5.1.2B Sensory responses to chemicals

The electrophysiological response of an insect to semiochemicals can be studied with the electroantennogram (EAG) of the whole antenna or the single-cell technique that measures responses of specific receptor cells (Payne, 1979). Each antennal receptor cell contains multiple acceptor sites that interact with the chemicals. Bark beetle olfactory cells on the antennae have been shown to be of several functional types, which probably are found in every species: (1) a highly specific type such as the ipsdienol-sensitive cells in I. paraconfusus and I. pini that is responsive only to one of two possible enantiomers (cannot be superimposed but otherwise identical structures, see alpha-pinene in Fig. 4), Fig. 4 - Host tree monoterpenes
Fig. 4. Major monoterpenes of conifers. Note that the enantiomers of alpha-pinene are identical except that they are non-superimposable (mirror images). Camphene, B-pinene, 3-carene, B-phellandrene, and limonene also have two enantiomers, although only (-)-B-pinene and (+)-3-carene are found in trees (Mirov, 1961). Myrcene and terpinolene are achiral.
 (2) a pheromone-sensitive type that is also responsive to some other synergists or inhibitors such as the frontalin cells of D. frontalis (the cells have at least two acceptor types each specific for one enantiomer of frontalin, Fig. 5), and (3) "generalist" types that respond to host monoterpenes as well as pheromones to some extent (Mustaparta et al., 1980; Payne et al., 1982; Dickens et al., 1985; Dickens, 1986).

Fig. 5 - Bark beetle pheromone components
Fig. 5. Pheromone components of bark beetles. Key: aggregation component (a), inhibitor of aggregation (i). Row 1: 2-methyl-3- buten-2-ol (a, I. typographus); 3-methyl-3-buten-1-ol (a, I. cembrae), 4-methyl-3-heptanol (a, S. multistriatus). Row 2: sulcatol (a, G. sulcatus, G. retusus); seudenol (D. pseudotsugae, D. rufipennis, D. simplex); MCH (i, D. pseudotsugae); lanierone (a, I. pini). Row 3: cis-verbenol (a, I. paraconfusus, I. typographus, I. calligraphus); trans-verbenol (a, D. ponderosae, T. minor; i, D. brevicomis); verbenone (i, Dendroctonus); chalcogran (a, Pityogenes). Row 4: ipsenol (a, Pityokteines curvidens, and many Ips: e.g. I. paraconfusus, I. grandicollis), ipsdienol (a, many Ips, e.g. I. paraconfusus, I. duplicatus, I. pini, I. calligraphus, I. avulsus); amitinol (a, I. amitinus); E-myrcenol (a, I. duplicatus). Row 5: (+)-exo-brevicomin (a, D. brevicomis, Dryocoetes); (-)-exo-brevicomin (a, Dryocoetes); methyl decadienoate (P. chalcographus). Row 6: frontalin (a, many Dendroctonus); endo-brevicomin (i, D. frontalis); multistriatin (a, S. multistriatus); lineatin (a, T. lineatum). References to above pheromones are in reviews by Borden (1982) and Byers (1989a), and the following (Bakke, 1975; Baker et al., 1977; Lanne et al., 1987; Borden et al., 1987; Byers et al., 1989b, 1990a, b; Teale et al., 1991; Camacho et al., 1993).
 The technique of "differential adaptation" first attenuates an electrophysiological response by exposing a receptor cell (or antenna) to a high concentration of a compound until specific acceptor sites are "saturated", then these sites are exposed to a different volatile to see if any responses are elicited (Payne and Dickens, 1976; Payne, 1979; Dickens et al., 1985). By using this technique with single-cell recordings it was shown that D. pseudotsugae has at least four olfactory cell types (Dickens et al., 1985). Three types are each most sensitive to either MCH, seudenol, or frontalin (Fig. 5), although they are all stimulated somewhat by all of these pheromone components. The fourth cell type is most sensitive to host compounds released by excavating beetles that are synergists of pheromone components. Acceptors can be specific for one enantiomer of a chiral mixture (Payne et al., 1983) and there may be either only one type of acceptor per cell (e.g. (+)- or (-)-ipsdienol in I. paraconfusus (Mustaparta et al., 1980) or both types of chiral acceptors on the same cell (e.g. (+)- and (-)-frontalin in D. frontalis (Payne et al., 1982) and D. pseudotsugae (Dickens et al., 1985).

The antennae of both sexes of I. paraconfusus are equally sensitive (EAG) to natural pheromone and to (+)-ipsdienol (Light and Birch, 1982). However, the males have been shown to be relatively less attracted by higher concentrations of synthetic pheromone components, and they were not as likely to fly directly toward the pheromone source as were females (Byers, 1983a). Thus, the sexual differences in behavioral response of I. paraconfusus to aggregation pheromone appear to be the result of differences in central nervous system (CNS) integration rather than differences in peripheral receptors. Cis-verbenol (Fig. 5) elicits a similar electrophysiological dose-response curve for both sexes of I. typographus, but some sexual differences exist for response to methyl butenol (Dickens, 1981). Similar to I. paraconfusus, males of I. typographus orient less directly to pheromone at the final landing on the host than females (Schlyter et al., 1987c); this behavioral difference could be due either to CNS differences between the sexes or to the lesser receptor sensitivity of males to methyl butenol. Mustaparta et al. (1980) have shown that eastern US populations of I. pini have separate cells for each of the two ipsdienol enantiomers that are synergistic attractants (Lanier et al., 1980). However, the exposure of the two cells to the enantiomers together did not synergistically increase the nerve impulse rate, and so it was concluded that synergism in this case acts at the CNS level.

Of the three or four receptor types found in most species it appears that individual receptors within a type also can vary in their response spectrum to various chemicals (Dickens, 1986; Dickens et al., 1985; Mustaparta et al., 1980, 1984; Tommerås et al., 1984). In the few cases so far known, the cell responsive to host-plant chemicals are present in both sexes, but the host- selecting sex (males of Ips or females of Dendroctonus) has a lower threshold to plant compounds (Dickens, 1981, 1986; Dickens et al., 1983). However, the role of plant compounds in long-range orientation of many species of Ips is not certain (as discussed in part 5.2). Probably short-range behaviors on the bark (e.g., gustatory responses) are influenced by host volatiles, but this also is poorly understood.

Some bark beetles have receptors sensitive to compounds they do not produce but are found in several other bark beetles as pheromone components (Tommerås et al., 1984). Lanne et al. (1987) showed compounds not found in T. piniperda (exo-brevicomin, ipsenol, and pheromone components of other bark beetle species) elicited an EAG response in the beetle. Some of these compounds are found in competing species of bark beetles, but other compounds are probably found only in species colonizing nonhost trees (e.g., Norway spruce). Three sympatric species of the southern U.S., Ips avulsus, I. calligraphus, and I. grandicollis, are most responsive electophysiologically to components of their own pheromones (ipsenol, ipsdienol, cis-verbenol); but they can also respond to components of their sibling species as well as frontalin and verbenone from D. frontalis and alpha-pinene from their host pine released by activities of other species (Smith et al., 1988).

The difficulties with electrophysiological methods for unraveling ecological phenomena during host finding and tree colonization are three-fold: (1) the electrical measurements may not be correlated to the behavior of interest, (2) the nerve impulses vary between cell types (Mustaparta et al., 1980) of which all are not located and tested, and (3) the electrical patterns are further integrated in the complexity of the CNS.
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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.