5.2.2 Orientation mechanisms for attraction to semiochemicals

5.2.2 Orientation mechanisms for attraction to semiochemicals

Primary attraction to the host tree can be considered to occur over a "long-" or "short-range." The concept of range differs between authors and depends also on the insect considered. Here I consider long-range attraction for bark beetles to be flight orientation over several meters to a semiochemical source. In reality the division is arbitrary, since bark beetles may orient over practically any distance depending on release rate of semiochemical, although at higher release rates the insect may not closely approach the source due to adaptation (Baker et al., 1988). However, the concept of range is still valid since at natural release rates the beetle will have a range of orientation distances which can be considered as long- or short-range in comparison to orientation distances elicited by other semiochemicals.

Attraction to pheromone is certainly long-range. Three parallel lines spaced 4.6 m apart and hung with sticky screens spaced every 1.5 m intercepted I. paraconfusus in a "V"-shaped pattern narrowing to a pheromone source of 50 males boring in a pine log (Byers, 1983a). In this experiment beetles appeared to be orienting over a distance of at least 17 m. S. quadrispinosus beetles were intercepted by passive traps 12 m from a girdled hickory tree that was attracting these beetles (Goeden and Norris, 1964). The distance over which beetles respond anemotactically depends primarily on the release rate of the volatile (under mild wind conditions). In Denmark, I once observed I. typographus flying slowly upwind (0 to 0.5 m/s ground speed) in 3 m/s gusty winds to a large fallen spruce tree under massive attack. During their orientation to pheromone, beetles were flying at 3-6 m height from at least as far as 50 m downwind from the tree. Jactel (1991) estimated that the maximum attraction distance of I. sexdentatus to pheromone-baited traps was 80 m.

Byers et al., (1989a) proposed the "effective attraction radius" (EAR) as an index of attraction strength for a semiochemical release rate from a trap. The EAR is the radius that a trap would need to be enlarged, as a spherical "passive" trap, in order to intercept as many dispersing insects as were actually caught on the trap when baited (Byers et al., 1989a). For example, the EAR of T. piniperda to a blend of three host monoterpenes, released at rates equivalent to a cut log of Scots pine from each of 10 traps along a 12 m high pole, was largest at the lowest trap (EAR = 1.3 m). The same design found an EAR of 3.2 m for I. typographus response to a blend of its pheromone components (Fig. 5). These comparisons indicate that the effective attraction radius can be larger for a pheromone than for host volatiles. However, both these values would be greater at higher chemical release rates.

The optomotor anemotaxis mechanism for orientating to pheromone sources proposed for insects, especially moths (David et al., 1982, Baker, 1989), also appears to function in bark beetles (Choudhury and Kennedy, 1980). In this theory, a bark beetle attempts to fly directly upwind when in contact with a packet of pheromone-laden air of the plume, but casts (flies from side to side with respect to the source) when contact is lost. The beetle senses the wind direction while flying by observing the ground below: in no wind, or head-on wind, the ground moves directly underneath during flight. However, if the visual ground field also moves from right to left somewhat, for example, then wind is coming from the left, and the beetle turns to the left to minimize the transverse ground shift and keep the ground moving directly underneath so that the insect heads upwind and toward the pheromone source.

Short-range attraction could be considered to occur within one meter such as when flying along the trunk as I have observed for T. piniperda; however, after landing the beetle must use a different mechanism than optomotor anemotaxis. During walking the ground does not move under the beetle due to wind, but the beetle probably can sense wind direction by mechanoreceptors and use pheromone- modulated anemotaxis combined with "casting" or circling movements to locate the odor source. Beetles walking in an arena with laminar airflow respond to a point source of synthetic pheromone (or air from an attacked log) by walking directly upwind within the odor plume. If they happen to walk outside the plume as it narrows to the source, they would experience a concentration gradient decline as they walked. By turning slightly with respect to the upwind angle (as detected by tactile hairs) they would either soon re- contact the odor or the concentration would further decline. In the later case they could reverse the angle or continue turning in a circle which would bring them into odor contact, whereupon they could walk directly upwind again. This mechanism is consistent with observations of beetles responding to pheromone or host odors in a laboratory olfactometer (see Birch, 1984) for species of Ips, Dendroctonus, Tomicus, and Pityogenes (Byers et al., 1979; Byers and Wood, 1981a; Lanne et al., 1987; Byers, 1983a; Byers et al., 1990a, b). Borden and Wood (1966) show tracings of tracks of I. paraconfusus walking upwind to pheromone.

Akers (1989) studied orientation of I. paraconfusus to pheromone in a laboratory olfactometer. He found that beetles increased their counterturning rate (turning left then right etc.) in relation to a decline in the rate of concentration increase as they approached the source. In a second study, beetles walked in all directions with respect to the wind without pheromone present, but when in a pheromone plume they decreased their angle to the source (although usually not heading directly upwind) and their turning rate increased (Akers and Wood, 1989a). These generally upwind walking angles and increased turning rates would be expected for beetles orienting to a pheromone source. An important finding was that beetles did not usually walk directly upwind but at slightly different angles. This was attributed to "inaccurate anemotaxis" rather than a preference for a specific angle with respect to wind and pheromone (anemomenotaxis). Bark beetles appear to have a third mechanism for finding odor sources in the absence of wind. Akers and Wood (1989b) discovered that I. paraconfusus can find pheromone sources in still air. The turning rate increased, but only slightly, as the beetles approached the diffusion source, while the mean heading angle to the source decreased as the beetles neared the source (but not for the last 15 cm). Thus, bark beetles are able to use any of several orientation mechanisms, depending on the environmental context, to locate hosts and mates.

Byers, J.A. 1995. Host tree chemistry affecting colonization in bark beetles, in R.T. Cardé and W.J. Bell (eds.). Chemical Ecology of Insects 2. Chapman and Hall, New York, pp. 154-213.