COS 115-1 - Leave the leaves? How autumn leaf litter management affects overwintering moth and parasitoid wasp communities

Abstract: Pollinator and natural enemy populations are declining due to urbanization. Residential landscaping practices such as leaf litter management could disrupt these insect’s life cycles during their vulnerable overwintering period. However, the effects and modulating factors of litter management have yet to be studied. These could depend on the type of management practice, the intensity of yard maintenance, and the diversity of the residential tree canopy. For example, diverse canopies could attract insects in summer only for litter disturbance to kill them in autumn.

For two years, we manipulated leaf litter and collected emerging Lepidoptera and parasitoid wasps, taxa important to home gardeners, in both residential yards and a tree biodiversity experiment ( BiodiversiTREE, a TreeDivNet site). In 14-20 homes, we examined the effect of litter removal in areas of different yard care intensities (maintained vs unmaintained zones). At BiodiversiTREE, we compared 12 single-species tree plots with 12 multi-species tree plots and added additional common management techniques (leaf shredding and addition).

Regardless of maintenance area, 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 within each maintenance area had an insignificant effect on parasitoid wasp emergence in both 2021 (χ2 (1, N= 66) = 0.34, p= 0.55) and 2022 (χ2 (1, N= 34) = 0.25, p= 0.61). In 2022, maintained areas had about 30% more emerging wasps than unmaintained areas (χ2 (1, N= 14) = 4.04, p= 0.04). At BiodiversiTREE, removing litter reduced Lepidoptera emergence by about 40% in both polycultures (z= 2.14, p= 0.03) and monocultures (z= 2.15, p= 0.03). Wasps were unaffected by canopy diversity (χ2 (1, N= 24) = 0.61 , p= 0.43) or litter management (χ2 (3, N= 96) = 0.19 , p= 0.97).

Across both studies, there is strong evidence that litter removal reduces Lepidoptera, but not parasitoid wasp, emergence regardless of canopy diversity or yard maintenance area. Interestingly, more maintained areas, like front yards, have higher total emergences. These results suggest that retaining litter in situ could combat urban insect population decline.