Attack spacing in bark beetles. The bark of Norway spruce Picea abies has been scrapped by hand to "sand" away the
surface to reveal the attack sites which have then been circled with a marking pen. Later a plastic sheet will be overlaid and the attacks marked on the
sheet for later measurements in the lab. Bark beetles that feed in the relatively thin phloem layer just under the
bark of conifers often must tolerate high levels of competition. It is always advantageous to avoid competition whenever possible.
Bark beetles probably have evolved several behavioral mechanisms to avoid competition, such as not boring into the bark
too close to neighbors that have already bored into the bark. Individuals of each bark beetle species appear to have
an inherent spacing distance which the beetles prefer to respect. As the distance between boring sites (called attacks) increases, the beetles
probably choose sites at random (or more or less at random due to random imperfections in the surface structure of the bark). The mechanisms for the
spacing are not known really, but may include sound (stridulation = squeaks by rubbing body parts together), by chemical pheromone components, by fighting, and
by visual inspection of boring holes. In any case, larger beetles have larger minimum allowed distances (MAD). For example, Ips typographus has
a MAD of 2.5 cm while the much smaller Pityogenes chalcographus has a MAD of 1.7 cm. Dendroctonus brevicomis and Tomicus piniperda have
a MAD of about 4.5 cm. The MAD was first proposed in Byers (1984) .
Another method for visualizing spacing and territoriality in organisms is called Dirichlet tessellation .
Images © 1996 by John A. Byers, Chemical Ecology.