The project was divided into two subprojects. In the first subproject, survey measurements were conducted at seven cockchafer infested sites in Central and Southern Germany during the vegetation period in 2017 (April – October) in close cooperation with relevant stakeholders in agriculture and forestry. Each site was visited once and measurements conducted at two to four randomly chosen plots (50 cm x 50 cm) per site. Measurements comprised soil audio recordings, excavation and characterization of larvae, and incubation of larvae to quantify CH4 emissions. In total, CH4 emissions of 91 cockchafer larvae were measured directly in the field. Over the entire dataset, average CH4 emissions were 1.3 µg CH4 h-1 larva-1, but there was a large variability within and between the different sampling sites. The highest CH4 emissions with up to 7.1 µg CH4 h-1 larva-1 were observed in a meadow in May 2017 where M. melolontha larvae had reached their final larval stage (3rd instar) with weights ranging between 2.0 and 2.7 g. For M. hippocastani, CH4 emissions were mostly below 0.5 µg CH4 h-1 larva-1. However, Melolontha spp. are pest insects and can reach abundance levels of more than 50 individuals m-2, thus upscaled fluxes can easily reach values of more than 10 µg CH4 h-1 m-2. Overall, emission strength and variability tended to increase with larval body weight and decrease with increasing soil excavation depth.
In the second subproject (October 2016 – October 2017), the effect of different M. melolontha larvae abundances on net soil CH4 fluxes were quantified under a wide range of environmental conditions in a controlled field experiment consisting of 27 soil mesocosms. These mesocosms were planted with either turf, carrots or a combination of both, and infested with cockchafers. Larval abundances ranged between 0 and 16 larvae m-2. On average every two weeks throughout the vegetation period, net soil CH4 fluxes were quantified with closed static chambers. These chambers were combined with a 13CH4 isotope pool dilution technique to non-invasively estimate gross CH4 production and CH4 oxidation in the soil. Since the soil in this experiment was well-aerated, any gross CH4 production was very likely attributable to the cockchafer larvae. Acoustic soil monitoring was conducted alongside the chamber measurements in cooperation with the University of York (England). Stable carbon isotope analysis of the gas samples was performed at the University College Dublin (Irleland) with a state-of-the-art high precision cavity ring-down spectrometer (Picarro G2201-i) equipped with a Small Sample Isotope Module (SSIM) for processing small discrete gas sample volumes. Four general trends were observed in this experiment: a) the soils acted as net sinks for atmospheric CH4, b) net CH4 uptake rates increased throughout the vegetation period, c) net CH4 uptake rates were higher in mesocosms covered completely with permanent vegetation, and d) net CH4 uptake rates tended to be higher in the mesocosms with the highest infestation level within each vegetation type, pointing towards a stimulation of gross CH4 oxidation rates in the presence of CH4 emission hotspots in the soil. With respect to the method development, two new data analysis routines for non-invasive soil monitoring were developed. One for the simultaneous analysis of 13C-CH4 and CH4 concentrations in small, discrete gas samples by cavity ring-down spectroscopy, and one for the estimation of cockchafer infestation levels based on active larval communication (=stridulation).
