Host Selection by Bark Beetles: Playing the Odds in a High-Stakes Game

Abstract:
Bark beetles face challenges and trade-offs during host selection, imposed by lethal tree defences, lower nutrition and higher competition in less well-defended trees, scarcity and ephemeral distribution of susceptible hosts, limitation of suitable hosts to one beetle generation, and relatively short lifespan and vulnerability of adults during host searching. Beetles contend with these challenges by using multiple, integrated sensory modalities, and sequential decision making. They incorporate both negative and positive information at multiple scales to locate susceptible hosts across large and heterogeneous landscapes. Some of the olfactory sensory neurons that convey non-host signals are relatively broadly tuned, whereas those that underlie intraspecific communication and host quality assessment are more specific, an arrangement that maximizes the use of antennal space while retaining high fidelity of detection channels strongly linked to survival and reproduction. The pertinent co-localization of neurons within sensilla provides the means for odour mixture processing in the periphery and enhances odour source discrimination and evaluation of chemical ratios in host tissue. Bark beetles show high behavioural plasticity in their orientation sequences, which allows them to adjust to environmental variability. This plasticity is modulated by environmental, genetic, and geneenvironment drivers. Behavioural plasticity allows individuals to best realize the benefits that can be derived from pheromone-mediated cooperative attacks when populations are high. Cross-scale linkages among neurons, sensilla, orientation sequences, populations, and habitat structure underlie the landscape-scale environmental and socioeconomic impacts bark beetles exert. They also underlie rapid outbreaks in response to human-induced environmental alterations, such as climate change, habitat manipulation, and global transport.