Timing and mulching frequency affected the number of nests of cavity-nesting wasps that hunt for aphids in forest meadows

Mulching, cutting of the vegetation without plant biomass removal, is a common alternative to mowing. The aim of our study was to find out if the mulching of forest meadows at different time points affects cavity-nesting bees and wasps. We exposed trap nests for cavity-nesting bees and wasps at 24 forest meadows in south-western Germany over 2 years and applied four experimental mulching treatments with six replicates: (i) mulching in June, (ii) mulching in September, (iii) mulching in June and September, and (iv) no mulching as control. Nests were collected throughout the growing period. The insects were sorted and analyzed according to functional groups. Mulching in June and September reduced the nest number of all cavity-nesting insects in the second but not in the first year. The separation of insects into three functional groups (bees, herbivore-hunting wasps and carnivore-hunting wasps) showed that the number of herbivore-hunting wasp nests was reduced by mulching in September in both years and by mulching in June and September in the second year. Specifically, aphid-hunting wasps were influenced by mulching in September or mulching twice in the second year. Aphid-hunting wasps likely find their larval food in the vegetation of the forest meadows, while the other studied groups likely find their main larval food in the surrounding forests and are therefore not negatively affected. Implications for insect conservation For maintaining the reproductive success of cavity-nesting wasps that hunt for aphids, we recommend mulching once in June rather than mulching in September or twice a year.