5.3 FEEDING STIMULANTS AND DETERRENTS

Presumably, the beetle must not only determine that the bark underneath is the proper host and is suitable for reproduction, but Pityogenes chalcographus it must also judge potential competition by whether nearby areas have bark beetles beginning their attacks. Many species of bark beetle bore their entrance holes in a spaced or uniform pattern, indicating the beetles are territorial in order to avoid competition (Byers, 1984, 1992c). In some cases the beetle will bore through the outer bark, regardless of the host, until it encounters the phloem. For example, I. paraconfusus will bore through the outer bark of the nonhost white fir, Abies concolor, as readily as through bark of the host ponderosa pine. However, the beetle only bores about 1 mm in white fir phloem and then leaves (Elkinton and Wood, 1980). At this time gustatory stimulants and deterrents (also possibly olfactory cues) are balanced in a decision whether to continue feeding and excavating the gallery. The beetle probably can determine whether the host tissue is of good quality in terms of nutritional and moisture factors (Webb and Franklin, 1978). The phloem of ponderosa pine, sugar pine (P. lambertiana), Douglas-fir (Pseudotsuga menziesii), red fir (Abies magnifica), and several other conifers contain about equal amounts of glucose, fructose and sucrose (Smith and Zavarin, 1960). Bark beetles have been induced to feed or lay eggs on several diets, but the most successful contain some percentage of host (usually phloem) tissue (Jones and Brindley, 1970; Richeson et al., 1970; Whitney and Spanier, 1982; Conn et al., 1984; Byers and Wood, 1981b), indicating the presence of feeding or ovipositional stimulants. Sucrose was found in preliminary experiments to increase feeding by I. paraconfusus in powdered cellulose diets (Byers and Wood, 1981b).

Few studies have attempted to isolate feeding stimulants in conifer-feeding bark beetles, and none have isolated specific compounds. Elkinton et al. (1981) extracted ponderosa pine phloem successively with diethyl ether (partitioned with water), water and then methanol, and added these extracts to powdered cellulose diets. I. paraconfusus beetles were then given a choice between a control diet and a diet with extract. The diet with the ether extract did not cause beetles to remain longer, but did cause more feeding but no preferential boring. The water partition of the ether extract only caused beetles to remain longer. The water extract elicited more boring and feeding, while the methanol extract was inactive since feeding stimulants had already been extracted by the ether and water treatments. These results indicate that several compounds function in gustatory preferences. Solvent (methanol-water-benzene) extracts of lodgepole pine bark (Pinus contorta) were absorbed by tissue paper and shown to induce feeding by D. ponderosae (Raffa and Berryman, 1982a). The benzene fraction induced biting but not feeding while the polar fraction (water- methanol) caused continued feeding. Differences in feeding preferences for bark extracts were large between trees, but these differences could not be attributed to amounts of 13 monoterpenes as determined by gas chromatography (GC). Also, extracts of trees judged resistant, because beetles that had been forced to attack in cages either refused or discontinued attack, were as stimulatory to feeding beetles as those from susceptible trees. In contrast, Hynum and Berryman (1980) found greater feeding preferences for extracts of bark of susceptible than of resistant lodgepole pine. However, the susceptible trees had been killed by the beetles before solvent extraction, which might have allowed microorganisms to produce additional feeding stimulants. White (1981) also found differences in gustatory deterrent and stimulatory properties of bark extracts from different trees of loblolly pine, P. taeda.

In beetles of deciduous trees, most work on feeding stimulants and deterrents involves the elm bark beetle, S. multistriatus. Vanillin and syringaldehyde are short-range attractants inducing feeding in S. multistriatus (Meyer and Norris, 1967b). Feeding stimulants were isolated from the bark of American elm, Ulmus americana, of which one was partially identified as a pentacyclic triterpene (Baker and Norris, 1967). Later some lignin intermediates and phenolics were indicated (Meyer and Norris, 1974). Doskotch et al. (1973) succeeded in identifying another feeding stimulant in elm bark as a catechin xyloside. A tritiated catechol feeding stimulant was shown to penetrate the gustatory receptor of S. multistriatus (Borg and Norris, 1971). Scolytus rugulosus are stimulated to feed in fruit trees by several phenolic compounds (Chararas et al., 1982).

S. multistriatus was induced to feed on sucrose pith disks when volatiles from benzene extracts of bark of nonhost trees (Eastern cottonwood, Populus deltoides, and yellow buckeye, Aesculus octandra) were placed 7 mm away (Baker and Norris, 1968). However, these nonhost trees were not fed upon since they contained nonvolatile feeding deterrents as shown by lowered feeding on a mixture of host and nonhost extracts compared to host extracts. S. multistriatus beetles do not attack the nonhost hickory, C. ovata, due to the presence of juglone (5-hydroxy-1,4-napthoquinone), a feeding deterrent (Gilbert et al., 1967). Elm tree tissue infected by the fungus Phomopsis oblonga are avoided by S. multistriatus due to several feeding deterrents of complex structure, e.g., oblongolide (isomer of dimethylnapthofuranone), a norsesquiterpene lactone, two tiglic esters of 5,6-dihydro-5-hydroxy-2-pyrones, nectriapyrone, 4-hydroxyphenylethanol, 5-methylmellein as well as acids of 2-furoic, orsellinic, 3-nitropropanoic, and mellein-5-carboxylic (Begley and Grove, 1985; Claydon et al., 1985).

Diterpene acids (e.g., abietic, levopimaric, neoabietic and palustric acids) have been isolated from ponderosa pine oleoresin (Anderson et al., 1969; Himejima et al., 1992) and these are known from other conifers to be antifeedants against aphids and sawflies (Wagner et al., 1983; Schuh and Benjamin, 1984; Rose et al., 1981), but have not been tested on bark beetles. Tannins, phenolics and terpenoids that can inhibit feeding or digestion in other insects (Berenbaum and Isman, 1989) could also affect bark beetles, but again there are no reports. Ponderosa pine bark extracted first with ether yields behenic and lignoceric acids, fatty alcohols, resin acids, and flavonols (quercetin and myricetin, pinoquercetin, pinomyricetin and dihydroquercetin), a subsequent acetone extract contains tannins and phlobaphenes, while a hot-water extract has tannin (6-11 % dry weight of bark) and carbohydrates (Anderson, 1962; Anderson et al., 1969). Many of these compounds are found only in the outer bark and although they may be important in deterring nonhost bark beetles, at least the host-adapted I. paraconfusus does not eat the outer bark (Elkinton and Wood, 1980).
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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.