Importance of Ecological Variables in Explaining Population Dynamics of Three Important Pine Pest Insects

Abstract

Climate change challenges forest vitality both directly by increasing drought and heat periods and indirectly, e.g., by creating favorable conditions for mass outbreaks of phyllophagous insects. The large forests dominated by Scots pine (Pinus sylvestris L.) that cover the lowland regions in northeast Germany have already been affected regularly by cyclic mass propagations of defoliating insect species in the past with climate projections implying an even more advantageous environment for devastating outbreaks in the future. To improve predictive and responsive capacities we have investigated a wide range of ecological parameters to identify those most strongly related to past outbreak waves of three central species. In total, we analyzed 3,748 variables covering stand and neighborhood properties, site quality, and climatic conditions for an area of roughly 750,000 ha of pine forests in the period 2002–2016. To reflect sensitivity against varying climate, we computed “floating windows” in relation to critical phenological phases of the respective insects. The parameters with the highest explanatory power resulted from the variable importance measures of the Random Forest (RF) methodology and have been evaluated by a 10-fold cross-validation process. Our findings closely reflect the known specific gradation patterns and show that relative variable importance varies with species. While Lymantria monacha L. feeding was mainly dependent on the surroundings of the respective stand, Diprion pini L. proved to be almost exclusively susceptible to climatic effects in its population dynamics. Dendrolimus pini L. exhibited a mixed pattern of variable importance involving both climatic and forest structure parameters. In many cases the obtained statistical results support well-known ecological cause-effect relations and long-term population change dynamics. The RF delivered very high levels of sensitivity and specificity in the developed classifications and proved to be an excellent tool to handle the large amounts of data utilized for this study. While the presented classification approach may already support a better prognosis of the amplitude during the outbreak culmination, the obtained (most important) variables are proposed as preferable covariates for modeling population dynamics of the investigated insect species.

Forest site conditions and the threat for insect outbreaks in the Scots pine stands of Polissya

“…The foci of mass propagation of foliage browsing insects are formed in favourable ecological conditions, among which the most important is the microclimate, the availability of preferred food, and the absence of natural enemies (Kosunen et al 2017;Hentschel et al 2018;Meshkova et al 2019).…”

First Record of Dendrolimus pini Outbreak on Aleppo Pine in Croatia and Severe Case of Population Collapse Caused by Entomopathogen Beauveria bassiana

“…Higher survival rate as a result of earlier hibernation ending under favorable climatic conditions means avoiding certain risks related to overwintering in the soil (e.g. infestation with soilborne fungi or bacteria) and can also be attributed to higher daily sun duration [14].…”

Pine Pitch Canker and Insects: Relationships and Implications for Disease Spread in Europe

“…Insect-mediated encounters between pines and F. circinatum are also strongly determined by the temporal and spatial patterns in the development of insect populations. Environmental factors, especially temperature, are key determinants of this synchrony [190]. Because insects are ectothermic, their development and reproduction are readily affected by changes in temperature regimes.…”