Summary
Wireworms are the larvae of click beetles. The economically most important pest species in Europe are currently thought to be found within the genus Agriotes (family Elateridae). Most crops will suffer damage when wireworms feed on roots or tunnel through the lower stem parts and belowground plant organs. Because wireworms live belowground, they are a challenging target to control; and this is compounded by the fact that the larvae of some species are morphologically indistinguishable but differ in their damage potential and disease resistance. The amount of crop damage as a result of wireworms has increased over the past few decades, likely due to changes of cultivation methods and land use and restrictions in pesticide use.

The present thesis is dedicated to the broad subject of wireworm control, with a focus on classifying the available control methods and testing single control tools. The objectives of this research were as follows: (a) to scan the literature for efficient or promising control strategies, and to analyse present research needs in the field of wireworm control, (b) to test the use of calcium cyanamide (CaCN₂) as a crop protection tool against wireworms and (c) to analyse wireworm behaviour to broaden the biological knowledge of species-specific features.

First, relevant work on the occurrence and management of wireworms (Agriotes spp.) in European agriculture and horticulture over the last decade was synthesised. This included relevant work on population monitoring, damage prediction, agronomic strategies and both the chemical and biological control of Agriotes wireworms. The review showed that the most widespread and destructive species in Europe is A. lineatus, followed by A. obscurus and A. sputator. Furthermore, it indicated that the development of species-specific control methods was advisable, which means that it is important to reliably determine wireworm species and to map their regional distribution. When selecting an insecticide, it is crucial to consider the need for both stand protection and wireworm reduction, as certain insecticides act as repellents or allow wireworm recovery. The most promising alternative wireworm control methods seemed to be the use of trap crops and crop rotation changes, as well as the combination of entomopathogenic fungi with either nematodes or insecticides; however, products for their commercial use still need to be developed. Damage prediction techniques and reliable control strategies are currently challenging and require further investigation, and economic thresholds for susceptible crops also need to be established, particularly for the most destructive Agriotes species. Finally, additional species-specific knowledge about the biology and ecology of these pests is still required.

To determine the effect of CaCN₂ on wireworms, two different experimental designs were used. First, the contact effects of this fertiliser on wireworms were observed in small transparent containers filled with CaCN₂-treated soil. The repellent effect of CaCN₂ was then evaluated using horizontally placed pipes that contained untreated soil on one side and CaCN₂-treated soil on the other, with five wireworms placed in the middle of each pipe. The results of these experiments indicated that CaCN₂ has a non-toxic but repellent effect on wireworms. Therefore, the combined findings from the review and the CaCN₂-experiments could be used as guidance for extension services and agriculturalists when deciding which control measures will be most suitable/acceptable for wireworm control.

To analyse the antipredator behaviour of four wireworm species, three behavioural phases were defined: tonic immobility, orientation period and digging period. Larvae were picked up with featherweight forceps and individually dropped onto standardised white peat in walk-in climate chambers at three different temperatures (7°C, 20°C and 27°C), and the time taken for each phase was then measured and compared. The following behaviour types were observed, reflecting different escape tactics: (a) distinct tonic immobility (A. lineatus), (b) brief/inconsistent tonic immobility (A. obscurus, A. sordidus), and (c) immediate fleeing/burying (A. ustulatus). In addition, significant differences were found between A. lineatus larvae originating from the Netherlands and those from Germany with respect to their morphology, behaviour and genetics (PCR results). This biological information for each species will increase our understanding of these insects and improve interpretation of experimental data. In addition, the described behavioural differences between Agriotes wireworms may represent a useful additional criterion that could be used to distinguish between live A. lineatus and A. obscurus larvae when using morphological characteristics for species identification.

Finally, the results of this thesis indicate that a holistic approach is required to solve the wireworm problem on a long-term basis. Similar to medical science, treating the symptoms will bring short-term relief but does not address the problem itself. With regard to wireworm control, standardized monitoring of the larvae that are present and environmental parameters should allow the potential risk to be assessed before planting/sowing crops, which will guide to selection of suitable preventive measures for a given cropping and farming system. However, therapeutics (i.e. single strategies in the fields of chemical, mechanical or biological control) should not replace natural systems, but rather serve as complements to them in temporarily unbalanced systems. The interactions between different components need to be further explored to find the most suitable matches that will restore natural balance (e.g. pest-predator), whilst also being mindful of increasing food demands and economic pressures.