Katharina Wechselberger

Planthoppers transmit pathogens to various economically important crops. In Central Europe, the sugar beet, Beta vulgaris L. ssp. vulgaris var. altissima (Döll) (Amaranthaceae: Caryophyllales) and potato production are threatened by diseases associated with the γ-proteobacterium “Candidatus Arsenophonus phytopathogenicus” and the phytoplasma “Candidatus Phytoplasma solani”. Infections cause yellowing and wilting, reduce sugar yield in sugar beet, induce rubbery taproots in potato, and can lead to plant decline. In Austria, the cixiid Reptalus quinquecostatus (Dufour) is considered a main vector of these pathogens. While several plant species have been reported as feeding or shelter plants for adult R. quinquecostatus, no reproductive host plant supporting nymphal development has previously been confirmed. In this study, we investigated whether sugar beet is a potential host plant for the development of R. quinquecostatus. Host plant trials were conducted using potted sugar beet plants placed outdoors in insect cages. Four cages were infested with a total of 15 adult Reptalus spp. (10 females and 5 males) per cage to allow oviposition. Egg masses and nymphs were observed in 75% of the cages during the trial period. After 366 ± 3.46 (SD) d, 4 adult R. quinquecostatus emerged from 2 out of 4 cages. Our findings provide the first experimental evidence that sugar beet can function as a reproductive host plant for R. quinquecostatus.

Wireworms within the genus Agriotes (Coleoptera: Elateridae) can cause substantial damage to agricultural crops. The vertical movements of these pest insects in the soil make the timing of control measures a difficult task. The forecast model SIMAGRIO‐W utilizes soil temperature and moisture data to predict the migration of Agriotes wireworms to the upper soil layer. The model distinguishes between two risk levels: low risk (less than 30% of the wireworm population in the upper soil layer) and high risk (more than 30%), which are considered adequate for practical purposes. In German field sites, the model demonstrated an 80% success rate across four soil types. The SIMAGRIO‐W model was tested under the warm and dry climate in eastern Austria. The validation process revealed an overall accuracy of just 46%, primarily due to the fact that SIMAGRIO‐W assumes a maximum activity level for wireworms at 11°C. However, high activity levels were observed at soil temperatures of up to 26°C at the experimental sites. The discrepancy in the prediction power of the model between the German and Austrian field sites may be explained by differences in temperature tolerance between the Agriotes species occurring in eastern Austria (e.g. A. ustulatus) and in western Germany (e.g. A. obscurus ). Our findings demonstrate that the thermal preferences of different wireworm species must be taken into account to make the SIMAGRIO‐W model a widely applicable decision support tool for predicting vertical movements of Agriotes wireworms.

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.

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.

Inundative mass application of Metarhizium brunneum BIPESCO 5 (Hypocreales, Clavicipitaceae) is used for the biological control of Diabrotica v. virgifera (Coleoptera, Chrysomelidae). Long-term field trials were performed in three Austrian maize fields—with different cultivation techniques and infestation rates—in order to evaluate the efficacy of the treatment to control the pest larvae. In addition, the indigenous Metarhizium spp. population structure was assessed to compare the different field sites with BIPESCO 5 mass application. Annual application of the product Granmet-P^TM (Metarhizium colonized barley kernels) significantly increased the density of Metarhizium spp. in the treated soil above the upper natural background level of 1000 colony forming units per gram dry weight soil. Although a decrease in the pest population over time was not achieved in heavily infested areas, less damage occurred in treated field sites in comparison to control sites. The Metarhizium population structure was significantly different between the treated field sites. Results showed that inundative mass application should be repeated regularly to achieve good persistence of the biological control agent, and indicated that despite intensive applications, indigenous populations of Metarhizium spp. can coexist in these habitats. To date, crop rotation remains the method of choice for pest reduction in Europe, however continuous and preventive application of M. brunneum may also present an alternative for the successful biological control of Diabrotica.