New pest management solutions are needed to control soil invertebrates (insects, nematodes, mollusks) in order to implement the goals of the European Green Deal. Natural volatile organic compounds (VOCs) such as 1-octen-3-ol and 3-octanone, emitted by the entomopathogenic fungus Metarhizium brunneum could be part of the solution. Three major crop pests, Agriotes lineatus (wireworm), Diabrotica virgifera virgifera (corn rootworm) and Phyllopertha horticola (garden chafer), were susceptible to fumigation with 1-octen-3-ol and 3-octanone. The toxicity of the VOCs was tested in soil-free arenas and in soils which differed in moisture content and porosity. The mortality rates were dependent on the interaction between the VOC treatment, dose and pest species. The insects differed in their sensitivity to these VOCs. A dose of 1.25 µl of 1-octen-3-ol applied in a closed glass tube was sufficient to kill D. v. virgifera and P. horticola in soil trials whereas 5 µl was needed to kill A. lineatus. The highest dose (20 µl) was highly toxic to all insects. Soil moisture content slightly influenced mortality rates whereas porosity had no obvious impact. The mode of action of the VOCs is unknown but the compounds are likely to cause tissue damage and loss of body fluids. This may explain the shrivelled appearance of corn rootworm and garden chafer and melanisation in wireworm. Both 1-octen-3-ol and 3-octanone show promise as biofumigants.

Wireworm, the larval stages of click beetles, are a serious pest of tubers, brassicas and other important commercial crops throughout the northern hemisphere. No effective control agent has been developed specifically for them, and many of the pesticides marketed as having secondary application against them have been withdrawn from EU and Asian markets. Metarhizium brunneum, an effective entomopathogenic fungus, and its derived volatile metabolites are known to be effective plant biostimulants and plant protectants, although field efficacy has yet to be validated. Field validation of a combined M. brunneum and derived VOC treatments was conducted in Wales, UK, to assess the effects of each as a wireworm control agent and biostimulant. Plots were treated with Tri-Soil (Trichoderma atroviridae), M. brunneum, 1-octen-3-ol or 3-octanone, or combinations thereof. Treatments were applied subsurface during potato seeding (n = 52), and potatoes were harvested at the end of the growing season. Each potato was weighed individually and scored for levels of wireworm damage. Applications of both the VOCs and the M. brunneum individually were found to significantly decrease wireworm burden (p < 0.001). Combinations of M. brunneum and 3-octanone were also found to significantly decrease wireworm damage (p < 0.001), while no effect on yield was reported, resulting in an increased saleable mass over controls (p < 0.001). Herein, we present a novel ‘stimulate and deter’ wireworm control strategy that can be used to significantly enhance saleable potato yields and control wireworm populations, even under high pest pressure densities.

The volatile organic compounds (VOCs) 1-octen-3-ol and 3-octanone produced by the entomopathogenic fungus Metarhizium brunneum are known to have pesticidal properties at high doses against a range of invertebrate pests. Very little is known about their behavior-modifying (semiochemical) properties. This study focused on investigating the behavioral responses of three subterranean crop pests, wireworm (Agriotes lineatus), western corn rootworm (Diabrotica virgifera virgifera), and garden chafer (Phyllopertha horticola), to relatively low doses of 1-octen-3-ol and 3-octanone. The behavior of wireworms and corn rootworms were slightly influenced by the VOCs, yet not significantly. Western corn rootworms appeared to be slightly attracted by 100 µl and 200 µl 1-octen-3-ol and 100 µl dose of 3-octanone, respectively but slightly repelled by the higher dose of 3-octanone. Wireworms appeared to be slightly repelled by 1-octen-3-ol and high dose 3-octanone, but slightly attracted by the 100 µl dose of 3-octanone. The VOCs had no significant impact on garden chafer. In silico studies showed that corn rootworm odorant binding proteins (OBPs) had a strong binding affinity of 1-octen-3-ol and high dose 3-octanone, indicating that these VOCs can be detected and recognized by corn rootworm. OBPs are well conserved between species; thus, wireworm and garden chafer OBPs should also be able to bind with the VOCs. Further trials will be done to confirm that VOCs could be used as semiochemicals. Appropriate formulation of the VOCs should increase their efficacy and prevent rapid dissipation of the VOCs.

Wireworms, the larvae of the click beetle are a major polyphagous soil pest, damaging crops such as corn, wheat, or potatoes. The withdrawal of registration of many chemical insecticides left farmers with a limited number of solutions against wireworms and soil pests. Entomopathogenic fungi (EPF) can be useful against wireworm; yet they are slow acting and species dependent. To override these limitations, stress and kill strategies were tested, combining spinosad and different Metarhizium strain against three different wireworm species from the genera Agriotes. Combining spinosad and M. brunneum could increase the mortality of the EPF strains; yet, the susceptibility of the Agriotes species differs for each combination (Chapter I). The aim of the PhD was to develop new integrated pest management strategies against wireworms to avoid important corn yield losses. Developing pest management strategies using EPF seemed unreliable; thus, fungal volatile organic compounds (VOCs) were tested for their fumigant and attractant/ repellent activity (Chapter II and III). In Chapter II, fumigation using 1-octen-3-ol and 3-octanone was tested against wireworm (Agriotes lineatus), corn rootworm (Diabrotica virgifera virgifera) and garden chafer (Phyllopertha horticola). The VOCs were highly efficacious in a closed environment (Chapter II); yet, in open containers, no mortality was observed (Chapter III). In this experiment, the VOCs had attractant/ repellent properties depending on the species. In silico study, confirmed that corn rootworm could detect and recognize the VOCs with their odorant binding proteins. As odorant binding proteins are well conserved between species, it is expected that wireworm and garden chafer can also detect and recognize the VOCs. In chapter IV, the VOCs were formulated as slow-release granules. These granules facilitate application of the VOCs in the field and increase the remanence of the VOCs in the soil. In fact, to avoid economic damage, wireworms need to be repelled from maize seedlings during the three first weeks of growth. Four doses of 1-octen-3-ol cyclodextrin granules were tested to reduce wireworm damage on maize. The lowest dose (2.26kg/ha) was attractive and increased damage; but doses of 4.52kg/ha or higher, significantly reduced damage for 18 days. The granule formulation needs to be optimized to increase the duration of the protection, but early indications suggest that VOCs granules appear to be a possible candidate to achieve wireworm control.