Caterpillar loss through grassland harvest differs between two related butterfly species of conservation concern

1. Temperate, semi-natural grasslands emerged from traditional land use and are recognised for their outstanding biodiversity. However, these grasslands severely declined due to agricultural industrialisation. Accordingly, specialised species of nutrient-poor grasslands like the closely related butterflies Nickerls Fritillary Melitaea aurelia Nickerl, 1850 and Marsh Fritillary Euphydyras aurinia (Rottemburg, 1775) (Lepidoptera: Nymphalidae) have experienced strong declines in the last century due to grassland loss and deterioration. Here, we studied for the first time the effects of the complete harvest process in grasslands from mowing until the removal of the cut vegetation on the survival of their caterpillars. 2. To assess survival rates, we sampled larvae of both species and environmental conditions before and after the harvest process and analysed the results using generalised linear mixed-effects models. 3. Our study revealed that the harvest process in grasslands mown once by bar mowers resulted in substantial caterpillar loss influenced by (i) growth form of the host plants and (ii) timing of grassland harvest. Accordingly, the host plant of E. aurinia, Succisa prantensis, lost nearly half of its leaves during the harvest process, while late harvesting was especially harmful for caterpillars already commencing hibernation. Overall, E. aurinia had higher losses compared with M. aurelia, and these were most severe in late-mown moist meadows. 4. To increase the survival rate of E. aurinia, we recommend mowing before the beginning of hibernation using bar mowers. Complementary, or if mowing is done later, at least 20% of the meadow should be left as an untouched refuge in a spatially rotating manner.