Management of Forest Pests and Diseases

Abstract

The occurrence patterns of forest insect pests and diseases have been altered by global events such as climate change. Recent developments in improved monitoring methods and tools for data analyses provide new opportunities to understand the causes and consequences of such changes. Using a variety of management tools, forest pest management programs can mitigate the influence of global changes on forest health. The goal of this Special Issue is to improve our understanding of the root causes of changes that have induced global changes. Fifteen papers are included in this Special Issue, covering several issues in forest pest management. One paper reviews the causes of Korean oak wilt, and another paper discusses fourteen invasive tree pests in Russia. The remaining thirteen papers cover issues related to the monitoring and management of forest pests. These studies provide a better understanding of the causes of change in the patterns of forest pests under the influence of global changes. These reviews also contribute to the development of forest-pest-management strategies to mitigate such impacts on forests due to global changes.

1. Introduction

Changes in the distribution of forest pests, increases in the numbers of and damage from invasive species, and outbreaks of new species have been observed in many countries, apparently due to climate change and the increase in international trade [1]. Responses of forest pests to global changes are complex, and their prediction is difficult [2]. Recent developments in remote sensing technology offer the ability to rapidly monitor large areas damaged by forest pests [3,4]. Additionally, recent developments in data analysis using machine learning offer tools better suited to understanding the causes and impacts of pests at forest scales. Various management options, including biological control, offer a chance to develop a management strategy to mitigate the influence of global changes on forest health.

The applicability of remote sensing to assess damage from forest pests has been reviewed recently [3,4]. Images acquired from satellites or unmanned aerial vehicles (UAVs) are potentially available to assess forest damage from forest insect pest and disease. Moreover, changes in forest damage at the spatial and temporal scales can be monitored quickly and with precision. Hall, Castilla, White, Cooke and Skakun [3] used this technology to evaluate forest damage from several forest pests, including gypsy moth (Lymantria dispar [L.]), mountain pine beetle (Dendroctonus ponderosae Hopkins), and forest tent caterpillar (Malacosoma disstria Hübner). Detecting the damage caused by forest pests using UAV has also been widely attempted [5,6,7].

Newly developed data analysis techniques also offer fresh insights into the influence of global changes on the occurrence patterns of forest pests. Through a better understanding of the causes and patterns of forest pests, more effective management strategies can be constructed. For example, the potential distribution of pine wilt disease (PWD), caused by pine wood nematode (Bursaphelenchus xylophilus (Steiner and Buhrer) Nickle), in Korea was predicted though machine learning methods using local first occurrence data [8]. The authors found that new occurrences of PWD were associated with human activities and vehicle movement. This finding was supported by independent studies using a path-finding algorithm and dispersal models in Korea [8,9]. These results showed that movement of PWD through human activities was a major factor in the dispersal of PWD in Korea, and the analysis provided basic information useful in building a management strategy for PWD in Korea.

A variety of forest pest management options have been developed to the level of practical use, following laboratory and field evaluations. The aim of this Special Issue “Management of Forest Pests and Diseases” is to discuss proposed or current management strategies for the control of forest pests whose effects are worsening due to global changes. The monitoring and analysis of pest occurrences form the basis for evaluation of new pest-control methods. Based on the understanding of the causes of new pests, pest outbreaks, or pest distributions and the role of global changes in each case, the authors of the papers in this issue discuss potential mitigation and prevention tools.

2. Papers in This Issue

Fifteen papers are included in this issue, focusing on the monitoring and management of forest pests: Choi, et al. [10] reviews Korean Oak wilt (KOW) (a new syndrome of oak decline in Korea) and describes its cause, distribution, history, disease cycle, and management. An increase in the number of stressed trees among the potential hosts of KOW has been the major cause of KOW in Korea due to an increase in the average age of trees in oak stands, as well as some effect of climate change. Musolin, et al. [11] reports 14 species of invasive tree pests in Russian forest and urban ecosystems, including Leptoglossus occidentalis, Agrilus fleischeri， and Ips amitinus. All of these species (except for Ips amitinus and Acrocercops brongniardella) were associated with human activity and are expected to expand geographically in Russia.

Six papers discuss the management of forest pests, while seven focus on monitoring. Interestingly, four papers are on pine wilt disease caused by pine wood nematode, showing that this nematode is a severe threat to forest health globally. The six papers on forest pest management discuss diverse approaches, including chemical and biological control, mass trapping, and overall management strategy. Sun, et al. [12] extracted two chemicals with nematocidal activities (cyclo-[Pro-Phe] and 2-Coumaranone) from a bacterium, Lysinimonas sp., collected in the rhizosphere of Pinus thunbergii. The nematocidal activity of these two compounds was confirmed through in vivo experiments using pine seedlings. Kim, et al. [13] found a parasitoid, Spathius verustus Chao, parasitizing Monochamus spp., vectors of pine wood nematode in Korea. The wasp is a gregarious ectoparasitoid that prefers Monochamus alternatus to Monochamus saltuarius. The possible use of this wasp as a biological control agent against vectors of PWD is discussed. Bălăcenoiu, et al. [14] discuss the use of pesticides to control populations of Corythucha arcuata, an invasive species in Romania. The contact pesticide alpha-cypermethrin and a systemic pesticide, acetamiprid, were applied to oak forests by helicopter. Chemical control reduced the nymphal population by up to 96%, but the application of the systemic pesticide was more effective in reducing the pest population. Fora and Balog [15] present information on the population density of two bark beetles (Ips typographus and Pityogenes chalcographus) in spruce (Picea spp.) in forests of the Apuseni Natural Park in Romania. They found that the ranges of both species were expanding and that their densities were high regardless of the level of forest management practiced. They attributed this bark beetle problem to climate change and concluded that new approaches, such as biological control using natural enemies, were needed. Olivieri, et al. [16] present results on the control efficiency of two Bacillus thuringiensis kurstaki (Btk) strains for the control of gypsy moth larvae in Mediterranean cork oak forests. Aerial applications using helicopters reduced the density of gypsy moth larvae by approximately 70%, showing the possibility of the use of Btk as a biological control agent for this pest and forest. Resnerová, et al. [17] compare the efficiency of three trapping methods for the bark beetle Ips cembrae in European larch forests. Trap trees, pheromone traps, and insecticide-baited traps were used for experiments, and the trap tree method proved to be the most efficient method in terms of the number of bark beetles captured. Therefore, although all traps tested were useful for the monitoring of this bark beetle, trap trees were most effective in reducing the density of this bark beetle.

Finally, seven other papers focus on monitoring forest pests. Davydenko, et al. [18] studied emerald ash borer (EAB), Agrilus planipennis, as an invasive species in Ukraine. This species had the potential to kill European ash, Fraxinus excelsior. The ash tree is also threatened by an invasive pathogen Hymenoscyphus fraxineus. Ash trees infected by this fungus are more vulnerable to infestation by EAB. Zhang, et al. [19] identified an olfactory gene from Monochamus saltuarius, a vector of pine wood nematode in China, and found that this gene in the male beetle was down-regulated when the beetle was infected with pine wood nematodes. Male beetles infected by pine wood nematode became insensitive to olfactory stimuli from female beetles or host plants. The resulting increased period of maturation feeding before mating provided the nematode with a longer time to transfer into pine trees. Gathercole et al. [20] studied the leaf microbiome of oaks in relation to acute oak decline (AOD) in Quercus robur and Q. petraea in Britain. Four bacteria including Brenneria goodwinii were collected from a lesion of AOD. Whole genomic DNA analysis collected from 421 trees nationwide showed that these bacteria were found regardless of AOD symptom, suggesting these four bacteria are part of the normal oak microbiome rather than causes of AOD. Gray et al. [21] used a Bayesian hierarchical model to predict the occurrence of eastern spruce dwarf mistletoe (ESDM), Arceuthobium pusillum and assessed the predictive performance of the model. The model’s overall accuracy was only 52%. The model is useful for understanding the occurrence pattern of ESDM and aiding the monitoring and management of ESDM. de la Fuente and Saura [22] modeled the expansion of the distribution of pine wood nematode in the Iberian Peninsula using a process-based network model. Climate was not the most important factor for the dispersal of PWN because the nematode can disperse through areas that are already climatically suitable. Differences in the host-species-specific susceptibility to PWN were decisive for the expansion of PWN, showing the need for host-species-specific management strategies. Choi, et al. [23] quantified areas of gypsy moth defoliation in 2020 using Landsat satellite images in Wonju, Korea. About 13.0% of the forested area in Wonju was defoliated by gypsy moth in 2020. Moreover, in Wonju, the gypsy moth preferentially occurred in Larix kaempferi forests as compared to oak and other deciduous trees. Koreň, et al. [24] used machine learning algorithms (MLA) to model the spatial and temporal infestation patterns of the bark beetle Ips typographus in the Czech Republic. Among MLAs, the extra tree-classifier method was the most useful in assessing the spatial infestation of the beetle. This model is expected to be useful for designing new forest management strategies to reduce damage by this bark beetle.

3. Conclusions

New pest invasions or changes in the distributions of forest pests have been observed in many parts of the world. Impacts of invasive species on forest ecosystems are of major concern globally. PWD is an invasive species in Asia and Europe that has been a target of several research programs, focused variously on monitoring, modeling, and management. These studies suggest that PWD is one of the most severe threats to forest health globally. To mitigate the impacts of global changes on forest health, suitable forest pest management strategies need to be developed based on scientific monitoring and analysis. We hope that this Special Issue will lead to an improved understanding of the causes of changes in the occurrence patterns of forest pests and new ways to mitigate their damage.