2.2.3. IPM Thresholds

IPM implementation needs a standardized monitoring method combined with reliable damage thresholds. The aforementioned bait-trap monitoring method has given reliable results over sites and years and might be considered as a standard both for ordinary wireworm IPM implementation and for the assessment of damage thresholds for other wireworm species/crop combinations.

Although increasing literature about wireworms has been published over the last few years (Figure 1), to our knowledge, only four papers report practical IPM damage thresholds, with them being restricted to five species and two crops: *Melanotus communis* thresholds in sugarcane crops [82], *Agriotes brevis*, *A. sordidus*, *A. ustulatus* [35,57], and *A. lineatus* [80] in maize crops. Published thresholds are summarized in Table 2. Other papers supply information about crop susceptibility to wireworms that allows an indirect estimation of damage thresholds. Furlan et al. [20] carried out pot trials that introduced the same number of wireworms per pot for different crops. Results showed a large variation in crop susceptibility. A number (6/pot) of wireworms (*A. ustulatus*, *A. sordidus*) causing a 50% maize and sunflower plant loss, had a negligible effect on soybean but killed most of the sugar-beet seedlings. Likewise, Griffith [83] demonstrated differences in plant susceptibility to wireworm attacks in laboratory tests. Larvae of *Agriotes* spp. were presented with a choice between the seedlings of test plants and of wheat, which is known to be susceptible. Some plants, e.g., onion, were as susceptible as wheat to wireworm attacks, whilst others (mustard, cabbage, French marigold, clover, and flax) were attacked less often. All pea and bean plants exposed to wireworms were attacked, but most tolerated the attacks and continued to grow. Old generic thresholds based on larval density assessed by soil sampling have low scientific reliability and little practical potential [32], one reason being that none of the wireworm species studied were specified.


**Table 2.** Published damage thresholds according to click-beetle species, crop, and monitoring method.

\* Derived data published in the cited paper; plant damage was lower than 15% with 1–2 wireworms per trap. Wireworm plant damage lower than 15% in maize should not result in yield reduction [23].

#### **3. Pest Population Management**

The current resurgence of wireworm damage to various crops has resulted in a strong demand for new agroecological methods to control those pests, notably consequential to the reduced availability of pesticides, possibly in response to global changes and pressing demands by the general public for the implementation of more environmentally friendly agricultural practices. Accordingly, continuous advances in the knowledge of click-beetle biology and ecology have led to several new managemen<sup>t</sup> practices currently being tested or developed. New proposals mostly originate from (1) the field of agricultural sciences, with them promoting relevant cultural or mechanical methods (use of resistant/tolerant crops, design of bespoke tilling strategies or rotation); (2) the field of chemical ecology (use of pheromones for sexual confusion); (3) the field of trophic ecology (biological control); and (4) from the field of landscape ecology (large-scale habitat managemen<sup>t</sup> to reduce pest pressure at landscape scale).

#### *3.1. Cultural or Mechanical Control*

#### 3.1.1. Effect of Rotation

The first prevention strategy when controlling wireworm populations is to plan a diversified ecosystem that includes a rich rotation with crops and cover crops placed in the most suitable positions. Crops susceptible to wireworm damage should be placed after crops that do not favor or that reduce wireworm populations (e.g., incorporating barley and oats into crop rotations can reduce wireworm attacks [84]). Crop diversification can benefit wireworm control. For instance, mustard, cabbage, French marigold, clover, and flax are less susceptible to attack, while pea and bean plants tolerate attacks [83]. Hence, large intensively tilled (e.g., hoed) inter-row crops and/or biocidal cover crops directly reduce wireworm populations [85,86]. Generally speaking, cover-crop choice can contribute to wireworm cultural control both through its effect on soil biodiversity and ecosystem stability and through its biofumigant/biocidal effect. Crop choice can contribute to wireworm mechanical control by increasing larval mortality, either due to tillage interventions when preparing sowing beds or to hoeing in large inter-row crops.

#### 3.1.2. Effect of Tilling

As the life cycles of wireworm species last several years and take place largely in soil, tillage may impact several of their life-history traits. During the oviposition period in spring, females lay their eggs in the top soil-layer [10,20] in a steady environment, such as litter or grass, whenever possible, because of their own sensitivity to temperature fluctuations [6] and their eggs' sensitivity to desiccation. After hatching, larvae are exposed to soil tillage, in particular to ploughing, making them vulnerable to predation [55] or desiccation [87]. In 1949, Salt and Hollick [55] conducted a five-year experiment, which highlighted that the decline in wireworms was accompanied by an outstanding change in the distribution of larvae sizes, reflected by a decrease in the number of young larvae. It is currently acknowledged that, due to a lack of soil cover, oviposition might be reduced on row-crop compared with grassland [19,32]. Seal et al. [54] found that ploughing three times during the summer reduced wireworms collected at bait traps from 1.75 per bait trap to 0.2 per bait trap, compared to no change in unploughed control plots. This reduction was attributed to exposure to bird predation and desiccation. Larval mortality depends on tillage timing, which should match the egg-laying and first instar larvae periods, which are the most susceptible to unfavorable soil conditions. The best tillage timing for interfering with wireworm population dynamics varies with the species life-cycle. For example, in Italy, overwintering *A. sordidus* adults emerge from their cells in the soil from late March– early April and start to lay eggs from May onwards [10]; thus, susceptible instars (eggs and young larvae) occur in the soil from May to June, usually peaking in May. Therefore, tillage from mid-May to late June, as preparation of seed beds for the subsequent crop and hoeing in row crops can dramatically reduce subsequent wireworm populations.

#### 3.1.3. Effect of Water Management

The effect of drying and flooding has been studied mainly on the American West Coast [16,88–90] and in British Columbia [91]. Irrigation timing may play a role in interfering with wireworm population dynamics. The drying of the top-most soil layer just after eggs are laid can be an effective means of controlling wireworms. Soil drying could be achieved by withholding irrigation from alfalfa before harvest, but it is nevertheless more effective in lighter sandy soils [88]. The main challenge of water managemen<sup>t</sup> as a control lever is the different response of species according to soil moisture. While *Ctenicera pruinina* (Horn) has long been a pest of dryland wheat [92] and disappears as a pest when fields are converted to continual irrigation [93], *Limonius californicus* do not survive well in dry soil and prefer soil with 8–16 percent moisture [16,89]. Another challenge is the ability of some species to adapt to soil moisture [94]. Despite damage often being reported in soils that flood in the winter, field flooding can effectively reduce *Agriotes* wireworm populations when combined with high temperatures [91]. Lane and Jones [95] highlighted the relationship between soil moisture and temperature on the mortality of *Limonius californicus* larvae. At 30 ◦C, all larvae submerged under soil and water were killed in four days, whereas only 26 percent of larvae died after 21 days when temperatures dropped below 10 ◦C. It was also demonstrated that alternating periods of soil flooding and drying is effective for reducing wireworms [96].

## *3.2. Semiochemical Control*

Since the 1970s, regular progress has been made in elucidating the composition of clickbeetle pheromones. Synthetic mixtures are now available for several species of agricultural importance, opening new perspectives for using them in wireworm monitoring or even developing new control strategies that rely on adult sexual confusion or mass-trapping.

Besides their potential use for establishing wireworm populations (see Section 2.2), pheromone traps might be used to reduce populations, either through mating disruption or through mass-trapping. Mass-trapping was successfully implemented in Japan to control *Melanotus okinawensis* on sugarcane, with adult densities being reduced by approximately 90% after six years of mass-trapping with 10 pheromone traps per hectare [97]. By contrast, a similar study observed no reduction in *Melanotus sakishimensis* abundance [98]. For *Agriotes* species, the limited attraction range of pheromone traps exacerbates the challenge of mass-trapping and requires a dense network of traps to be set up if populations are to be reduced. Hicks and Blackshaw [70] estimated that suppressing *Agriotes* populations using mass trapping would be prohibitively expensive (2755 €/ha/year), requiring four years of trapping with 10 traps/ha for *A.obscurus*, 15 traps/ha for *A.obscurus*, and m for *A. sputator*. In a long-term experiment on potatoes, Sufyan et al. [99] captured 12,000 specimens belonging to three *Agriotes* species over a period of five consecutive years without any effect on the subsequent larval densities or on potato damage. In 2014, Vernon et al. [100] indicated that arrays of traps spaced 3 m apart potentially disrupted mating but also showed that only 85.6% of the released *A. obscurus* were recaptured. As pointed out by Ritter and Richter [101], mating disruption may be easier for short-lived adult populations that are protandrous and exhibit a short, well-defined swarming period. Work is still in progress on the use of pheromone traps [102] to estimate wireworm population levels for IPM programs [57,68].

#### *3.3. Biological Control of Wireworms*

Inundative releases of natural enemies to control pests have been implemented for many years and may be a way to control wireworms in the future. In Europe, this is successfully performed by mass releases of *Trichogramma* wasps against the European Corn Borer (*Ostrinia nubilalis*) in Germany, a *Metarrhizium* product for the control of June chafer larvae (*Phyllopertha horticola*) in Switzerland, or a *Metarrhizium* granule for control of black vine weevil larvae (*Otiorhynchus sulcatus*) as well as a variety of uses of entomopathogenic nematodes against different horticultural pests. Van Lenteren et al. [103]

describe a wide variety of further uses worldwide. Kleespiess et al. [104] showed there are also some potential candidates for wireworm control. Currently, the main focus is on entomopathogenic fungi, with some research also being done on nematodes and combinations of different organisms.
