**Contents**



### *Editorial* **Diversity and Distribution of Forest Insects**

**Dariusz J. Gwiazdowicz**

Faculty of Forestry and Wood Technology, Poznan University of Life Sciences, Wojska Polskiego 71c, 60-625 Poznan, Poland; dariusz.gwiazdowicz@up.poznan.pl

When we ponder or discuss present-day challenges faced by entomology, we focus first of all on climate change, since the warming climate contributes to a dramatic extension in the ranges of many invertebrate species. This leads to predictions concerning imminent health hazards because certain insect species are vectors of life-threatening diseases. For example, female mosquitoes from the genus *Anopheles* transmit protozoa from the genus *Plasmodium*, which cause malaria. These are the problems investigated by specialists in medical entomology. Another important field of research is connected with economic entomology, which is a subdiscipline of entomology and focuses on the effects of insects on the human economy, primarily with regard to agriculture or forestry. Food production is a key issue for the survival of human populations; however, large-area crop cultures are exposed to a considerable risk of infestation by numerous pests. The control of their populations while preventing any potential deterioration in food quality is an extremely important and complex task, particularly considering consumer expectations to be provided healthy, quality food while meeting the demand of the global food market.

However, when analysing the challenges faced by present-day forest entomology, it may be concluded that this problem is far more complex that the examples given above might indicate. On the one hand, we observe outbreaks (mass-scale emergence) of many insect species threatening not only single trees but also large forest complexes. The resulting losses reported by foresters may be substantial and amount to millions of cubic metres of timber annually. A dramatic example we have seen in recent years is connected with the dynamic population growth of the European spruce bark beetle *Ips typographus*, which is leading to the die-back of spruce forests in Europe. This forces foresters to implement measures inhibiting the bark beetle outbreaks and thus minimising economic losses. At the same time such measures are sometimes considered controversial. Many conservationist groups and NGOs oppose foresters' interference, arguing that these are natural processes and, as such, they should be accepted. It is true that many insect species, including the European spruce bark beetle, create specific conditions since their feeding galleries are microhabitats colonised, e.g., by several dozen fungal species and over 100 mite species. For this reason, the role of insect species commonly considered to be pests while being negative, at the same time, may also be considered positive in other respects.

Another aspect of forest entomology is connected with studying the richness of nature. The forest environments, particularly tropical forests, are habitats for many thousands of species, among which insects are the most numerous group. Unfortunately, even today we do not know the exact number of all species living on our planet; instead, we may only talk of rough estimates of that number. Assuming that, at present, we know approx. 2 million species, it may be stated that most species are still unknown to science. In this vast extent of our ignorance, an important part relates to invertebrates, particularly insects. For this reason, it is urgent to identify, describe and name species new to science. However, this is a long-term, tedious process, and while present-day taxonomy uses new research methods, such as, e.g., molecular techniques, there are justified concerns that some species will become extinct before we have a chance to identify them.

Apart from the numerous, abundant species posing a threat to economic prosperity, there are other species, which are rare and endangered. We are responsible for their protec-

**Citation:** Gwiazdowicz, D.J. Diversity and Distribution of Forest Insects. *Forests* **2022**, *13*, 1857. https://doi.org/10.3390/f13111857

Received: 31 October 2022 Accepted: 2 November 2022 Published: 7 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

tion, and in order to ensure their survival, we need extensive knowledge on their biology and ecology, which may be provided by advanced entomological research conducted also in forested areas. Knowing their localities, ranges of occurrence and habitat preferences we may protect these tiny, sometimes hardly visible animals more effectively. Sometimes to ensure such protection for invertebrate assemblages we identify "umbrella species", which are the species selected as the basis for conservation-related decisions, because protecting these species indirectly protects the many other species that make up the ecological community of their habitat (the umbrella effect). In order to analyse environmental changes—both of natural origin and those caused by anthropogenic factors—bioindicator species are frequently used, such as many invertebrate species, also including insects.

This relatively superficial overview of problems investigated by forest entomology clearly shows its complexity and multidimensionality. If we additionally consider the outstanding richness of species colonising the forest environment in different geographical regions, as well as correlations between a multitude of fungal, plant and animal species, it becomes obvious that we face a long-term cognitive process, a practically never-ending quest for scientific knowledge. This is what the Special Issue of the *Forests* journal, entitled "Diversity and Distribution of Forest Insects" and comprising 10 papers, aims to do.

The publication by Gwiazdowicz et al. [1] concerning invertebrates found on a relict tree species *Zelkova abelicea* (Lam.) Boiss on the Greek island of Crete is fully in line with the trend focusing on nature conservation. Relict tree species (the term stemming from Latin *relictum*, "that which is left/forsaken/abandoned/left untouched", the participle form *relictus*—"abandoned, having been abandoned") were widely distributed on Earth thousands and even millions of years ago. As a result of changing climatic and environmental conditions, at present, they are found only sporadically and solely in those locations with conditions conducive to their survival. There is a general consensus among the scientific community that relict trees play a significant role, since they constitute specific microhabitats and, as such, promote biodiversity.

In the analysed material coming from eight experimental sites, representatives of Collembola (10,285 individuals) were most numerous. Moreover, representatives of 11 orders belonging to the class Insecta were also reported, among which Psocoptera (422), Hymenoptera (245) and Thysanoptera (163) were the most abundant. In turn, the class Arachnida was most numerously represented by individuals from the orders Acari (2237) and Araneae (212), while representatives of Pseudoscorpiones (20) were the least abundant. A total of 33 Collembola species were reported, among which *Xenylla maritima* was most abundant (3844 individuals). Moreover, 11 species new to science were identified, for which descriptions will be presented in a separate publication.

Among other things, based on the PCoA analysis (the centroid in the central part of the graph), it was stated that the most stable assemblage is found at the Gerakari locality, where old, magnificent trees are found, the trunks and branches of which are covered by abundant lichens and mosses. In the other localities, the trees were less showy, occasionally forming low thickets composed of shrubs browsed on by goats. This has obviously determined the richness of microhabitats and, as a consequence, also the species richness of micro-arthropods and the unique character of the assemblage.

Present-day forestry focuses on satisfying social needs, both tangible or material (e.g., providing timber) and non-tangible or spiritual (such as the forest being a source of creative inspiration), while simultaneously preserving natural richness (e.g., biodiversity). In this way, the multi-faceted role of the forest environment is being underlined, particularly stressing the fact that the forest is a habitat ensuring survival for endangered species. Effective protection of invertebrates frequently consists of the preservation of specific sites and microhabitats, including, e.g., rotting wood colonised by rare insect species.

Resources of dying and dead trees, decaying fragments of stems, stumps and branches, i.e., coarse woody debris (CWD), are an important structural element of biocenoses and are drivers of biodiversity. The aim of a study published by Mazur et al. [2] was to describe assemblages of saproxylic beetles in pine stands of western Poland in view of rotting wood

resources. The authors present faunistic (species identity) and quantitative (species and individual counts) data from two types of stands:


The authors reported 2006 individuals, which were classified to 216 insect species, and they showed the impact of the adopted forest management method on biodiversity. Managed stands with the implemented sanitation cutting regime, in which trees that are naturally dying are removed, are characterised by lower species diversity indices. Moreover, the authors noted that unmanaged stands were characterised by a high share of zoophagous, mycetophagous and saproxylic species. In contrast, managed stands showed a high share of xylophagous beetles [2].

In turn, ambrosia beetles (which live in a nutritional symbiosis with ambrosia fungi) were investigated by Holuša et al. [3], who focused on oak stands. Those authors presented a hypothesis that a limited amount of sunlight and a greater moisture content in those stands promote the growth of fungi, which determines an increased abundance of ambrosia beetles. In addition, the biodiversity of phloexylophagous insects is greater in old-growth oak stands than in many other types of forest stands because old-growth oak stands have more rotting wood, including dry branches in treetops. The authors tested the hypotheses that:


Those authors captured beetles into traps in 10 control plots in Czechia, where they caught over four thousand individuals of ambrosia beetles classified to six species. The two most abundant species, *A. dispar* and *X. saxesenii*, represented 98% of the trapped beetles. Both of these ambrosia beetle species were more abundant in oak-dominated forests with a high canopy closure rate, indicative of a stable and humid environment suitable for the growth of ambrosia fungi, compared to oak forests with a low canopy closure level. Furthermore, a greater abundance of dead oak branches in the canopy was found to be an important factor promoting the occurrence of *A. dispar*. Although the abundance of some species was slightly higher in unmanaged forests, no statistically significant differences in ambrosia beetle abundance was found in managed vs. unmanaged forests [3].

An important trend in forestry research comprises studies investigating the biology and ecology of potential pest species. An example in this respect may be provided by the red-haired pine bark beetle *Hylurgus ligniperda*, which is a well-known forest insect that colonizes the phloem of pine species and which can attack stumps, freshly cut logs and stored timber. Usually, *H. ligniperda* does not kill trees and is considered a secondary pest. Previously, when researchers recorded its original distribution, it included Europe, Russia, the Mediterranean and the nearby Atlantic islands. It was also reported as introduced to South Africa, Japan, South Korea, Sri Lanka, Australia, New Zealand, the USA (NY and CA), Brazil, Uruguay and Chile. It should be noted that the distribution of *H. ligniperda* in East Asia is ambiguous in the literature. For this reason, Lin et al. [4] wanted to clarify its status, asking whether it is a native or invasive species in China or Korea. To answer this question, those authors not only collected the material for analyses in the forest habitats, but they also examined specimens deposited in museum collections.

The first reports on *H. ligniperda* in this geographical region come from the 1930s in Japan. Currently, the distribution of this beetle covers the entire Honshu Island. Those authors' investigations and analyses showed that the previous record of the occurrence of *H. ligniperda* in China was likely misplaced, whereas new observations suggest it has recently invaded and successfully colonized parts of the Shandong Province, China. In South Korea it has spread rapidly in recent years, and its current distribution effectively covers a major part of this country [4].

In turn, the spongy moth (formerly known as a gypsy moth) *Lymantria dispar* is a polyphagous pest that defoliates various species of trees in the genera *Populus*, *Salix*, *Quercus*, *Acer* and *Pinus*. This species has been introduced to several continents and is now found in Europe, Africa, Asia, North America and South America. Its polyphagous larvae live on a variety of deciduous and coniferous trees and can cause severe damage in years of mass reproduction. Due to these features, *L. dispar* is listed among the world's 100 worst invasive alien species.

Every year *L. dispar asiatica* destroys hundreds of hectares of forests, resulting in thousands of dollars in losses. To conserve forests from spongy moth invasions, Akram et al. [5] studied the features of this pest through morphological, molecular and flight analyses to gain insight into its spread rate. The authors hypothesized that the age of moths and locality, by which morphological features differ, influence the flight ability of female *L. dispar asiatica* from five distinct localities in China.

An experiment showed that the female Asian spongy moth from the Xifeng region in China has a significant potential for quick dispersal because of its flying range. The authors also found that 1-day-old females travel faster and have more potential to fly strong than 2 and 3-day-old adult females. They concluded that age is the most important factor affecting an insect's flight, predominantly for Asian spongy moths [5].

As a result of climate warming, the environmental conditions change and become conducive to the occurrence of alien species, frequently being invasive species. Alien species from the Scolytinae subfamily naturalized in Europe represent over 12% of all European Scolytinae, of which most are ambrosia beetles. Ambrosia beetles (Coleoptera, Scolytinae and Platypodinae) are a polyphyletic group covering numerous species from the Scolytinae subfamily, mainly Xyleborini and Xyloterini tribes, and from the Platypodinae subfamily, which differ from bark beetles in the foraging type: bark beetles feed on the phloem, while ambrosia beetles feed on fungi growing in tunnels or galleries created within a host plant. These species usually infect weakened or dead trees and rarely kill healthy plants. However, fungi carried in their mycangia might be pathogenic to particular host species, leading to economic losses.

*Gnathotrichus materiarius* is one of the alien ambrosia beetles spreading across Europe since 1930. This is a technical pest of coniferous wood in the USA and Europe and due to excavation galleries in the lower part of the trunk it is of economic importance as a pest, decreasing the technical quality and economic value of affected timber. Witkowski et al. [6] developed a theoretical model for the distribution of this species in Europe considering forecasted climate change by the years 2050 and 2070.

The authors indicate that the species has not yet already reached its maximum range within the climatically suitable area and can reach more sites because a low level of niche saturation is typical of the beginning of the second stage of invasion—the 'log phase', which comes after the 'lag phase'. The prediction developed for current climate conditions pointed out the most likely areas where the species can spread. The Balkans and the Baltic countries are highly suitable regions situated a short distance from the known locations of *G. materiarius*, and there are no natural barriers that would prevent the colonisation of this area. Results of these investigations may be used by phytosanitary service agencies to limit the spread of this species to new areas, e.g., together with sold timber [6].

In temperate forests within Europe, early-flushing (EF) deciduous trees are often heavily infested by early spring leaf-eating Lepidoptera, while late-flushing (LF) trees are phenologically better protected against such heavy infestations, as spring moth larvae begin to appear before their buds burst. Sarvašová et al. [7] studied whether the infestation of LF trees by spring Lepidoptera can be affected by EF ones if they grow in their immediate vicinity. The authors compared spring assemblages of leaf-eating larvae of Lepidoptera on LF *Quercus cerris* with those on EF *Q. pubescens* in several microhabitats in Slovakia.

Those authors found that the species composition of larval assemblages on the two oak species was similar. In contrast, on small groups and on lone trees, the lepidopteran larvae were significantly less abundant on LF trees than EF ones. In the case of young trees, the abundance of larvae and the composition of their assemblages on both oaks were comparable in the forest. In the open habitat, LF trees were less infested by larvae than EF ones, and the assemblages of moth larvae differed between the two. The authors revealed the effect (associational susceptibility) of EF trees on LF ones when growing in a close vicinity. This means that the phenological protection of LF trees may not be sufficient if they grow close to or are surrounded by EF ones [7].

The European spruce bark beetle *Ips typographus* is a dangerous pest of spruce stands, sometimes leading to huge economic losses. The activity of this species is stimulated, e.g., by climate warming, local droughts or air pollution, which weakens local spruce populations and makes them more susceptible to infestation by this beetle. For this reason, an important role is played by methods applied to control its population and measures aimed at increasing the effectiveness of currently used traps. Heber et al. [8] conducted an experiment, in which three main components of current monitoring techniques were tested in terms of their potential for mass trapping of *I. typographus*. These are attractant composition, application rate and trap type.

The conclusions of this article state that during early spring, pheromone traps are highly attractive to individuals in search of breeding sites, meaning that it is the best time for mass trapping. A temporary increase in the application rate for this first swarming period could contribute to a more marked reduction in population sizes, while trap types with a high selectivity would spare antagonists. This would increase the chances to sustainably reduce the population of *I. typographus* to a level not harmful for standing trees or at the least mitigate the peak of a mass outbreak. However, the significant increase in total trap catches achieved by the three-directional approach of this study does not seem sufficient to effectively reduce the number of beetles in an outbreak situation. Thus, the replacement of salvage logging and sanitation felling as the most effective treatments in bark beetle management by mass trapping is not yet an option [8].

The Special Issue contains information concerning not only insects, but also microarthropods closely related with these insects, such as mites. A study by Błoszyk et al. [9] analysed the character of assemblages of mites from the order Uropodina colonising a nature reserve in Poland. Mites from the suborder Uropodina are tiny arthropods, typically of max. 1 mm in body size. They inhabit diverse forest microhabitats such as, e.g., leaf litter, rotting wood, subcortical feeding galleries of insects, ant nests but also bird nests, in which they feed on fungi growing there. They typically spread through a specific zoochory, called phoresis, in which one organism attaches itself to an insect's body and uses it for transportation.

No economic activity is conducted in the Jakubowo nature reserve and the natural environment is affected generally only by natural factors. In that area, monitoring of environmental changes has been carried out for over 40 years, and the primary aim of this work was to assess stability of uropodid mite assemblages depending on environmental changes such as, e.g., secondary plant succession or increases in the mass of decaying wood.

It was observed that within the 40-year monitoring period, the species composition of uropodid mites has changed. Certain species recorded in 1982 retreated, such as, e.g., *Cilliba rafalskii* and *Trachytes lamda*. In turn, the appearance of species previously not reported, i.e., *Oodinychus obscurasimilis* and *Neodiscopoma splendida*, in the opinion of the authors, is a result of climate warming and increased ranges of these species. Moreover, the natural succession of plant cover observed in each research plot undoubtedly has a bearing on the changes in the communities of the discussed group of mites in terms of both their species diversity and abundance. The observed trend showing an increase in the number of *Olodiscus minima* and *Trachytes pauperior*, i.e., species with higher moisture requirements, in the reserve is most likely caused by the formation of an undergrowth layer, leading to additional shading and helping to maintain greater soil moisture [9].

*Bursaphelenchus xylophilus*, known as the pine wood nematode or pine wilt nematode (PWN), is a nematode feeding in resin canals of pines thus leading to their clogging and resulting in tree death. This species originally was found in North America; however, in the 20th century, it was also recorded in Asia, e.g., China, Japan and Korea, as well as Europe, in Portugal and Spain, among others. The pine wilt nematode is spread by a number of bark beetles and wood borers, and it is most often associated with beetles from the genus *Monochamus*, the pine sawyers. In view of serious concerns related to the spread of this pest, new studies have been conducted on insects being vectors of PWN. An example in line with this research may be provided by a publication of Chu et al. [10], who investigated the colonization sites (phloem and xylem) and colonization sequences of xylophagous beetles on pine trees and analysed the species richness and spatial distribution of these beetles along PWN-infected tree boles during different stages of the pine wilt disease.

The research was conducted on selected trees infected with PWN in a *Pinus massoniana* forest of the Fujian province and a *Pinus thunbergii* forest of the Shandong Province in China. According to molecular identification of the COI gene and comprehensive identification of the morphological characteristics, those authors identified 20 insect species, and *B. xylophilus* was isolated only from *Monochamus alternatus* in both *P. massoniana* and *P. thunbergia*. No PWN was detected from other xylophagous beetles and other insects in this study. This important information may facilitate a limitation of the spread of this dangerous nematode [10].

Summing up the subject matter presented in the Special Issue of the *Forests* journal entitled "Diversity and Distribution of Forest Insects", it is evident that it is in line with the main trends in forest entomology research. It covers the two primary research trends related to the protection of rare species colonising the forest environment and methods of effective biodiversity preservation, as well as ecology of species causing damage in the forest environment and potential for a more effective limitation of such damage. All these aspects indicate that in the nearest decades, these two main research trends will continue to be of importance within the broadly understood forestry sciences.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The author declares no conflict of interest.

#### **References**

