Byers, J.A. 1996. An encounter rate model of bark beetle
populations searching at random for susceptible host
trees. Ecological Modelling 91:57-66.
Iterative equations were developed that predict the encounter rate
between a population of moving animals and a population of stationary objects,
where the animals cease to search upon finding an object. The encounter rate
through time depends on the number of searching animals (bark beetles), number
of stationary objects (host trees), average speed of the animals, average
radius of the object, and area of the search arena. The iterative equations
were used in a computer program to vary these parameters with regard to flight
dispersal of the bark beetle Ips typographus (Coleoptera: Scolytidae) during
their search for susceptible host trees of Norway spruce (Picea abies).
Realistic parameters of flight speed, numbers of beetles and susceptible host
trees, tree diameters, density of healthy trees, search area, and time
searching were held constant while certain of these parameters were varied in
computer model runs. In most cases, significant proportions of the modelled
bark beetle population (of which individuals fly forward with a random
component) found the relatively few susceptible host trees suitable for
colonization. Only at very low effective flight speeds (due to longer stays on
trees) or with widely distributed hosts of smaller diameter, did relatively
few beetles find suitable hosts. Once the "pioneer" beetles find susceptible
hosts, release of aggregation pheromone would greatly increase the effective
radius of the host and allow rapid concentration of the population on these
trees. The model suggests that primary attraction to host tree volatiles is
not mandatory for host finding and selection in many species of bark beetle.
Evolution of an olfactory response to host volatiles is more probable in
species with low population densities or widely dispersed host plants, or
both. Also, there would be little selection pressure on a bark beetle species
to evolve aggregation pheromones if they can respond over some meters to plant
volatiles that reveal the location of a susceptible host.
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