Retaining leaf litter protects Lepidoptera in suburban yards

Leaf litter management is a common landscaping practice in suburban ecosystems, yet we have little understanding of its impacts to overwintering Lepidoptera and parasitoid wasps, taxa important to urban land managers. These impacts could depend on the area in which it is conducted and the diversity of the residential tree canopy. To assess this disturbance, we manipulated litter and collected emerging Lepidoptera and parasitoid wasps in both residential yards and a tree-biodiversity experiment (BiodiversiTREE). In suburbia, we studied litter removal in maintained vs unmaintained areas. At BiodiversiTREE, we compared single-species with multi-species tree plots. In suburbia, about 75% more Lepidoptera emerged when litter was retained, in both 2021 (χ2 (1, N= 66) = 5.52, p= 0.01) and 2022 (χ2 (1, N= 34) = 7.56, p= 0.005). More Lepidoptera emerged from maintained areas than unmaintained areas, two times more in 2021 (χ2 (1, N= 26) = 6.04, p= 0.01) and 1.4 times more in 2022 (χ2 (1, N= 14) = 19.07, p= < 0.001). Litter management had an insignificant effect on parasitoid wasp emergence. At BiodiversiTREE, removing litter reduced Lepidoptera emergence by about 40% in both polycultures and monocultures (z= 2.15, p= 0.03). Wasps were unaffected by canopy diversity or litter management. There is strong evidence that litter removal reduces Lepidoptera, but not parasitoid, emergence regardless of canopy diversity or yard maintenance area. These results suggest that retaining litter in situ could combat urban insect population decline.