Host-tree finding in bark beetles. It is not known how bark beetles find host trees - Two hypotheses: (1) do beetles land at random on trees (both healthy and weakened) and test them to see if they can be colonized, or (2) are beetles attracted from some distance (several meters) to susceptible or damaged trees that are less resistant to attack? Some bark beetles are known to be attracted to tree chemicals (monoterpenes found in resin) from some distance (several meters or tens of meters). A good example is Tomicus piniperda which is attracted to alpha-pinene, 3-carene, and terpinolene (monoterpenes in wound resin). Other bark beetles such as Dendroctonus brevicomis seem to find trees at random, since sticky-traps showed that landing was similar on dead trees and living trees. A theoretical question that can be posed is: How far would a beetle fly in a "normal" Norway spruce forest of 30 cm diameter trees at the usual density? The answer is 67 m on average. This can be visualized if we let loose 1000 beetles and measured how far they flew before striking a tree - then the average distance would be 67 m (see Byers (1996) ).

Images © 1996 by John A. Byers, Chemical Ecology.