Relative impacts of gypsy moth outbreaks and insecticide treatments on forest resources and ecosystems: An experimental approach

Abstract

1. Gypsy moth outbreaks cause severe defoliation in Holarctic forests, both in North America where it is invasive, and in its native range in Eurasia. Defoliation can hamper timber production and impact ecological communities and processes. Aerial insecticide applications are regularly performed in outbreak areas to mitigate economic losses. These operations can be financially costly and harmful to non-target species and may disrupt species interaction networks. However, replicated studies of the relative impacts of gypsy moth outbreaks and insecticide application on forest growth and animal communities are rare and have yet to be carried out in the species' indigenous range. 2. Here, we review the pathways in which gypsy moth outbreaks and the chemical control of these outbreaks affect forest ecosystems. We then present an experimental design established in South Central Germany in early 2019, aiming to study the ecological and economic consequences of gypsy moth eruptions and insecticide application in oak forests. The study's full factorial design comprises forest stands at high and low defoliation risk, either treated with tebufenozide or left unsprayed, within 12 experimental blocks. Measurements of forest growth and structure, tree mortality, gypsy moth density, and composition of lepidopteran, bird, bat, ground beetle, and canopy arthropod communities will be conducted for several years. 3. One-year intensive monitoring of gypsy moth populations and damage across the selected sites showed substantial differences in population density between plots at high and low defoliation risk and high efficacy of tebufenozide in suppressing gypsy moth populations in treated plots. In the first year of the experiment, gypsy moth density and defoliation in predicted outbreak plots differed strongly, confirming the This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Tracking the temporal dynamics of insect defoliation by high‐resolution radar satellite data

“…The plots were selected based on a gypsy moth survey of egg mass conducted in 2018 by regional forest offices on the 2,802 transects (10 trees per transect). The ‘defoliation risk index’ (DRI) was calculated with the mean egg mass density on transects divided by the critical egg‐mass density (high risk if DRI > 1 and low risk if DRI < 0.5; for details, see file S1 in Leroy et al., 2021). Candidate plots required conditions of non‐recent spray history, structural homogeneity and at least 5 ha.…”

“…In this study, we made use of a well‐replicated experimental manipulation of caterpillar densities in mixed oak forests (see more details of the experimental design in Leroy et al., 2021) to test the applicability of publicly available Sentinel‐1 radar (henceforth, simply referred to as ‘radar’) data to quantify the temporally and spatially highly variable herbivory and tree response by refoliation (Figure 1). To validate our approach, we used terrestrial laser scanning (TLS) data and caterpillar counts by canopy fogging and compared radar‐derived indices with Sentinel‐2 optical data.…”

Assessment of defoliation and subsequent growth losses caused by Lymantria dispar using terrestrial laser scanning (TLS)

“…The experiment took place in mixed oak forests in different stands in Franconia-Bavaria in Germany (Leroy et al 2021). The experimental design area consisted of 11 blocks (A, B, D, F, G, H, J, M, N, O, and S), varying spatially and dominated by oak trees.…”

“…For this classification, a defoliation risk index was used, which calculation was based primarily on gypsy moth egg-mass density (Leroy et al 2021, Appendix). The survey of egg-mass density was conducted by regional forest offices in 2018 (Leroy et al 2021). In each block, an insecticide treatment (Mimic; M) was randomly assigned to one plot per defoliation risk level, while the second plot was left unsprayed and used as a control (C).…”

“…In contrast to the TLS leaf-wood separation approaches available, we evaluated our calculated foliation parameters based on inter-annual stem growth since stem growth and leaf area are correlated (Rolland et al 2001). To show the effects of defoliation, we used a large-scale field experiment in gypsy moth-infested oak-mixed forests in Germany (Leroy et al 2021). The experimental design comprises two defoliation risk levels based on gypsy moth egg-mass occurrence and two treatment types sprayed to protect the trees from defoliation or unsprayed control.…”