Host choice mechanisms in tree killing bark beetles

Bark beetles with the capacity to kill and colonize living trees are main pests of coniferous forests around the world. The European spruce bark beetle, Ips typographus, belongs to a group of bark beetles possessing facultative attack patterns. For long periods I. typographus is living in low populations only on seriously weakened hosts. However, following disturbances like storms or drought periods, the amount of suitable breading material increases dramatically and the population size of beetles may rise within a short time to epidemic levels. In the epidemic phase I. typographus behaves aggressively, attacking healthy trees and destroying large areas of Norway spruce forest. Bark beetles with this pattern of behaviour are exploiting a different ecological niche than during the endemic, low population phase: in broken or seriously weakened hosts they exploit a relatively poor source of nutrients without dealing with host defences. Instead they have to deal with a higher degree of interspecific competition. On the other hand, a living host provides breeding material, which is rich in nutrients and relatively free from interspecific competition, but there beetles have to fight the defence responses of the host tree. Host defences can be overcome by mass attack of thousands of beetles on the same tree, which is coordinated by aggregation pheromones. The trade-off between evaluating host quality and risk rating host defences is a great challenge for the pioneering beetles, that initiate an attack before any pheromone plumes exist. The host choice mechanism can be divided in several distinct steps:1.habitat location during flight2.host location during flight3.host evaluation after landing, involving detection of host suitability and spacing between species and individuals to avoid inter- or intraspecific competition.4.risk rating host defences after entrance into host phloem.The question whether the beetles’ attack behaviour is ruled by pheromone attraction alone or is guided by markers for host suitability has been an issue of debate for decades.The peripheral nervous system of bark beetles possesses olfactory receptor neurons for both pheromones, host volatiles and non-host volatiles. Several studies support the theory that non-host volatiles cues play an important role in habitat location, while host volatiles are involved in host finding and host acceptance, either alone or through modulating pheromone attraction. Host defence responses consist of physical barriers and chemical compounds that may be detrimental to invaders. Conifers maintain a certain level of preformed (constitutive) defence that can be enhanced during and after the attacks, e.g. by the increased production of defensive metabolites and development of defensive structures. The different metabolites involved in a trees defence responses may be important markers for beetles to evaluate the trees’ defensive ability, and guide them in their decision to enter a host.Because the population density is an important factor influencing the beetles’ success in overwhelming a trees defence, it has been hypothesized that it also will modulate beetles behaviour, either directly or through its influence on the beetles’ quality. This introductory paper is written to obtain an overview over the literature dealing with different aspects of host choice in bark beetle species exhibiting so called aggressive host colonisation behaviour. Different hypotheses and aspects of importance for the host choice behaviour are discussed. Finally, I propose a host choice model for the European spruce bark beetle Ips typographus.