Chemical signal interactions of the bark beetle with fungal symbionts, and host/non-host trees

Abstract

The symbiosis between the bark beetle (Ips subelongatus) and its fungal symbiont (Endoconidiophora fujiensis) poses a serious threat to larch forests. However, the signaling pathways between these symbiotic partners and their host/non-host trees are not fully understood. Inoculation of the host larch (Larix principis-rupprechtii) with two strains of E. fujiensis induced a rapid and long-term release of monoterpenes. Althouh the fungi had a level of tolerance to these compounds, many monoterpenes inhibited fungal culture growth. Moreover, monoterpenes with stronger inhibitory effects on fungal growth exhibited weaker synergistic effects on the attraction of I. subelongatus to aggregation pheromone. Surprisingly, individual isomers of aggregation pheromone components promoted fungal symbiont growth in a culture medium. Non-host volatiles (NHVs) were tested and shown to completely inhibit the growth of fungal symbionts in culture but had no effects on beetle responses to aggregation pheromone, with the exception of (Z)-3-hexen-1-ol. These results reveal convergence and mutualism patterns in the evolution of I. subelongatus and E. fujiensis with respect to host tree volatiles but not in response to NHVs. Ultimately, we put forward a hypothesis that host plants are ecological and evolutionary determinants of bark beetle–fungus symbioses in terms of their complex signaling interactions.

Comparative Analysis of Eight Mitogenomes of Bark Beetles and Their Phylogenetic Implications

“…Bark beetles (Coleoptera: Curculionidae: Scolytinae) feed mainly on the phloem and xylem of their hosts (e.g., pines, spruces, larches), severely affecting forest ecology and reducing forestry production [ 1 , 2 ]. Because they bore cryptically under the bark and into the wood, they are difficult to detect at the onset of the threat, resulting in many bark beetle species becoming established outside their original range [ 3 ].…”

Functional investigation of monoterpenes for improved understanding of the relationship between hosts and bark beetles

“…(2007). In a previous study, R ‐(+)‐α‐pinene significantly reduce the pheromone trap catches of I. subelongatus , whereas S ‐(–)‐α‐pinene showed no effect (Fang et al., 2020), suggesting that different Ips bark beetles exhibit different behavioural preferences towards α‐pinene enantiomers. In the current study, α‐pinene consistently enhanced the attraction of the clerid beetle T .…”

“…in Europe (Mayer et al., 2015). The major hosts contain monoterpenes as their predominant phytochemical volatiles that play an important role in the interaction between Ips bark beetles and conifer trees (Fang et al., 2020; Schlyter & Birgersson, 1999; Seybold et al., 2006). Moreover, monoterpenes are involved in the constitutive and induced defence of spruce trees against I. typographus (Everaerts et al., 1988; Mageroy et al., 2020).…”

“…Host myrcene is highly toxic to I. typographus ; however, more importantly, myrcene can abolish the attraction of the beetle to its pheromone‐baited traps in the field, potentially suggesting that myrcene shows a strong inhibitory effect on I. typographus , similar to or higher than the effect of S ‐(–)‐verbenone or non‐host volatiles (Bakke, 1981; Unelius et al., 2014; Zhang, 2003). By contrast, myrcene significantly synergized I. subelongatus responses to its pheromone source [ S ‐(–)‐ipsenol/ S ‐(+)‐ipsdienol] (Fang et al., 2020). To fully understand the disparate behavioural regulation of myrcene in sympatric and sibling Ips bark beetles, further field‐based investigation is needed to test all potential compounds, including myrcene, S ‐(–)‐verbenone and non‐host volatiles, as well as their different combinations.…”

“…The analysis procedure here followed Fang et al. (2020) in general, but with improvement as follows. All hindgut extracts ( n = 10) were analysed using the Agilent 6890 GC System (Agilent Technologies), equipped with a flame ionization detector and a chiral CycloSil‐B capillary column (30 m length × 0.25 mm internal diameter × 0.25 μm film thickness; JW Scientific).…”