of Pest Science, 2016.