5.2.4 Avoidance of unsuitable host trees and nonhosts

5.2.4 Avoidance of unsuitable host trees and nonhosts

Host plant suitability in insects has been reviewed by Scriber (1984). A host plant's suitability to bark beetles varies with its nutritional quality and composition of anti-insect toxins. Nonhost trees are probably less nutritional, and the beetle in most cases would not be adapted to detoxifying some of the nonhost toxins that have evolved for use against other herbivorous insects. A beetle would save much time and energy if it can discriminate the host from the nonhost and determine the suitability of a host by olfactory means without the need to land. Sometimes host and nonhost trees are in virtual contact and the beetle could land by mistake on the nonhost; however, short-range olfactory cues might indicate the inappropriateness of the bark substrate. If the beetle still could not decide, boring a short distance into the nonhost might reveal the lack of feeding stimulants or the presence of deterrents so that the beetle would leave.

According to studies in the previous section, bark beetles find their host tree by attraction to host volatiles (or after random landing and probing). However, it is becoming increasingly apparent that many beetles avoid nonhost trees due to specific odors. It is inherently more difficult to isolate repellents and inhibitors used in avoidance behavior than to isolate attractants since tests of avoidance require one to first isolate the attractive host odors and then present these with and without the possibly inhibitory nonhost odors. Several studies indicate that at least some species of bark beetle avoid nonhost volatiles during their search for host trees. The attraction of both T. piniperda and H. palliatus to ethanol (1-6 g/day) was reduced by odors from cut logs of nonhost trees, birch, Betula pendula, and aspen, Populus tremula (Schroeder, 1992). In future experiments, host logs (or monoterpenes and ethanol) should be tested instead of ethanol alone to simulate the host tree. Dickens et al. (1992) reduced the attraction response of D. frontalis, I. grandicollis and I. avulsus to aggregation pheromone by release of the green-leaf volatiles, 1- hexanol and hexanal. T. domesticum colonizes wood of deciduous trees (e.g. Fagus sylvatica, Quercus spp. Betula spp.) and is known to be attracted to ethanol (Magema et al., 1982; Paiva and Kiesel, 1985). Conifer monoterpenes of Scots pine and verbenone (from decaying conifers) reduced response of this species to ethanol (Byers, 1992a) and would provide a mechanism for avoiding nonhosts and unsuitable colonization areas. This also is valid for the hardwood-breeding Anisandrus dispar (Schroeder and Lindelöw, 1989).

In addition to their ability of discriminating the host from among nonhost trees, flying beetles are capable of determining the suitability of their host. Verbenone is found in relatively large amounts (æg) in male hindguts of several pest bark beetles of North America, D. frontalis, D. brevicomis, D. ponderosae, and D. pseudotsugae (Renwick and Vité, 1968; Rudinsky et al., 1974; Byers et al., 1984; Pierce et al., 1987), but in low amounts (ng) in T. piniperda (Lanne et al., 1987), or essentially absent in I. paraconfusus, I. typographus, and P. chalcographus (Byers, 1983b; Birgersson et al., 1984, 1990). Verbenone inhibits the attraction of these beetles to their respective aggregation pheromones (Renwick and Vité, 1969, 1970; Byers and Wood, 1980; Bakke, 1981; Byers et al., 1989c; Byers, 1993b). Some microorganisms isolated from bark beetles or their gallery walls, may convert alpha-pinene to cis- and trans-verbenol (Bacillus cereus, Brand et al., 1975), or trans-verbenol to verbenone (various yeasts, Leufvén et al., 1984). A fungal culture isolated from the mycangium of D. frontalis was able to convert trans-verbenol to verbenone, and it was proposed that this process may account for termination of attack (Brand et al., 1976). Verbenone is increasingly released from ageing logs of spruce and pine colonized by bark beetles (I. typographus, Birgersson and Bergström, 1989; T. piniperda, Byers et al., 1989c), possibly due to the activity of microorganisms.

Byers (1989a, b) speculated that if verbenone is a consistent signal of microbial activity in decaying hosts, then a bark beetle species may have evolved an avoidance to this compound (a kairomone) in order to avoid unsuitable hosts. This species then could have evolved to produce verbenone as a pheromone that reduced intraspecific competition, since the avoidance response was already existent. Other bark beetle species might then evolve to avoid species that produced verbenone (as an allomone), and so avoid interspecific competition. Sympatric species on the same host might coevolve response to and/or production of verbenone since the chemical could serve as a signal for several types of beneficial information (kairomone, pheromone, and allomone). Verbenone does not always inhibit bark beetles, for example, H. palliatus feeds in unhealthy or dying Scots pines that probably are releasing verbenone (Byers et al., 1989c), and the beetle's attraction to ethanol was not inhibited by verbenone (Byers, 1992a). Deciduous trees in a state of decay may not release verbenone since they probably do not have alpha-pinene, thus T. domesticum could evolve to avoid decadent nonhost pines by avoiding verbenone (Byers, 1992a).

In the case of conifer bark beetles, verbenone is increasingly implicated as a general sign of host unsuitability (due to microbial decay or competition with bark beetles). Therefore, it is paradoxical that conifers have not evolved the capacity of converting alpha-pinene, which they have in abundance, to verbenone in order to become repellent to bark beetles and thereby increase resistance.

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.