Mario Oswald

Direct Insect Mortality by Different Mowing Practices: A Biomass-Based Quantification in Agricultural Grasslands

Advisor: Harald W Krenn

Master's Defensio

Thursday, June 29th 2023, 14:00 CET
SR 1.5, UBB
Djerassiplatz 1, 1030 Vienna


Rapid intensification and change of agricultural used land are frequently identified as important causes for today’s global biodiversity decline, including insects. Agriculture practices need to be as biodiversity friendly as possible if we want to preserve insect biodiversity and associated ecosystem functions. This study is therefore conducted to classify various common mowing techniques regarding their direct impact on insects in agriculturally differently managed meadows. To classify the different mowing techniques in terms of their negative effects on insects, the biomass of dead insects is tested as a potentially standardizable parameter. Insect biomass and orthopteran diversity of freshly mown plots of an intensively and an extensively managed meadow are compared. A main aspect is the evaluation of error rates regarding overseen individuals during the sampling, to determine the representativeness of the tested classification methods. 
Results show that the choice of mowing technique used, has strong impact on the direct insect mortality and accumulated dead insect biomass in tested meadows. Proportion of dead insect biomass to total insect biomass per plot is significantly higher (Kruskal-Wallis H, p < 0.001, n = 120) if a rotary disc mower with conditioner (median = 21,25 %) is used in contrast to a double-knife cutter bar mower (median = 0,58 %) or a simple rotary disc mower (median = 1,96 %). Different time of day of mowing seems to have minor influence on the direct insect mortality as no significant differences were measured. Insect biomass does not differ significantly between the two studied sites. Diversity of Orthoptera in extensively versus intensively managed meadow plots was higher (in total 10 vs. 4 species) as was the number of individuals (104 vs. 38). The biomass of insects that were initially overlooked and subsequently discovered during laboratory re-examination of "emptied in the field" collection bags was compared to the total biomass of
insects collected per plot, sampling time, and collection method. The resulting percentage, representing the error rate of overseen insect biomass, was consistently very low, with < 0.001 % in 95 out of 173 counts and less than or equal to 10 % in 145 of 173 counts.
The tested evaluation methods show sufficient robustness to allow objective comparisons and classifications, confirmed by the low error rate observed. Finally, biomass might serve as a valid and easily implementable classification parameter to evaluate the direct negative impact of different mowing techniques on insects.