Biological and other factors that influence whether a bark or ambrosia beetle presents a potential risk of entry, establishment, spread, and impact for the EPPO region were identified through expert opinion and based on the selected species. Each factor was ranked by expert judgement as of major, medium, or minor importance. Risk factors considered to be of major or medium importance are summarised below, along with illustrative examples. The risk factors were developed in detail in the EPPO study, giving more examples of species. Minor factors were not considered further. For some factors, key elements that are especially important to the risk were identified for some factors and rated against each species.
Table 2 presents the ratings of the elements most important to the risk factors for the representative species.
3.2.1. Biological Factors
Many Scolytinae are inbreeders, which means that they mate with their siblings in the galleries where they developed (sib-mating). This is the case of species in the Xyleborini genera Ambrosiodmus, Cnestus, Euwallacea, Xyleborinus, Xyleborus, and Xylosandrus. Consequently, nearly all females leaving the tree have already mated. Inbreeding therefore facilitates entry (transfer to a host) and establishment.
The ability to colonize live trees (whether stressed or apparently healthy) is key to the risk of entry (transfer to a host), establishment, and impact. All bark and ambrosia beetles considered in the EPPO study have some association with live trees. For example, Megaplatypus mutatus attacks only live standing trees, while Austroplatypus incompertus attacks apparently healthy trees and Cnestus mutilatus has been recorded on live stressed trees. Some species attacking live trees have also been recorded attacking timber, which is an advantage for entry (association with wood commodities). This is, for example, the case of Phloeotribus liminaris. For initial colonisation in a new area, some species may be able to utilize various material, from recently dead or weakened to living healthy trees.
Polyphagy and the ability to attack new hosts in new areas are advantages for entry, establishment (higher probability of finding a suitable host), and impact. Polyphagy was considered in terms of the number of host families recorded for the bark and ambrosia beetle species. The most polyphagous species are recorded on plants in over 50–60 families (e.g.,
Hypothenemus eruditus) (
Table 2). Even bark and ambrosia species that are somewhat restricted in their host selection can typically utilize many species within host families or within higher ranks, and such species can colonize confamilial hosts in the newly occupied ranges. Most bark and ambrosia beetles considered in the study have been recorded on new hosts when introduced into new areas (e.g.,
Xyleborus glabratus).
Ambrosia beetles are always associated with fungal symbionts [
17] and may also carry other fungi. Bark beetles are also associated with fungi, with relationships ranging from casual associations or more consistent but non-obligate associations to coevolved nutritional mutualism [
18]. Fungus-farming as carried out by ambrosia beetles is indirectly favourable to establishment as a mechanism for broadening host range. More importantly, an association of a bark or ambrosia beetle with a pathogenic fungus increases the potential impact. Unfortunately, this may not be recognised until the fungus is transported to a new invasion area or a new host, leading to unexpected impact such as took place for
Geosmithia morbida (associated with
Pityophthorus juglandis).
The climatic conditions to which a bark or ambrosia beetle is adapted is important for the potential risk (especially establishment and spread). Species known to occur in areas climatically similar to those occurring in the EPPO region are very likely to establish. Many bark and ambrosia beetles originate from tropical or subtropical areas, and establishment is likely to fail because of an unsuitable climate. Nevertheless, the Mediterranean Basin has already proven to be sufficiently warm for many such species (e.g., Xyleborus bispinatus). In addition, the trend of increasing temperatures in the EPPO region may increase the likelihood of establishment of cold-intolerant species.
The capacity to disperse, either naturally or human-assisted, increases the risk of entry, establishment (finding a host far from the entry point), and spread for all bark and ambrosia beetles. The size of the pioneer population will influence dispersal capacity (because there will be more individuals dispersing long distances among the members of a larger population). All bark and ambrosia beetles have some natural dispersal capacity through flight in at least one of the sexes, but the flight biology and capacity remain essentially unknown for most species. Among the representative species,
Platypus quercivorus is recorded as having a good flight capacity (in experiments in a flight mill, some individuals flew over 25 km [
19]). Human-assisted dispersal through the movement of wood is, in any case, the main factor for establishing new infestations of bark and ambrosia beetles.
Most, if not all, bark and ambrosia beetles infesting non-coniferous wood can establish at low population densities (just a single female in sib-mating species) on recently dead, weakened, or apparently healthy hosts. However, mass-attack, based on aggregation pheromones, can facilitate the establishment of some species (e.g.,
Acanthotomicus suncei [
20]) by allowing them to overcome the defences of living trees. Mass attack as a necessary requirement (which is mostly known for species on coniferous wood) can be unfavourable to establishment, while solitary colonizers have displayed high success in entry and establishment. However, mass attacks once a species is established and the epidemic threshold is reached (e.g.,
Pityophthorus juglandis or
Xylosandrus crassiusculus) can result in higher impact.
The ability to develop several separate or overlapping generations (e.g., Xylosandrus compactus) increases both the risk of spread and impact through higher population density. This also occurs in the case of sister broods (subsequent oviposition by the same female in different galleries).
3.2.2. Additional Factors
All of the representative bark and ambrosia beetles have some association with non-coniferous wood commodities, thus increasing the risk of entry. Bark and ambrosia beetles are commonly associated with wood commodities in trade and are frequently intercepted. This is the case for example for
Euplatypus parallelus,
Xyleborinus artestriatus, and
X. octiesdentatus. The association of bark and ambrosia beetles with different wood commodities is considered in
Table 3.
There are uncertainties about the association of bark and ambrosia beetles with some wood commodities, especially wood chips, hogwood and processing wood residues. However, given the small size of bark and ambrosia beetles (adult body size ranges from <1 mm to 9 mm for the representative species), such an association remains a potentially important risk.
There are also uncertainties about the association with wood commodities for species that preferably attack small diameter material such as branches or twigs, because they would mostly not be present in tree trunks. However, such species may also attack larger diameter material, and some wood commodities may also contain small diameter material or whole trees.
Wood packaging material not treated according to ISPM 15 [
11] was considered a potential pathway for all species considered, given the large number of historical interceptions and introductions apparently linked to this pathway. Finally, a potential association with plants for planting (and possibly cut branches) was identified, with variable levels of uncertainty, for all bark and ambrosia beetle species in the EPPO study including those that attack small-diameter material such as twigs.
The past history of introductions of a bark or ambrosia beetle documents a risk of entry and establishment. Most species used as representative examples are known to have moved to new areas including the three
Euwallacea spp.,
Monarthrum mali, and
Scolytus schevyrewi. In a review of the literature, a total of 138 exotic Scolytinae species have been recorded to have established in more than one land mass [
21].
The existence of a trade is an essential factor for the risk of entry of bark and ambrosia beetles. At the scale of the EPPO region, there is no complete picture of all wood commodities or genera and species of wood imported into the region. Nevertheless, a huge diversity of non-coniferous species is known to be used for their wood. Data available in FAOSTAT [
22] for EPPO countries show a broad trade of non-coniferous wood commodities including tropical and non-tropical round wood, sawn wood, and wood chips and particles. The wood commodities imported from outside the EPPO region come from a large diversity of origins in the northern and southern hemispheres (
Table 4).
Bark and ambrosia beetles are difficult to detect and identify, thus increasing the risk of entry and establishment. They are generally small and cryptic, and may be transported in small numbers, making them difficult to detect. Detection may be facilitated where there are known specific attractants (e.g., pheromones or kairomones). Commonly used attractants (e.g., ethanol) allow for a large range of bark and ambrosia beetles to be detected. However, the identification of bark and ambrosia beetles is difficult and requires either specialized taxonomic expertise or access to gene sequencing, which is not always available throughout the EPPO region.
Eradication and containment of bark and ambrosia beetles is very difficult in nearly all circumstances, increasing the risk of spread and impact. Most non-native species of bark and ambrosia beetles are detected long after establishment, when the population density has increased, and the species may already have started spreading naturally or through human-assisted pathways. In addition, there are few control measures that are effective due to the beetles’ cryptic life cycle.
The existence of suitable habitats increases the risk of entry, establishment, spread, and impact. Areas where the diversity of trees is high are more favourable for entry and establishment because non-indigenous beetles are more likely to find a suitable host. Species may also shift habitats, for example, between forest and orchards (e.g., Euwallacea fornicatus sensu lato, and Ambrosiodmus rubricollis). Finally, urban environments provide high host diversity and trees that are often in stressed condition, enabling colonisation by opportunistic wood borers.
Bark and ambrosia beetles may cause economic damage including the degradation of wood value, negative impacts on exports of wood commodities, tree decline or tree death (e.g., Austroplatypus and Platypus species); environmental damage such as ecosystem changes through the death of trees (e.g., Xyleborus glabratus) or social damage including job losses (e.g., due to processing units closing because of losses of wood production) and effects on the recreational value of an area (e.g., Pityophthorus juglandis). Previous high impact is an indicator of potential risk. However, there are often uncertainties in terms of the hosts that will be attacked in a new invaded area, and their susceptibility and the potential damage cannot be accurately predicted. This is especially the case for beetle species for which data is lacking such as Acanthotomicus suncei and Gnathotrupes spp. of Nothofagus.
Good management practices can reduce susceptibility to infestation by bark and ambrosia beetles and would protect plants from attacks by secondary pests. Conversely, management leading to increased tree stress reduces resistance to attack, and/or resilience following an attack.