Exotic trees at risk of native insect attack

In this post, Manuela Branco talks about her recent paper “Host range expansion of native insects to exotic trees increases with area of introduction and the presence of congeneric native trees”

Also, see Manuela’s cartoon illustrating how host range expansion of native insects to exotic trees increases with area of introduction and the presence of congeneric native trees

Exotic tree species are used worldwide for planting, often because they are fast growing species with good wood properties. Common wisdom also assumes that being planted outside their native range, exotic trees may benefit from the loss of their natural pests. This observation has led to the so called “Enemy Release Hypothesis”. However exotic tree species may also be attacked by insect species native from the country of introduction, in particular by those which are able to expand their feeding niche. To evaluate this risk, we conducted a systematic review of the scientific literature.

Feeding guilds and major families of native European insects, which expanded their host range to feed on exotic tree species introduced to Europe. The sizes of the circles are proportional to the relative numbers of insects recruited from each feeding guild. — Feeding guilds and major families of native European insects, which expanded their host range to feed on exotic tree species introduced to Europe.
The sizes of the circles are proportional to the relative numbers of insects recruited from each feeding guild.

A total of 590 native European insect herbivores have been observed to feed on one or more of the main 28 exotic tree species introduced to Europe as planted forests. Feeding guilds recruited by exotic tree species well reflect the insect communities commonly associated to living trees in temperate forests. Thus, host expansion is not confined to specific feeding guilds. Yet, some feeding guilds, such as the highly specialized gall makers are under-represented.

The presence of congeneric native trees in the area of introduction increases the risk of exotic trees being attacked by native insects, especially by specialist herbivores (Fig. 1). An emblematic example is Pinus radiata, introduced to Europe from North America. It is one of the most common exotic trees used in European planted forests and has several native congeners in Europe. Its plantations are reported to be severely damaged by Thaumetopoea pityocampa in southern Europe.

Effect of the geographical area over which exotic tree species were planted on the total number of native European insect herbivores that expanded their host range to the same trees.

The geographical extent of forest planted with exotic trees is also critical: more native insect herbivores are reported to feed on those exotic trees that are planted across large areas (Fig. 2). For example, Pseudotsuga menziesii, which although has no congeners in Europe, was extensively planted across Europe (in at least 12 countries) and is the species with the greatest number of native insects recruited (94).

Host expansion by native, and potentially damaging, herbivores may thus occur more frequently than previously thought. Such a finding emphasizes the need for careful evaluation of risks when introducing exotic tree species for plantation purposes, for example when searching for safe alternatives to native trees in the context of climate change adaptation. We recommend that risk assessments be conducted prior to the introduction, considering several factors such as invasiveness, hybridization and susceptibility to native insect species or pathogens. Also we suggest that exotic tree species should not be planted as large monocultures.