Gypsy moth outbreaks cause severe defoliation in Holarctic forests, both in North America where it is invasive, and in its native range in Eurasia. Severe 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.

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 with 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.

One-year intensive monitoring of gypsy moth populations and damage across the selected sites showed substantial differences in population density between plots with 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 importance of using many replicates and blocking to control spatial heterogeneity. The experiment will be running continuously during the coming years to produce short- and medium-term economic and ecological data to improve our understanding and management of gypsy moth outbreaks.

One-year monitoring of gypsy moth population and defoliation during an outbreak in German oak forests. Data collected from March to August 2019 at all life-stages of the gypsy moth (egg, larva, pupa, imago). Details on the sampling methods and timing and processing of the various data are provided in the associated article.