Host selection paradigm

It should now be clear that the initial stimuli involved in host selection may be initially directional (primary attractants) or non-directional (random landing) depending on the species, but that highly directional stimuli (pheromones) take over once a tree is attacked. Using the example of 7. paraconfusus, initial positive host selection seems to occur only after landing and feeding, but 

This distinction clarifies the surprising differential sensitivity of male and female 7. paraconfusus to components of their pheromone and to that of I. pint. Males and females are equally sensitive to their natural pheromone and to its component, (+ )ipsdienol, over a wide range of concentrations. However, females are significantly more sensitive to pheromonal ( + )ipsdienol than to allomonal (-)ipsdienol (the pheromone of I. pini), whereas males are more sensitive to allomonal (- )ipsdienol than to their own pheromonal (+ )ipsdienol (Light and Birch, 1982). There is a clear adaptive advantage for males to locate new host material quickly and to avoid resources occupied by /. pini, unless nothing else is available, since if the two species do co-colonize a resource, the reproductive potentials of both are reduced. This premium on selection of unoccupied resources underlies the high sensitivity of males to allomonal ipsdienol. The interruption of response in 7. paraconfusus by verbenone from D. brevicomis may have a similar basis. 

Other species are deterred from material infested by 7. paraconfusus in the same way that 7. paraconfusus is deterred from attacking occupied resources. Throughout colonization, until the progeny leave the host, there is constant interaction with other species, particularly parasites and predators of parent adults, larvae and emerging adult beetles. 

During the evolution of bark beetles, their hosts and associated fungi, parasites, predators and organisms, the composition of bark beetle pheromones will probably have been modified as other species evolved responses to components of the pheromone and exerted selection pressures on the beetle population. There is only circumstantial evidence that this might have happened. For example, in the experiments on widely separated populations of 7. pini (Lanier et al., 1972), California beetles attracted more local 7. pini than did New York beetles, and vice versa in New York. However, New York beetles in California attracted far more of the local predator E. lecontei than did the local 7. pini. California and New York 7. pini use different ratios of the enantiomers of ipsdienol as their pheromones. The high resolution of (-)ipsdienol in California populations versus the blend in New York supports the idea of coevolution of chemical systems of predator and prey production of and response to (+ )ipsdienol being eliminated in California by a predator which had evolved specific responses to it. Perhaps in the absence of E. lecontei in New York there was no pressure to resolve the blend. 

The small proportion of (- )ipsdienol in the pheromone blend of 7. paraconfusus might similarly be advantageous in spite of it being the pheromone of its competitor, 7. pini, because (- )ipsdienol also appears to deter 7. latidens. The exploitation of pheromone components by predators need not necessarily select for different attractive blends but could result in selection for components that interrupt the predator response. For example, the highly specific response of T. chlorodia to exo-brevicomin from female D. brevicomis is interrupted by 

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Wang H, Li J, Bostock RM, Gilchrist DG (1996) Apoptosis A Functional Paradigm for Programmed Plant Cell Death Induced by a Host-selective Phytotoxin and Invoked During Development. Plant Cell 8 375... 

Fig. 12.4 Behavioral paradigm for host selection in Ips paraconfusus from adult emergence to construction of galleries and egg laying.

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