Minimizing insect mortality during grassland mowing: The potential of insect chasing devices

<b>Abstract</b>1. Early and frequent mowing is a threat to grassland arthropods. To date, measures to counteract these losses have largely been limited to extensification concepts, which are not always feasible on intensive grasslands.2. One option for insect-friendly mowing is the use of chasing devices, which are designed to remove arthropods from the grassland before mowing. Scientific research on their effectiveness is limited to one study on parasitic microhymenoptera. Studies on other groups are lacking.3. In a three-year experiment we have investigated the effectiveness of three devices, i.e. two mechanical flushing bars (truck tarpaulin, hanging down metal brackets) and a blowing device. We tested different driving speeds and distances of the flushing bar from the mower on seven abundant arthropod groups in grassland that differ in their escape strategies.4. We found the blowing device to be the most effective. At the maximum driving speed of 12 km/h it had an effect on almost all taxonomic groups studied except Araneae and Hymenoptera. At this speed, the metal bracket flushing bar was only effective on Diptera and Coleoptera. The truck tarpaulin flushing bar was only effective at a low driving speed of 5 km/h, albeit on four out of seven taxa.5. Chasing devices have a great potential to remove arthropods prior to mowing, thereby reducing mortality. Whilst a costly blowing device works at higher speed, inexpensive mechanical flushing bars have a limited effectiveness (metal brackets) or only work at low speed (truck tarpaulin).<b>Materials and Methods</b>The experiments took place in the years 2022 to 2024 on a study site of the Kleinhohenheim Agricultural Experiment Station of the University of Hohenheim, Stuttgart, Germany (48.734 °N, 9.204 °E). In each year, the study site was mown twice, once in June/July and once in September (Figure 1). The experimental design consists of five blocks, each separated into five plots. This design allows to repetitively sample five different treatments per block (von Berg <i>et al.</i>, 2024). More detailed information on the study sites and design can be found in von Berg <i>et al.</i> (2024)Here, we compared the direct effects of two flushing bars and a blowing device together with a conventional disc mower (Disco 320, without conditioner, working width 3 m, Power Take-Off (PTO) speed 850-1000 rpm, Claas Saulgau GmbH, Bad Saulgau, Germany) on the arthropod fauna. We tested the following treatments: (a) disc mower with flushing bar made of truck tarpaulin, (b) disc mower with hanging down metal brackets (c) disc mower with a blowing device and (d) a disc mower without flushing bar as control. The first two devices were self-made prototypes (instructions and exact measurements are publicly available: dryad link available after acceptance) while the blowing device was supplied by a manufacturer of mowing machines (leaf blower type F4052, capacity 240 m³/min, PTO 850 rpm, Fischer Maschinenbau GmbH &amp; Co. KG, Gemmrigheim, Germany) (Figure 2).The sampling took place twice a year from end of June to beginning of July 2022–2024 and end of September 2022–2024, depending on the weather (Table 1). The experimental plots were treated and sampled in similar time intervals. For detailed sampling description see von Berg <i>et al.</i> (2024)The devices were attached in either 1 m (truck tarpaulin in 2022, 2023), 5.5 m (truck tarpaulin and metal brackets) or 6.8 m (blowing device; in 2024) distance to the mower. Based on the first year’s results attempts have been made in subsequent years to continually optimise the design of the flushing bars, the driving speed and the distance between the flushing bar and the mower: Flushing bars were installed either directly to the mower (truck tarpaulin in 2022, 2023) or at the front of the tractor (2024) at a distance of 5.5 m and 6.8 m from the mower, for the mechanical flushing bars and the blowing device, respectively. They were installed so that the lower edge of the mechanical flushing bars and the blowing device were 15 cm and 30 cm above the ground, respectively. The driving speed of the tractor was 5 km/h (in 2022) and 12 km/h (in 2023 and 2024) with a cutting height of 7 cm (Table 1, Figure 2).Three randomly placed 1m² isolation squares (Mühlenberg, 1989) were set up per plot and completely vacuumed for a total of 5 min with an insect vacuum (ecoVAC Insect vacuum, ecoTech Umwelt-Messsysteme GmbH, Bonn, Germany). Further, we removed the mown grass from the isolation squares by hand and weighted the grass with a crane scale. In 2022, the whole content of each insect vacuum sample bag was transferred into vials filled with 80% denatured ethanol. From 2023 onwards samples were transferred to zip-lock plastic bags and stored at -18 °C until further processing (for detailed information see von Berg <i>et al.</i>, (2024))In the first year, we sampled 60 isolation squares (disc mower: 30 samples, dm mower with truck tarpaulin: 30 samples; driving speed 5 km/h; Table 1). In the second year, we sampled 60 isolation squares (disc mower: 30 samples, disc mower with truck tarpaulin: 30 samples; driving speed 12 km/h; Table 1), and in the third year, 120 isolation squares (disc mower: 30 samples; disc mower with truck tarpaulin: 30 samples; disc mower with metal brackets: 30 samples; disc mower with blowing device: 30 samples; driving speed 12 km/h; Table 1). All 240 samples were sorted for the following taxonomic groups: Araneae, Orthoptera, Cicadina, Heteroptera, Hymenoptera (parasitoid wasps and Anthophila), Coleoptera, and Diptera. All non-target taxa (e.g. Collembola, Aphididae, Formicidae) from the analysis. Only individuals that were not damaged or injured by the mower were considered for the subsequent data analysis (von Berg <i>et al.</i>, 2024). All sorted individuals were stored as voucher specimens in 80% denatured ethanol at the Universities of Hohenheim and Tübingen. All data have been deposited in and are available from Dryad repository (link available after acceptance).All data analyses were performed in RStudio (Posit team, 2023) using R 4.3.2 (R Core Team, 2023)following Santon <i>et al.</i> (2023) to conduct generalized linear mixed models (glmmTMB; (Brooks <i>et al.</i>, 2017)) using Template Model Builder (TMB; (Kristensen <i>et al.</i>, 2016)). Due to the different experimental setups, each year was analysed separately and samples with missing values for one or more of the predictors were excluded from the analyses. This resulted in a sample size of n = 30 for all treatments in all experiments, except for n = 29 in case of the truck tarpaulin flushing bar in 2022 and 2023. In addition, one outlier for Diptera (92 individuals per m²) was excluded from the analyses for the 2022 truck tarpaulin flushing bar (n = 28). Date was used as factor predictor, daytime and grass weight were included as numeric predictors and block and plot were used as random factors. All models were tested for interaction effects between date and treatment and were included where necessary, i.e. for Heteroptera, Coleoptera and Diptera in the 2024 experiment. Tukey post hoc tests using ‘multcomp’ (Hothorn <i>et al.</i>, 2008) for models without interactions or ‘emmeans’ (Lenth, 2023) for models with interactions were used to compare data from the various scaring devices in 2024. Pairwise comparisons (mean and 95% compatibility intervals), considering the model predictions, were used to determine the percentage differences between the individual treatments (Santon <i>et al.</i>, 2023).Referencesvon Berg, L., Frank, J., Betz, O., Steidle, J.L.M., Böttinger, S., Sann, M. (2024). Disc mower versus bar mower: evaluation of the direct effects of two common mowing techniques on the grassland arthropod fauna. <i>Journal of Applied Ecology</i>, <b>n/a</b>.Brooks, M.E., Kristensen, K., Benthem, K.J. van, Magnusson, A., Berg, C.W., Nielsen, A., Skaug, H.J., Mächler, M., Bolker, B.M. (2017). glmmTMB Balances Speed and Flexibility Among Packages for Zero-inflated Generalized Linear Mixed Modeling. <i>The R Journal</i>, <b>9</b>: 378–400.Hothorn, T., Bretz, F., Westfall, P. (2008). Simultaneous Inference in General Parametric Models. <i>Biometrical Journal</i>, <b>50</b>: 346–363.Kristensen, K., Nielsen, A., Berg, C.W., Skaug, H., Bell, B.M. (2016). TMB: Automatic Differentiation and Laplace Approximation. <i>Journal of Statistical Software</i>, <b>70</b>: 1–21.Lenth, R.V. (2023). emmeans: Estimated Marginal Means, aka Least-Square Means.Mühlenberg, M. (1989). <i>Freilandökologie</i>. 2nd ed. Quelle &amp; Meyer Verlag, Heidelberg.Posit team. (2023). RStudio: Integrated Development Environment for R.R Core Team. (2023). R: A Language and Environment for Statistical Computing.Santon, M., Korner-Nievergelt, F., Michiels, N.K., Anthes, N. (2023). A versatile workflow for linear modelling in R. <i>Frontiers in Ecology and Evolution</i>, <b>11</b>.<b>Description of the dataset</b>Numbers of individuals for the seven assessed arthropod groups (Araneae, Orthoptera, Cicadina, Heteroptera, Hymenoptera, Coleoptera and Diptera) recorded for the different treatments: disc mower (dm), disc mower with truck tarpaulin (dm), disc mower with metal brackets (dm_brackets) and disc mower with blowing device (dm_air) in three consecutive years (2022, 2023, 2024). Sampling was carried out using 1-m² isolation squares and an insect vacuum on the following sampling dates: 2022-07-04, 22.09.2022, 2023-06-15, 2023-09-19, 2024-07-08, 2024-09-25. The study site was divided into five blocks (B1-B5). Each block contains up to 5 sampled subsections (plot P1-P5). Per subsection and treatment three samples were taken at the same time. The time of sampling (daytime), the driving speed of the tractor (speed in km/h), the cutting height (in cm), the weight of the grass in each isolation square (grass weight in kg) as well as the distance between the flushing bar and the mower (in m) were recorded. Distance between the flushing bar and the mower is not applicable (N.A.) for the disc mower only (dm) treatment.