Introductions of exotic forest insects can exert substantial ecological and economic impacts on natural forest systems. The mountain pine beetle (Dendroctonus ponderosae Hopkins), an aggressive bark beetle native to western North America, kills mature pines at outbreak levels and is currently expanding its geographic, altitudinal and host ranges across the continent. Its generalist feeding behavior on pines, and its ability to kill novel hosts in newly invaded areas of Alberta, Canada, suggests that this insect could threaten pine forests in other regions of the world as well. Little is known of the susceptibility of Scots pine (Pinus sylvestris L.) to mountain pine beetle. Scots pine is a potential novel host common to forests across Eurasia. We measured outcomes of an outbreak by mountain pine beetle in mixed stands of mature Scots pine and the insect’s historical host, ponderosa pine (P. ponderosa Dougl. ex. Laws.) in the Black Hills of South Dakota, U.S.A. We conducted a retrospective assessment of 93 trees (54 Scots pine and 39 ponderosa pine) of similar size and proximity that experienced high beetle pressure over a period of three to four years ending in 2015.
Results/Conclusions
Our results show that mountain pine beetle is able to detect and attack live trees of Scots pine. Notably, we found strong attraction and preference for the novel Scots pine relative to the historical host, to the point of excluding the otherwise highly attractive and susceptible historical host from attack in mixed stands. Preference for one host to the point of exclusion of a known host has not been shown for this insect previously. However, we found that Scots pines demonstrated half the attack density and fifteen fold less likelihood of mortality in one year’s time relative to ponderosa pines in nearby stands. Thus, while Scots pines in this study were highly attractive to mountain pine beetle, they were less likely to succumb rapidly to an attack, suggesting defensive capacity unexpected in a host with no coevolutionary history with a tree-killing bark beetle in its native range. These results are important for assessing the risk of hypothetical mountain pine beetle outbreaks in Eurasia, and naturalized Scots pine stands throughout North America.