Mixed messages across multiple trophic levels: the ecology of bark beetle chemical communication systems

Abstract

Chemical, physiological and behavioral components of pheromone communication have been described for a number of bark beetle species, yet our understanding of how these signals function under natural conditions remains relatively limited. Development of ecologically based models is complicated by the multiple functions and sources of variability inherent in bark beetle semiochemistry. This discussion addresses four ecological issues of chemical signaling in bark beetles: the effects of aggregation on individual fitness, the possibility of cheating, how plants can defend themselves against herbivores that employ aggregation pheromones, and the implications of variability in chemical communication systems to predator avoidance.

Stimulation of tree defenses by Ophiostomatoid fungi can explain attack success of bark beetles on conifers

“…The stimulation of these syntheses is achieved basically through beetle aggregation on the target trees (Berryman, 1972(Berryman, , 1976Raffa, 2001;Raffa and Berryman, 1983a;Wood, 1982), which multiplies the points of aggression and thus increases the mechanical stress inducing and stimulating the tree's hypersensitive response . Above a critical threshold of attack density (Berryman, 1976;Raffa, 2001;Raffa and Berryman, 1983a;Safranyik et al, 1975;Thalenhorst, 1958), tree resistance is depleted to such a low level that it is not able to stop aggressors anymore, finally resulting in beetle establishment and tree death. If it allows limiting interspecific competition because a low number of beetle species are able to attack living trees, 801p3 Ann.…”

“…Sci. 66 (2009) 801 F. Lieutier et al such a strategy however can lead to intraspecific competition (Raffa, 2001;Raffa et al, 1993). The opposite is true for the strategy of avoiding host defenses, which would be used by the true parasites.…”

“…Such a threshold has been quantified for some bark beetle species and, although it is influenced by genetic and environmental factors affecting the trees' level of resistance, it can be used to rank these species according to their aggressiveness. For example, the critical threshold of attack density, expressed in number of attacks per m 2 of bark, was estimated, depending on tree vigor, between 50 and 120 for D. ponderosae on lodgepole pine (Raffa and Berryman, 1983a;Raffa, 2001;Waring and Pitman, 1983), around 75 for D. rufipennis on Picea engelmanii (Raffa, 2001), around 45 for S. ventralis on Abies grandis (Raffa and Berryman, 1982a) and between 200 and 400 for I. typographus on Norway spruce (Mulock and Christiansen, 1986), while on Scots pine, that of I. acuminatus was around 850 and that of Tomicus piniperda during its trunk attacks around 400 Långström et al, 1992).…”

Effects of drought and heat on forest insect populations in relation to the 2003 drought in Western Europe

“…After a prolonged stress, the vibrations that result from the cavitation of xylem tissues could attract insects such as bark beetles or woodborers [8,60,75]. First mechanical defense of conifers against woodborer attacks, the constitutive resin flow is reduced under severe water stress but is increased under a moderate drought [75]; and see references in [53]). In severely stressed hosts, the concentration of secondary metabolites implicated in host resistance is also reduced.…”

Changes in a Primary Resistance Parameter of Lodgepole Pine to Bark Beetle Attack One Year Following Fertilization and Thinning

“…The increasing magnitude and extent of bark beetle outbreaks has been placed into a context of thresholds and feedback processes that included scales at the tree and forest stand levels [3]. Beetles must overcome a threshold of tree resistance parameters (including resin flow) to succeed in colonizing a tree and for reproduction to be successful [38,39]. Within this context, increasing available nutrient levels to trees through stand manipulations may result in modified resistance parameters that must be considered when making management decisions.…”