2.2.1. Adult Monitoring

Monitoring soil-dwelling pests is difficult and expensive; thus, efforts have been made to assess population levels of click beetles in the hope of inferring larval abundances or crop damage. The identification of click beetle pheromone goes back to the 1970s in the USA for *Limonius* species [60,61] and the 1990s in Europe for *Agriotes* species [62]. Pentanoic acid and hexanoic acid were identified as pheromone compounds for *Limonius* species. Esters of geraniol are the main components of *Agriotes* natural sex pheromones [63], given that female pheromone glands contain up to 24 substances [62]. Varying the mixture formulation allows each species to be caught selectively or, alternatively, several of them to be attracted to the same trap [64]. Recently, several kinds of pheromone traps have been developed and used as research tools to monitor populations in both Europe and North America [24,26,65]. The female sex pheromones of most major European click-beetle pest species (*A. brevis*, *A. lineatus*, *A. obscurus*, *A.proximus*, *A. rufipalpis*, *A. sordidus*, *A. sputator*, *A. ustulatus*, *A. litigiosus*) have been characterized [64]. YATLORF (Yf) sex pheromone traps (Figure 3A) were designed for a range of *Agriotes* species, including all of the most harmful ones in Europe and part of the *Agriotes* pests in North America. In addition, a groundbased pheromone trap for monitoring *Agriotes lineatus* and *A. obscurus* was developed to catch *A. obscurus* and *A. lineatus* in North America [66]. The apparent ease with which pheromones can be used and their potential as a pest managemen<sup>t</sup> tool have made them attractive for pest monitoring. However, relating click beetles' catches to larval densities requires a good understanding of the pest behavior, pheromone lure reach, and effects of various abiotic factors on trapping [67]. Pheromone traps for *A. lineatus* and *A. obscurus* may have a very short attraction range (below 10 m) [68,69] with no directional bias [70,71].

Significant association was found between male click-beetle catches in pheromone traps and subsequent wireworm abundance and maize damage in the nearby area for three species: *A. brevis*, *A. sordidus*, and *A. ustulatus* [57]. For example, when Yf *A. ustulatus* catches exceeded 1000 beetles per season, there was a 20-fold higher probability that the trapped wireworm density exceeded five larvae per trap. The procedure and thresholds described in Furlan et al. [57] allow both farm-scale and area-wide monitoring, resulting in the drawing of risk maps in cultivated areas and enabling IPM of wireworms to be implemented at a low cost. They make wireworm risk assessment highly reliable, especially when it is associated with agronomic risk factor assessment. In contrast with these results, Benefer et al. [72] concluded that the proportion and distribution of adult male *A. lineatus*, *A. obscurus*, and *A. sputator* species may give a very misleading picture of the proportion and distribution of wireworm species in the soil, at least when they are caught with sex pheromone traps. However, this study had major constraints, including the fact that fields were observed for one year only while click beetles are associated with wireworm populations in the subsequent years. A longer period of study using more consistent methods might have revealed significant associations between click beetles trapped in previous years and wireworm population levels at year zero. In any case, as noted in a review on their use [73], pheromone traps are sensitive enough to detect low-density populations, and trapping systems are able to inform growers about the presence or absence of wireworm infestation.

**Figure 3.** Illustrations of trapping systems. ( **A**) Click-beetle pheromone trap YATLORF. (**B**) Wireworm bait trap (right pot) and sequential filling of the trap with an empty trap (left pot), a trap with a layer of vermiculite (second pot from left), a trap with a layer of vermiculite and a layer of germinating maize and wheat (second pot from the right).
