Phytochemicals as mediators for host range expansion of a native invasive forest insect herbivore.

Abstract

Mountain pine beetle (MPB) has recently invaded jack pine forests in western Canada. This invasion signifies a climate change-induced range expansion by a native insect. The mechanism underlying this invasion is unknown, but likely involves phytochemicals that play critical roles in MPB biology. Thus far, studies have investigated the compatibility of jack pine chemistry with beetles and their microbial symbionts. I have identified three phytochemical mechanisms that have likely facilitated the host range expansion of MPB. First, jack pine chemistry is overall similar to that of the historical hosts of MPB. In particular, jack pine chemistry is compatible with beetle pheromone production, aggregation on host trees and larval development. Furthermore, the compatibility of jack pine chemistry maintains beneficial interactions between MPB and its microbial symbionts. Second, compared with historical hosts, the novel host not only has lower concentrations of toxic and repellent defense chemicals, but also contains large concentrations of chemicals promoting host colonization by MPB. These patterns are especially pronounced when comparing novel hosts with well-defended historical hosts. Finally, before MPBs arrived in jack pine forests, they invaded a zone of hybrids of novel and historical hosts that likely improved beetle success on jack pine, as hybrids show chemical characteristics of both hosts. In conclusion, the phytochemistry of jack pine has likely facilitated the biological invasion of this novel host by MPB.