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Introductions of alien 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 oligophagous feeding behavior and its ability to kill novel hosts in newly invaded areas of Alberta, Canada suggest that this insect could threaten pine forests in other regions of the world. 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 and introduced to North America. Laboratory studies indicate the insects can colonize and reproduce in harvested logs of the host. We measured outcomes of an outbreak by mountain pine beetle in mixed stands of mature Scots pine and ponderosa pine, P. ponderosa Dougl. ex. Laws. var. scopulorum Engelm., a historical host for the insect, in the Black Hills of South Dakota, U.S.A. We conducted a retrospective assessment of beetle attack and tree mortality of 165 trees (54 Scots pine and 111 ponderosa pine) of similar size and proximity that experienced high beetle pressure for three to four years ending in 2015. Our results show that mountain pine beetle can detect and attack live trees of Scots pine. Notably, we found that nearly 90% of Scots pines showed signs of attack, while no evidence of attack was found on the historical host in mixed stands. However, we found that Scots pines received half the attack density and demonstrated fifteen fold less likelihood of mortality in one year's time relative to ponderosa pine in nearby stands. These results are important for assessing the potential for mountain pine beetle to kill trees in Eurasia and North America in Scots pine stands.