Flight dispersal

After emergence, bark beetles begin a dispersal flight in which they seek suitable trees for feeding and reproduction. Factors thought to regulate the abundancy of bark beetle populations are the availability of brood material and the density of the beetle population. The disperal flights of the different generations of one beetle species show overlap and cannot be exactly seperated in time. To avoid interspecific competition, the moment that most beetles from one generation start to disperse varies between species {[344],[352]}. Dispersal and finding a new host is a risky phase within the beetles' life cycle. The mortality rate during this search is over 50 %. Finally, only 15-25 % of the adults will succesfully locate a host tree, bore into the bark and produce offspring in the phloem-cambium layers. The average life expectancy of a bark beetle when dispersing is one to a few days {[458]}. Swedenborg et al.{[444]} reported that H. rufipes in Minnesota dispersed to uninfested brood wood mainly during the late afternoon (between 17:30 and 19:30 hours).

Bark beetles generally fly from release sources in all directions. European elm bark beetles are reported to fly toward light, especially UV light radiation from the sky {[230]}. When winds are strong the beetles appear to passively drift with the wind {[361]}. Choudhury et al. {[469]} reported that newly-emerged unfed and previously unflown S. multistriatus fly upwards and downwind. The longer beetles have spent in the air, the greater the chance that they will fly upwind. Von Keyserlingk {[347]} observed that S. scolytus flies downwind in open space. In shaded areas S. scolytus tends to fly into the open sky through an opening in the canopy regardless of wind direction. The beetle courses upwind when approaching an odour source. By flying downwind beetles can cover more area with less energy expense, specially at higher wind speeds (Photo 37, {[361]}).