Improved Outbreak Prediction for Common Pine Sawfly (Diprion pini L.) by Analyzing Floating ‘Climatic Windows’ as Keys for Changes in Voltinism

Katrin Möller,Jens Schröder,Rainer Hentschel,Aline Wenning

doi:10.3390/f8090319

Abstract

The biology and population dynamics of pine sawfly Diprion pini L. are extremely complex and variable. Among other factors, climatic conditions determine the potential for mass outbreaks of the species. In this paper, we investigate this influence and describe a statistical approach to identify responsible climatic variables in floating time windows, thus identifying the factors responsible for the transition from latency to outbreak events. Analyses were built upon a data base comprising outbreak events and fine-scaled climatic data for the period 2002–2016 for a model region in the state of Brandenburg, Germany. By applying Random Forest statistic classification analyses, we isolated a set of four variables. They cover precipitation, temperature, and potential evapotranspiration in distinct periods during the current and the previous year. These periods are not fixed in their position but attached to the floating phenological date of bud burst of the host species Pinus sylvestris L. The complete set of variables was able to distinguish forests likely to be defoliated from those not threatened at high probabilities (95% true-positive rate, 98% true-negative rate). The identified climatic windows offer insights into population dynamics in the study region, support adjustments in current monitoring algorithms, and indicate starting points for further investigations covering other regions or different years.

Highlights

The forests in the Northeast German lowlands have suffered for many decades from frequent mass outbreaks of phytophagous insects
The implemented variable importance implemented variable importance measure of this method allows us to determine the most measure of this method allows us to determine the most important climate variables and timescales important climate variables and timescales related to the observed mass outbreaks of D. pini
Little is known about a possible link between forest structure and parasitism of pine sawfly larvae in the cocoon phase [15]

Summary

Introduction

The forests in the Northeast German lowlands have suffered for many decades from frequent mass outbreaks of phytophagous insects. The German state of Brandenburg (see Figure 1) is a representative example of the environmental conditions and the challenges to forest management and protection that exist throughout this region. The climatic and pedologic characteristics of the area further aggravate the risks for mass propagations of forest pests associated with the widespread even-aged monocultures. Several remedial activities such as species diversification and improvement of structural diversity have partly begun to relieve the situation [1]. The central tasks of the forest protection service of the Brandenburg forestry state agency are to determine pest densities, to detect the beginning of gradations, to give a precise forecast of feeding damages, and to prevent total defoliation and forest decline by applying insecticides. To further optimize monitoring and preventive measures, a thorough understanding is Forests 2017, 8, 319; doi:10.3390/f8090319 www.mdpi.com/journal/forests needed of the of biological and species ecological ofreaction the relevant andinofthe their possible characteristics the relevant andcharacteristics of their possible to thespecies processes ecosystems reaction to the processes in the ecosystems caused by global and regional climate change [2,3]

Methods

Results

Discussion

Conclusion