Weathering the storm: how lodgepole pine trees survive mountain pine beetle outbreaks

Abstract

Recent mountain pine beetle outbreaks in western North America killed millions of lodgepole pine trees, leaving few survivors. However, the mechanism underlying the ability of trees to survive bark beetle outbreaks is unknown, but likely involve phytochemicals such as monoterpenes and fatty acids that can drive beetle aggregation and colonization on their hosts. Thus, we conducted a field survey of beetle-resistant lodgepole pine (Pinus contorta) trees to retrospectively deduce whether these phytochemicals underlie their survival by comparing their chemistry to that of non-attacked trees in the same stands. We also compared beetle attack characteristics between resistant and beetle-killed trees. Beetle-killed trees had more beetle attacks and longer ovipositional galleries than resistant trees, which also lacked the larval establishment found in beetle-killed trees. Resistant trees contained high amounts of toxic and attraction-inhibitive compounds and low amounts of pheromone-precursor and synergist compounds. During beetle host aggregation and colonization, these compounds likely served three critical roles in tree survival. First, low amounts of pheromone-precursor (α-pinene) and synergist (mycrene, terpinolene) compounds reduced or prevented beetles from attracting conspecifics to residual trees. Second, high amounts of 4-allyanisole further inhibited beetle attraction to its pheromone. Finally, high amounts of toxic limonene, 3-carene, 4-allyanisole, α-linolenic acid, and linoleic acid inhibited beetle gallery establishment and oviposition. We conclude that the variation of chemotypic expression of local plant populations can have profound ecological consequences including survival during insect outbreaks.

Anatomical defences against bark beetles relate to degree of historical exposure between species and are allocated independently of chemical defences within trees

“…contorta 's relatively lower proportions of the host compounds that D . ponderosae exploits to coordinate mass attacks during pheromone signalling (Erbilgin et al, ; Raffa et al, , ). Thus, these higher resin‐based defences may contribute to the greater impediment that P .…”

“…These particular metrics have been associated with greater likelihoods of survival during bark beetle outbreaks in several pine species in the western US (Ferrenberg et al, 2014;Kane & Kolb, 2010). The higher investment in morphological defence structures by P. contorta appears to complement its higher levels of constitutive and induced defence compounds against D. ponderosae and its microbial associates, and P. contorta's relatively lower proportions of the host compounds that D. ponderosae exploits to coordinate mass attacks during pheromone signalling (Erbilgin et al, 2017a;Raffa et al, 2017Raffa et al, , 2013. Thus, these higher resin-based defences may contribute to the greater impediment that P. contorta poses, relative to P. albicaulis, to the initiation of beetle aggregation (Bentz, Boone, & Raffa, 2015).…”

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

“… Minimum and maximum ranges of means were reported for each chemical compound from the following studies: Erbilgin & Colgan (); Erbilgin et al . (, ,b); Taft et al . () (data only from Alberta); Ishangulyyeva et al .…”

“…three‐fold higher from jack pine than from lodgepole pine (Erbilgin et al ., ), likely as a result of higher α‐pinene content in jack pine phloem (Table ). Higher pheromone production leads to a rapid and successful aggregation on jack pine (Erbilgin et al ., , ). Second, the fitness of MPB emerging from jack pine was superior in terms of body size relative to MPB emerging from lodgepole pine (Lusebrink et al ., ).…”

“…These chemicals act as both physical and chemical barriers against beetles and their microbial associates. However, historical hosts show large inter‐ and intra‐variations in their phytochemical profiles and resistance to MPB (Erbilgin et al ., ; Raffa et al ., ; Roth et al ., ). Second, pheromone production is closely linked to host chemicals (Blomquist et al ., ).…”