Introduction 

Climate change may have positive or negative effects on forests, depending on local initial conditions ([1]). Positive effects are perceived for example by the so-called “energy-limited forests”. These are forests growing in cold climates, thus limited by a short growing season, like sub-alpine forests. In this case, a growing season that is prolonged by climate change and ensuing warmer temperatures, may positively affect tree growth ([45]). On the other hand, most forests are limited by water availability. In fact, water is the main limiting factor for plant productivity all over our planet ([34]). In this type of forests, a warmer climate, associated with increased drought, has generally a negative effect on tree health ([45]). Hereafter, in this paper we will refer in particular to the forests of the temperate and boreal regions.

Although the negative effects of climate change are numerous, variable, and interacting with each other, they can be roughly distinguished into “direct” and “indirect”. As regards direct adverse effects on forest ecosystems, the acting factors are temperature, precipitation, wildfires, and extreme weather events. For example, higher temperatures and/or water stress have been identified as the most probable causes of increased tree mortality in southern European and western North American forests ([1]). Higher mortality is observed in different forest types, independently of tree size or plant genera ([5]).

The increase in wind damage is another phenomenon caused by climate change ([30]). Windstorms represent one of the main abiotic disturbances in European forests and the main cause of forest damage ([30]). However, it is not clear if this increase in damage is due to higher storm frequency and intensity or to higher tree susceptibility. In fact, temperature and precipitation changes alter soil proprieties and it is supposed that trees in these conditions are more prone to wind damage ([30]).

Finally, an increase in wildfires is also expected because of climate change ([37]). In fact, due to warmer and drier climate conditions occurring in various regions of the globe, an increase in fire-prone areas is expected globally, particularly in temperate and boreal climates ([38]). Furthermore, warmer seasons, as well as the lengthening of the potential fire season, and thus drier vegetation, cause an increment in the frequency and duration of burning fires ([46]) and finally higher wildfire damage.

Indirect negative effects are to be added to the aforementioned direct ones. Climate stressors can have deleterious effects on tree vigour, acting through different mechanisms. For example, water stress reduces carbon availability leading to carbon starvation, which threatens plant survival ([1]). In addition, tree vigour can be reduced also through effects on their symbionts. In fact, although the impact of climate change on plant microbiota is poorly understood ([41]), it is hypothesized that climate stressors can alter the diversity and structure of microbial communities ([13]), possibly causing plant microbiota dysbiosis ([41]). In both cases the final effect is an increase in plant susceptibility to biotic factors ([1], [45], [31]), particularly pathogens and insect pests. However, since climate changes affect at the same time trees, insect pests, as well as their interactions, the combined effect is very complex and hardly predictable. As mentioned above, some warm-adapted insect pests can benefit from higher temperatures and drier climates ([7]), leading to increased damage to forests. Conversely, other pest species may be negatively affected and, as a result, their population densities may decrease, ultimately leading to reduced forest damage ([28], [33]).

Effect of insect pest response to climate change on forests

Climate change may alter the frequency and intensity of forest pest outbreaks ([44]), however, these effects may vary, as outbreaks can be enhanced or decreased by the changed climatic parameters ([22]). The response of insect pests, in fact, may depend on the intensity of climate stressors as well as on the insect species (Tab. 1). Effects on certain species are well studied, however, generalization is not possible, as each species reacts very differently to the changing climate ([31]). In addition, the same species may have multiple and contrasting responses to climate change, in other words, its damage may increase or decrease according to the local community and geographical region ([22], [12]). Some species increase the outbreak intensity in the northern part of their range, while reducing it in the southern parts ([22], [18]). Many insects are shifting their range poleward and to higher elevations ([12]). In addition, climate factors may act in different ways on insects, affecting their survival (directly or through the activity of their natural enemies), their life cycle, their dispersal capacity, their distribution, and their trophic interactions ([4]).

  Negative impact of climate change on insect populations 

Temperatures

Insects, being ectothermic organisms, are strongly affected by temperature. Each species has a specific temperature range within which finds the proper conditions to survive. Thus, summer higher temperatures linked to climate change may negatively affect insects. If the temperature increases above this range, adverse effects may occur, such as decreased individual size, fecundity, dispersal capacity, and increased mortality rates ([8], [47], [15], [32]). Mild winter temperatures may affect insects differently. Certain insect species require chilling periods to enter the obligate winter diapause, so the privation of these lowering temperatures may lead to higher mortality. In addition, higher winter temperatures lead to a snowfall decrease and an earlier snowmelt. This may negatively affect the survival of species that benefit from snow protection during overwintering in the soil ([28]).

Insect-plant interactions

Changing climate may affect interactions between insect pests and their host plant. For example, due to climate change some specialist insects, strictly synchronized with host plant, may be negatively affected by a change in plant phenology. Some defoliators may have a short timeframe within which leaves provide proper quality food ([26]). Thus, if climate change acts differently on these species’ phenology than on that of its host, a desynchronization may occur ([48], [31]). Lack of synchrony leads to a reduction in insect abundance ([2], [33]) and thus plant damage is also reduced.

Severe water stress, which is expected with climate change, can modify the content of primary and secondary metabolites in plants, reducing nitrogen and water content, and thus affecting plant-feeding insects ([12]). Based on the most accepted hypothesis, the effects on insects depend on their feeding guild and the intensity of the stressors. Sap-sucking insects, as well as defoliators and leaf-mining insects, for example, are negatively affected by severe droughts as the weather-stressed host offers them low-quality food ([28], [9], [12]).

Natural enemies

Finally, since the insect pest density also depends on natural control agents, the responses of their natural enemies to climate change should be considered. Effects are not easily predicted and authors have contrasting opinions about them. Some researchers argue that a warming climate can be more deleterious on predators or parasitoids than on their prey or hosts, with beneficial effects on pest populations ([47], [12]). Conversely, others think that reduced food quality for insect pests, with the resulting lengthening of their life cycle, exposes them for longer periods to their natural enemies ([39], [28]). Based on this last theory, pest population densities would be reduced by climate change.

In conclusion, some insect pests may be negatively impacted by climate change, either directly, with effects on their survival or performances, or indirectly, through changes in host plant phenology or their natural enemies’ performance. This kind of effect may lead to a reduction in the population density of insect pests and in some cases to their range contraction ([22], [12]), with a consequent reduction of damage to forests.

  Positive impact of climate change on insect populations 

Temperatures

Warmer climate may favour some insect pests, particularly in regions where their optimal temperatures are not reached. Insect development rates strictly depend on temperatures: it is faster at optimal temperatures, while it slows down at suboptimal temperatures, regardless of them being higher or lower. Each species has a different sensibility to temperatures and a different optimal level. Some insect pests may be favoured in some regions, particularly those at higher latitudes, where temperatures are lower than their optimal level ([22]). Here, an increase in temperatures would accelerate their metabolic rate, thus increasing their development and movement capability, as well as reducing their mortality, ultimately leading to an increase in population density ([6], [16], [49]).

A higher development rate leads to shorter life cycles and, for multivoltine species, to a higher number of generations per year. This is also enabled by an extended growing season, which generally occurs due to climate change ([21], [19], [16], [12]). Therefore, clearly, if an insect pest has more generations per year, the damage it causes to plants increases too ([16]).

In addition, in areas where winter low temperatures are a limiting factor for insects, an increase in temperatures may reduce mortality rates of insects without an obligate winter diapause ([6]). This was demonstrated both for species that overwinter as eggs and those overwintering as larvae. Some examples are forest defoliators like the winter moth Operophthera brumata L., the gypsy moth Lymantria dispar (L.), and the European pine sawfly Neodiprion sertifer (Geoffroy), whose egg mortality rates have been reduced by higher winter temperatures ([43], [17], [28]). In the case of defoliators which overwinter as larvae the effect is more complex. One of the most studied cases is that of a Mediterranean species, namely the pine processionary moth, Thaumetopoea pityocampa (Denis et Schiffermüller). Its larvae actively feed during winter if temperatures allow it. This winter feeding reduces overwintering mortality and increases insect performances, leading to an increase in females’ fecundity ([3], [2], [40]).

With the warming climate, many insect pests are expanding their distribution and outbreak areas in regions that previously exceeded their thermal tolerance. In these new environments, they may have a higher impact due to the lack of coevolution, with higher damage to plants. This has been confirmed for many species of bark beetles and defoliators, which have recently increased their performances at higher elevations ([31]). Again T. pityocampae may serve as an example, as its outbreaks have been recently reported beyond both the northern and the altitude limits of its historical occurrence ([2]).

Insect-plant interactions

The multiple negative effects of climate change make forest trees prone to pest attacks. Adverse effects of warm temperatures and drought reduce plant defences, making them susceptible to pests. Indeed, while sap-sucking insects and defoliators may be negatively affected by severe droughts, generally, xylophagous insects benefit from them ([31], [28], [9], [12], [42]). In fact, many outbreaks of bark beetles have recently been reported after drought events. The main species involved in this phenomenon belong to the Tomicus, Ips, and Dendoctronus genera ([10], [1], [20]).

Finally, outbreaks by bark beetles and wood boring insects can be triggered by increasingly frequent windstorm and wildfire disturbances. As a result of these events abundant breeding material is made available for those insect species that exploit felled and broken trees, partially burned trees, and stressed trees in general. This kind of material is suitable for pest colonization, causing the population to build up rapidly ([35], [25], [14], [37], [15]). Especially for bark beetles, this situation is even more complex because their aggressiveness changes with population density. Once the population has reached high density the pest is even able to attack healthy trees, not only stressed ones ([24]). Initially, this occurs in single “spots”, near weakened trees, then on multiple spots, and finally in widespread areas ([36]).

An example is the European spruce bark beetle, Ips typographus (L.) which has caused severe outbreaks after windstorms in Europe on various occasions ([29], [24]). The more recent one is the “Vaia” storm occurred in the Alps in 2018, which has caused a huge infestation (still in progress) in the Italian spruce forests, even without the contribution of other climatic stressors ([27]).

Although adverse effects on insect pests are possible, many outbreaks of forest insects have been related to climate change ([1], [31]). In fact, outbreaks of many insect pests, mainly bark beetles and defoliators, have been recorded in different areas of the world after extreme droughts. For example, the outbreaks of the two aggressive bark beetles Dendroctonus ponderosae Hopkins and I. typographus were associated to higher availability of trees stressed by droughts or to faster biological cycles favoured by higher temperatures ([10], [21], [19]). Similarly, favourable climate promoted the survival and speeded up the development of some main forest defoliators, like the web-spinning sawfly Cephalcia arvensis Panzer ([2]). To these, we may add the aforementioned bark beetle outbreaks recorded after large and more frequent windstorms and wildfires ([29], [24], [27]).

  Conclusions 

The impact of climate change on forests is difficult to predict, particularly when it is mediated by insect pests, as it can vary depending on various factors ([15]). In any case, many outbreaks of forest insects have been recently recorded as related to climate change ([2], [6], [23], [33]). Generalisation is not possible, as many studies agree that insects belonging to different feeding guilds have different reactions to climate change. While the majority of sapsuckers and defoliators may be negatively affected by severe drought ([31], [11], [12]), they may benefit from higher temperatures, if these do not exceed their optimal threshold ([28], [15]). On the contrary, bark beetles and wood-boring insects generally benefit from both types of climate modifications ([10], [21], [23], [20]); furthermore, they may be favoured by the occurrence of wildfires and extreme windthrows ([24], [37], [15]). Anyway, the impact of insect pests varies depending on the insect species and the type of forest, in addition, the same insect species may decrease or increase both outbreak frequency and intensity in the different regions of its distribution range ([22], [12], [18]).

  Acknowledgments 

The research has been carried out within the project “LIFE MODERn(NEC) - new MOnitoring system to Detect the Effects of Reduced pollutants emissions resulting from NEC Directive adoption” - LIFE20 GIE/IT/000091.

FB contributed to bibliography curation, as well as writing, reviewing and editing of the manuscript. MB and TP contributed equally to the manuscript preparation.

  References