Trees Wanted—Dead or Alive! Host Selection and Population Dynamics in Tree-Killing Bark Beetles

Abstract

Bark beetles (Coleoptera: Curculionidae, Scolytinae) feed and breed in dead or severely weakened host trees. When their population densities are high, some species aggregate on healthy host trees so that their defences may be exhausted and the inner bark successfully colonized, killing the tree in the process. Here we investigate under what conditions participating with unrelated conspecifics in risky mass attacks on living trees is an adaptive strategy, and what this can tell us about bark beetle outbreak dynamics. We find that the outcome of individual host selection may deviate from the ideal free distribution in a way that facilitates the emergence of tree-killing (aggressive) behavior, and that any heritability on traits governing aggressiveness seems likely to exist in a state of flux or cycles consistent with variability observed in natural populations. This may have implications for how economically and ecologically important species respond to environmental changes in climate and landscape (forest) structure. The population dynamics emerging from individual behavior are complex, capable of switching between “endemic” and “epidemic” regimes spontaneously or following changes in host availability or resistance. Model predictions are compared to empirical observations, and we identify some factors determining the occurrence and self-limitation of epidemics.

Evolution of tree killing in bark beetles (Coleoptera: Curculionidae): trade-offs between the maddening crowds and a sticky situation

“…Such phenomena are known from other eruptive insect species, e.g., the migratory desert locust Schistocerca gregaria (Forskål) (Orthoptera: Acrididae) (Simpson et al 1999), as well as many bacteria (Ng and Bassler 2009), fungi (Rhome and Del Poeta 2009), and vertebrates (Cambui and Rosas 2012). In addition, there is evidence of a shift in the relative proportion of phenotypes associated with entering healthy trees within a bark beetle population during different population phases (Wallin and Raffa 2004), a relationship supported by modelling studies (Kausrud et al 2011) and articulated in natural history terms by early forest entomologists (Keen 1938;Evenden et al 1943). Thus, at endemic populations, most beetles tend to avoid highly defended hosts, whereas at epidemic populations many beetles accept such hosts (Boone et al 2011;Powell et al 2012).…”

“…Bark beetles that are facultative predators, on the other hand, have two quasi-stable population states, endemic and epidemic (Berryman 1982;Berryman 1983, 1987;Mawby et al 1989;Safranyik and Carroll 2006;Kausrud et al 2011). Even though trees are both abundant and long lived relative to bark beetles, their availability as hosts fluctuates as a result of the interaction between their defensive ability (genetic, environmental, phenological, demographic) and beetle population size (Berryman 1982;Berryman 1983, 1987;Safranyik and Carroll 2006;Kausrud et al 2011). Thus, predatory bark beetles cannot persist by solely using healthy trees, but like most predators must usually focus on weakened hosts, or even resort to scavenging when the availability of susceptible live hosts is poor (Lewis and Lindgren 2002;Wallin and Raffa 2004).…”

Group choices seemingly at odds with individual preferences

“…At low density, they attack trees that had blown over. At high density, the bark beetles still show a preference for such trees, but they are also able to aggregate on some living trees and to attack them [ 21 , 22 ].…”

Mass outbreaks of the spruce bark beetle Ips typographus in the context of the controversies around the Białowieża Primeval Forest

“…To some extent, such view is justified, as this takes place in the periods when european spruce bark beetle numbers are low, as well as under the conditions when forest ecosystems functioning is relatively stable. However, in subject literature, there has been stated that at increasing population numbers, the european spruce bark beetle also infests alive and healthy trees, as its mass attack overcomes tree resistance mechanisms and allows successful colonization leading to tree death (wermelinger 2004;Kausrud et al 2011). then a change of the status of the european spruce bark beetle is declared -it becomes a primary insect pest, because its attack on vigorous and healthy tree is the principal reason of host plant death.…”