2.1.1. Risk Factors

Farmers' expertise, studies and reviews dealing with wireworm biology and ecology, and control methods highlight different categories of the factors that drive wireworm infestation and result in crop damage (Table 1).

The feeding behavior of wireworms generally involves periods of inactivity in deep soil layers, mainly in summer or winter when soil environmental conditions are adverse. This inactivity alternates with foraging periods in autumn and spring when soil conditions become more favorable in the upper soil layers [9–11,16–18]. Climate, soil properties, and their interactions influence the vertical migration dynamics of wireworms, thereby influencing the damage they might cause to field crops.

As stated in the introduction, the multiannual biological life cycle of most wireworm species [9–11,19,20] features a prolonged period spent as larvae in the soil before pupation. It outlines the prominent influence of soil characteristics on wireworm infestation and damage. Jung et al. [21] showed preferred ranges of soil moisture by wireworms in relation to four soil types and for different *Agriotes* species. Lefko et al. [22] outline the importance of soil moisture in wireworm survival and spatial distribution, suggesting that soil moisture could reveal areas where wireworms are more likely to occur and could direct scouting within a field. Furlan et al. [23] conducted a long-term survey on maize fields (1986–2014), concluding that organic-matter content was the strongest risk factor for economic damage. The risk of damage increased considerably when its value was greater than 5%. Kozina et al. [24] reported that humus content (%), together with the current crop being grown, was the best predictor of high *Agriotes lineatus* abundance. They also found that soil pH was a strong predictor for the abundance of *A. obscurus* and *A. ustulatus*. Based on a large-scale survey carried out in 336 maize fields over three years in France, Poggi et al. [25] concluded that soil characteristics had a prominent influence on wireworm damage risk, ranking them third after the presence of wireworms and climatic variables, with both pH and organic-matter content also being major factors. The effects of soil texture, drainage, and other factors can be found in the literature (see for example Furlan et al. [23]).

The frequency and intensity of wireworm damage varies across regions. Fields exhibiting high larval populations tend to be spatially clustered [26,27]. The distribution of adult click beetles in the landscape is patchy and can be stable for several consecutive years [28,29]. On a smaller scale, Salt and Hollick [30] confirmed farmers' observation that damage can appear in the same area of the field over several years. Taken together, these features sugges<sup>t</sup> that regional and field characteristics, including agricultural practices and landscape context, are important factors in determining wireworm population (see Parker and Seeney [31]).

It is commonly stated that grasslands, as well as uncropped field margins and areas, provide the most favorable habitat for egg-laying and larval development [10,32], and may act as reservoirs from which larvae and click beetles disperse into adjacent crops [33,34]. Field history, plus landscape context through its effect on click beetle dispersal, may shape the pest abundance at the field scale.

Identifying which wireworm species are present (Figure 2) may be of importance, as wireworm damage is species dependent [35,36]. Several *Agriotes* species are the major contributors to wireworm damage in Europe, but species composition and co-occurrence with other wireworms vary, and other genera, such as *Selatosomus*, *Hemicrepidius*, and *Athous*, can also be very important locally [23,37–42]. In North America, several further genera, including *Selatosomus* (spp. formerly added to *Ctenicera*), *Limonius*, *Conoderus*, *Melanotus*, and *Aeolus*, are also economically important, as are native and introduced *Agriotes* [43–47]. In East Asia, *Melanotus* appear to be important, but there are also damaging species from other genera, e.g., *Agriotes* [48,49]. In a long-term study conducted in northeast Italy, Furlan [35] showed that damage symptoms, and thus crop damage, differed according to species. About the same damage level was observed for one larva of *Agriotes brevis* per trap, as for two larvae of *A. sordidus* or five larvae of *A. ustulatus* per trap. Feeding activity may vary significantly between species, thus calling for managemen<sup>t</sup> strategies that should be tailored to their seasonal dynamics [50]. Similarly, click beetle species differ in their preferences for soil properties and climate characteristics [51]. When studying the effect of factors on risk damage, researchers may fail to spot an effect when priori species have not been identified. Saussure et al. [52] justified their failure to identify an effect of

soil properties by the fact that they did not distinguish between the wireworm species present in the surveyed fields.

Eventually, agricultural practices alter the pest population and crop damage, thereby providing the components of putative prevention strategies (§3). For example, when appropriately applied, tillage reduces populations of eggs and young larvae by damaging them mechanically. Furthermore, delaying the sowing date may help reduce damage by desynchronizing the period of wireworm presence in the upper soil layers and the period during which the field crop is sensitive to wireworm attacks.

**Table 1.** List of risk factors driving wireworm infestation and resulting in crop damage. Cited references provide examples of studies evaluating the risk factor, without any claim for exhaustiveness. A considerable effort would be required to achieve an overview of all situations in terms of species × crop × location.



**Figure 2.** Variability in rear end for wireworm species from different genera. (**A**) *Melanotus punctolineatus*, (**B**) *Cidnopus aeruginosus*, (**C**) *Athous haemorrhoidalis*, (**D**) *Cidnopus pilosus*, (**E**) *Prosternon tesselatum*, (**F**) *Agrypnus murinus*, (**G**) *Adrastus* sp., (**H**) *Hemicrepidius niger*, (**I**) *Agriotes sputator*, and (**J**) *Selatosomus aeneus*.
