Schlyter, F., Byers, J.A. & Löfqvist, J. 1987a. Attraction
to pheromone sources of different quantity, quality, and
spacing: Density-regulation mechanisms in bark beetle
Ips typographus. Journal of Chemical Ecology 13:1503-1523.
The density of bark-beetle colonization of a tree could be regulated
by a quantitative effect of the pheromone signal from beetles in the
tree (cessation of release of attractive pheromone) or by a
qualitative effect (production of pheromone components inhibiting
attraction). The quantitative hypothesis was tested on Ips
typographus by varying the release rate of the two known attractive
compounds, 2-methyl-3-buten-2-ol (MB) and (4S)-cis-verbenol (cV). The
highest number of beetles were captured at traps with the highest
release rates. The catch was nearly proportional to the release of MB
and cV at a distance between traps of 12 m or more. At 6-, 3-, and
1.5-m distances between traps deployed in a triangular arrangement
there was still a good discrimination between release rates, but
relatively more beetles, especially males, were caught on the blank.
The lower release rates caught an equal sex ratio while the highest
release rate caught only about 30% males. The qualitative hypothesis
was tested by releasing the suspected inhibitors ipsdienol (Id) and
ipsenol (Ie), from traps in the same amounts as cV. Only small
effects were noted for I. typographus. However, the competitor I.
duplicatus was attracted to Id and inhibited by Ie, while the
predator Thanasimus formicarius was attracted to both compounds. On
the other hand, when the ratio of Id or Ie to cV was 10:1 or 0.1:1
rather than 1:1, they affected the numbers of I. typographus
attracted. A small amount of Id combined with the attractants
increased trap catch, while large amounts of Id or Ie decreased
attraction, especially when combined. Attack density regulation is
modeled as an effect of both quantitative and qualitative mechanisms
acting in sequence.