Journal of Chemical Ecology, VoL 18, No. 9, 1992

OLFACTORY RECOGNITION OF NONHOSTS ASPEN A N D BIRCH BY CONIFER BARK BEETLES Tomicus piniperda A N D Hylurgops palliatus

L.M.

SCHROEDER

Swedish University of Agricultural Sciences Division of Forest Entomology S-750 07 Uppsala, Sweden (Received January 15, 1992; accepted May 5, 1992)

Abstract--The field response of Tomicus piniperda (L.) and Hylurgops palliatus (Gyll.) (Coleoptera: Scolytidae) to the attractant ethanol in combination with volatile wood constituents released from the nonhost tree species Populus tremula L. (Salicaceae) and Betula pendula Roth (Bctulaceae) was studied using flight barrier traps. The attraction of both species decreased when aspen or birch wood was added to the ethanol bait. The same was true for Rhizophagus depressus (F.) (Coleoptera: Rhizophagidae), a predatory species associated with conifer bark beetles. Glischrochilus quadripunctatus (L.), Epuraea bickhardti St.-Claire Deville, E. unicolor (Oliv.) (Coleoptera: Nitidulidae), and Rhizophagus parvulus (Payk.) (Coleoptera: Rhizophagidae) were caught in higher numbers in traps baited with both ethanol and wood of aspen or birch than in traps baited with ethanol alone. In a separate experiment, landings of T. piniperda and H. palliatus on nonhosts (black plastic tubes) were demonstrated with sticky traps.

Key Words--Tomicus piniperda, Hylurgops palliatus, Rhizophagus depressus, R. parvulus, Glischrochilus quadripunctatus, Epuraea bickhardti, E. unicolor, Coleoptera, Scolytidae, Rhizophagidae, Nitidulidae, ethanol, nonhost volatiles, Populus tremula, Betula pendula, olfactory orientation, attraction, decreased attraction.

INTRODUCTION M o s t scolytids, as w e l l as m a n y o t h e r b a r k - a n d w o o d b o r i n g insects, r e p r o d u c e o n l y in d e a d o r w e a k e n e d trees. T h i s r e s o u r c e is g e n e r a l l y s c a r c e a n d t e n d s to b e d i s t r i b u t e d u n e v e n l y in s p a c e a n d t i m e . F u r t h e r m o r e , h o s t t r e e s are o f t e n 1583 00984)331/92/0900-1583506.50/0 9 1992 Plenum Publishing Corporation

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SCHROEDER

present in mixed stands with nonhost species. Thus, such insects need effective means for finding and recognizing suitable breeding material. Many bark beetles are attracted by host volatiles, alone or in combination with pheromones (e.g., Borden, 1982). Host odors may be characteristic of individual tree species. The major components of the volatile essential oils in trees responsible for characteristic odors are the terpenoids (see Harborne and Turner, 1984, and references therein). Some bark beetles, including both conifer and broad-leaved breeding species, are attracted by mono- and sesquiterpenes present in their host trees (e.g., Byers et al., 1985; Millar et al., 1986; Schroeder and Lindel6w, I989). Volatiles may also provide information about tree vitality. High concentrations of monoterpenes in the air may indicate the presence of damaged trees (Ikeda et al., 1980; Str6mvall and Pettersson, 1991). Degradation processes in dead or dying tree tissue result in the formation of substances that are absent or present in only small amounts in healthy trees. One such substance is ethanol, which is formed in dying tissue of both conifers (Moeck, 1970; Cade et al., 1970; Ikeda et al., 1980; Sj6din et al., 1989) and broad-leaved trees (Elliot et al., 1983). Ethanol attracts some scolytids reproducing in broad-leaved trees (Roling and Kearby, 1975; Montgomery and Wargo, 1983) as well as some scolytids breeding in coniferous trees (e.g., Moeck, 1970; Schroeder and LindelSw, 1989). Little is known about the mechanisms underlying bark beetle rejection of nonhost tree species. Rejection could be based on a lack of certain host characteristics and/or a negative response to some nonhost stimuli. Two ambrosia beetles reproducing in broad-leaved trees, Trypodendron domesticum (L.) and Anisandrus dispar (F.), respond negatively to c~-pinene, one of the major monotelpene constituents of both Scots pine, Pinus sylvestris L., and Norway spruce, Picea abies (L.) Karst. Both beetles are strongly attracted by ethanol alone, while e~-pinene-baited and unbaited traps catch a few or no individuals. When a-pinene is released along with ethanol, attraction to the latter compound is inhibited or decreases strongly (Nijholt and Sch6nherr, 1976; Schroeder and Lindel6w, 1989). Single cell responses to the odor from birch bark have been demonstrated in the conifer scolytids Trypodendron lineatum (Oliv.) and Ips typographus (L.) (Tcmmer~s, 1989; Tommer~s and Mustaparta, 1989). For some scolytids, laboratory studies demonstrate that the rejection of nonhost tree species can be based on gustatory cues. lps paraconfusus Lanier selected a host over a nonhost only after the beetles had bored through the outer bark (Elkinton and Wood, 1980). In the presence of certain nonhost substances, Scolytus multistriatus (Marsh.) rejected host-tree twigs suitable for maturation feeding (Gilbert and Norris, 1968). The present study was initiated to give information about the behavioral response of the two conifer bark beetles, Tomicus piniperda (L.) and Hylurgops

RECOGNITION OF NONHOSTS BY BARK BEETLES

1585

palliatus (Gyll.), to nonhost trees. Both species are common throughout Scandinavia. The flight periods of both species occur in early spring. In Scandinavia T. piniperda reproduces almost exclusively in Scots pine while H. palliatus reproduces in both Scots pine and Norway spruce, the two major conifers in this region. Both species are restricted to weakened or dead trees for their reproduction, H. palliatus preferring a somewhat more decayed breeding substrate than T. piniperda. The olfactory orientation to breeding material has been studied in both beetles in recent years. T. piniperda lacks an aggregation pheromone (Perttunen et al., 1970; Byers et al., 1985; Lanne et al., 1987). The only study conducted with 11. palliatus in this respect did not present any evidence of an aggregation pheromone (Kohnle, 1985). T. piniperda is strongly attracted by certain Scots pine monoterpenes and weakly attracted by ethanol, while the reverse is true for H. palliatus (Byers et al., 1985; Kohnle, 1985; Klimetzek et al., 1986; Schroeder, 1988; Schroeder and Lindel6w, 1989). Monoterpenes and ethanol can interact synergistically in attracting H. palliatus and T. piniperda (Vit6 et al., 1986; Schroeder and Lindelrw, 1989). In an initial experiment in the present study, the landing rates of T. piniperda and H. palliatus on hosts and nonhosts were estimated. In subsequent field experiments, the responses of T. piniperda and H. palliatus to volatiles from wood of the nonhosts European white birch, Betula pendula Roth (Betulaceae), and aspen, Populus tremula L. (Salicaceae), were studied.

METHODS AND MATERIALS

All experiments were conducted in 30- to 45-year-old Scots pine stands in the provinces of Uppland and G~istrikland in central Sweden. The landing frequencies of scolytids on vertical, black plastic tubes; healthy Scots pines; topcut pines; and upright-standing, newly cut logs of Scots pine were compared in t981. The diameter of the experimental trees was 14-17 cm. The top-cut trees were 6 m long, the logs 3 m long, and the tubes 1.5 m long. The top-cutting, which removed all green branches, was conducted in February, and the logs were cut in March 1982. The tubes (diam. 12 cm) were attached 0.5 m above ground. The four treatments were arranged in 14 randomized blocks. The distance between the treatments in each block ranged from 4 to 15 m and between blocks from 20 to 50 m. Each experimental tree and tube was surrounded by a trap cylinder (0.5 m high) constructed out of Stickem-Special-coated, 6.5-mm mesh wire netting. The cylinders were attached 1.5 m above ground. The distance between the trap and the surface of the trees and tubes was about 2 cm. All scolytids caught on the traps from April 4 to June 17, 1982 were collected. After the experiment, the top-cut trees and the logs were debarked and inspected.

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The number of scolytid egg galleries and the presence of emergence holes were recorded. The healthy trees were carefully examined for boring dust and pitch tubes without being felled and debarked. The attraction of beetles to combinations of 95 % ethanol (5 % water) and wood of aspen or European white birch was compared with the attraction to ethanol alone in 1990 and 1991. The attractiveness of the two tree species was studied in separate experiments, three with aspen and four with birch (Table 1). The treatments in each experiment were set out in pairs. The distance between the two traps in each pair was 10-15 m, and the distance between pairs was 20 m. In experiments 1 and 2 (1990), and 5 and 6 (1991) (Table 1), the same arrangements were used, but the positions of the treatments in each trap pair were switched between experiments. In each day of baiting, healthy aspen or birch trees were felled and cut into bolts (10-20 cm long), which were split with an axe. The cut wood was put into containers and covered with fine netting to keep bark beetles out. Wood volume differed between experiments (Table 1).

TABLE 1. OVERVIEW OF EXPERIMENTS WITH ASPEN AND BIRCH

Type of wood combined with ethanol bait" Year and Tree V o l u m e exp. species (liters) 1990 1 aspen 2 birch 3 aspen 4 birch 1991 5 aspen birch 6

7

birch

Trapping period

55 55 55

March17-20 March31 April 6-27

45

April 11-27

45 45

April 11-15 April 25-May 13 May 17-June 19

8

Bait replacements during tests new ethanol each day April 11, new ethanol; April 21, new wood and ethanol April 22, new wood and ethanol

(May 13-17, no trapping) May 17, new wood and ethanol; May 27, new wood and ethanol refilled; June 7, new wood and ethanol refilled April 29-May 13 (April29-May 13, no trapping) May 17-June 19 May 17, new wood and ethanol refilled; May 27, new wood and ethanol refilled; June 7, new wood and ethanol refilled

Replicates (N) 6 8 7 8 8 9

11

~Ethanol release rates given in text (estimated in laboratory)and in Table 2 (estimated in the field).

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RECOGNITION OF NONHOSTS BY BARK BEETLES

Ethanol was included in the experiments since previous studies demonstrated that, generally, only low numbers o f T. piniperda and H. palliatus are caught in unbaited flight barrier traps (Schroeder, 1988; Schroeder and Linde16w, 1989). Hence, a repellent effect of volatiles from wood of broad-leaved trees would be difficult to detect using unbaited traps as a control. Ethanol attracts both beetles, and the substance may be present in dying broad-leaved trees as well as in dying conifers. Beetles were caught in flight barrier traps modified from the type described by Chapman and Kinghorn (1955). The barriers consisted of 15-cm-high, transparent plastic sheets. Water-filled trap troughs were placed directly beneath the barriers, on top of the containers. Sex determinations were made by dissection for all species except Epuraea bickhardti St.-Claire Deville (Coleoptera, Nitidulidae), whose males can easily be recognized based on the appearance o f the middle tibia. The ethanol dispensers were placed in the containers just under the net lids. In experiments 1 and 2, 10-ml polyethylene vials (6 ml ethanol) with 9-mm-diam. openings were used. In all other experiments 200-ml glass jars (185 ml ethanol) with 45-mm-diam. openings were used. The release rates of ethanol from the dispensers in laboratory at 15~ were 3.0 _+ 0.3 g/24 hr for the polyethylene vials and 9.1 _+ 1.0 g/24 hr for the glass jars. In 1991, the field release rates during the trapping periods were estimated by measuring the amount of ethanol left in the dispensers at the end of each experiment, or before refilling during an experiment (Table 2). The mean air temperature varied between 5.0 and 8.4~ during the trapping periods in 1991.

TABLE 2. AMOUNTS OF ETHANOL RELEASED FROM DISPENSERS PLACED IN TRAPS BAITED WITH WOOD

(E & W)

AND 1N TRAPS WITHOUT WOOD (E) 1N 1991"

Amount released per day (X + SD, g) Expt. 5 6 7

Trapping period

Days (N)

E&W

E

April 11-15 April 25-May 13 May 17-27 May 27-June 7 April 29-May 13 May 17-27 May 27-June 7

4 18 10 11 14 10 11

7.3 _+ 1.6 3.5 • 0.9 4.2 • 0.7 3.2 • 0.8 2.0 ___0.8 2.4 • 1.4 1.4 • 0.5

5.8 • 0.7 3.0 • 0.4 4.0 • 0.6 2.8 • 0.4 1.5 ___0.7 1.4 • 1.1 1.0 • 0.4

~For number of replicates, see Table 1.

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SCHROEDER RESULTS

The scolytids T. piniperda, H. palliatus, Hylastes brunneus Er., H. cunicularius Er., and T. lineatum, were caught on the traps on the plastic tubes (Table 3). The numbers of beetles caught on the tubes did not differ significantly from the numbers caught on healthy and top-cut trees for any of the species. Much higher numbers of T. piniperda and H. brunneus were caught on the newly cut logs than in the other treatments, and the differences in catch were statistically significant for both species. At the end of the experiment, the only scolytid breeding attacks found were caused by T. piniperda on logs. Twelve of the 14 logs were attacked (X = 26 egg galleries/log), and in 10 of the logs the attacks resulted in the emergence of a new generation of beetles. The addition of aspen or birch wood to the ethanol bait resulted in reduced catches of T. piniperda, H. palliatus, and Rhizophagus depressus (F.) (Coleoptera, Rhizophagidae) (Tables 4 and 5). The reduction in catch in the different experiments varied between 58% and 96% for T. piniperda, 8% and 87% for H. palliatus, and 44% and 90% for R. depressus. The difference in number of beetles caught between treatments with wood and treatments without wood was statistically significant at the P = 0.05 level in one experiment for T. piniperda and in five experiments each for H. palliatus and R. depressus. In contrast to the species mentioned above, 1.6-3.4 times as many Glischrochilus quadripunctatus (L.) (Coleoptera, Nitidulidae), 2.7-8.3 times as many Epuraea bickhardti, 5.3-20 times as many E. unicolor (Oliv.) and about twice as many Rhizophagus parvulus (Payk.) were caught in the wood-baited traps as in the traps baited with ethanol alone. These differences were statistically significant for G. quad.6punctatus in two experiments, for E. bickhardti in six TABLE 3. MEAN NUMBER OF BEETLES CAUGHT PER STICKY TRAP ATTACHED TO HEALTHY, ToP-CUT, AND NEWLY CUT LOGS OF SCOTS PINE AND TO PLASTIC TUBES

Mean catch Species

T. piniperda H. palliatus H. brunneus 14. cunicularius T. lineatum

Host-tree species

Plastic tube

Healthy pine

Top-cut pine

Pine log

Scots pine Scots pine Norway spruce Scots pine Norway spruce Scots pine Norway spruce

2.2 3.9

1.5 4.0

3.9 4.1

31.4" 10.6

1.4 1.6 0.9

0.5 1.4 1.9

1.3 1.4 0.9

23.6" 1.6 2.5

"Significantly different from other treatments, P < 0.05, Newman-Keuls multiple-range test.

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RECOGNITION OF NONHOSTS BY BARK BEETLES

TABLE 4. SUMS OF BEETLES CAUGHT IN BARRIER TRAPS BATED WITH ETHANOL

ALONE (E) OR ETHANOLAND CUT WOOD OF ASPEN (E & A) Catch Year

Test

Species

E

E&A

P value~

Sex ratio (% females)

1990

1

T. piniperda 14. palliatus R. depressus G. quadripunctatus T. piniperda H. palliatus R. depressus G. quadripunctatus E. bickhardti T. piniperda H. palliatus R. depressus G. quadripunctatus E. bickhardti E. unicolor

96 40 18 22 27 89 27 62 58 171 65 82 10 240 14

14 10 9 56 4 22 13 98 172 61 21 28 34 797 280

0.067 0.012 0.237 0.058 0.056 0.036 0.044 0.008 0.006 0.023 0.014 0.029 0.059 0.010 0.013

47 31 52 67 69 33 63 52 52 50 46 82 57 47 76

3

1991

5

~

paired-sample t test.

experiments, for E. unicolor in three experiments, and for R. parvulus in the only experiment in which this species was present. In all but one case there was no significant (X2 test, P < 0.05) difference in sex ratio between beetles caught in treatments with wood and those without wood for any o f the beetle species in any o f the tests (Tables 4 and 5). The exception was R. depressus in experiment 6, where the female percentage was significantly lower in the wood baited treatment (58%) than in the treatment with ethanol alone (69 %). As a result o f the lower mean air temperature, the field release rates of ethanol (Table 2) are considerably lower than those measured in the laboratory at 15 ~ Rates o f release were somewhat higher from dispensers placed in traps with wood than from dispensers in traps without wood: 24% higher in experiment 5, 12% higher in experiment 6, and 48% higher in experiment 7.

DISCUSSION The present study shows that rejection o f the nonhosts aspen and birch by the bark beetles T. piniperda and H. palliatus may be based on volatiles emitted from these tree species. Both aspen and European white birch are among the

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SCHROEDER

TABLE 5. SUMS OF BEETLES CAUGHT IN BARRIER TRAPS BAITED WITH ETHANOL ALONE (E) OR ETHANOL AND CUT WOOD OF BIRCH (E • B)

Catch Year

Test

Species

E

E& B

P value ~'

Sex ratio (% females)

1990

2

14. palliatus 17. depressus E, bickhardti H. palliatus R. depressus G. quadripunctatus E. bickhardti T. piniperda H. palliatus R. depressus R. parvulus E, bickhardti E. unicolor 14. palliatus R. depressus E. bickhardti E. unicolor

30 29 18 33 18 36 21 50 97 298 10 43 10 53 201 44 10

4 3 48 11 5 77 175 21 41 167 21 198 95 49 99 159 53

0,019 0.068 0.046 0.107 0.010 0.045 0.001 0.337 0,002 0.050 0.010 0.001 0.001 0.758 0.027 0.006 0.025

35 77 42 56 61 57 60 63 51 65 40 52 75 45 67 53 53

4

1991

6

7

"Two-tailed paired-sample t test.

most common of the broad-leaved tree species in Sweden. Furthermore, they are often found in mixed stands together with Norway spruce and Scots pine. Thus the probability of T. piniperda and H. palliatus, as well as other conifer bark beetles, encountering aspen and birch is quite high. An olfactory response allowing the beetles to quickly reject nonhosts after or just before landing would save them valuable time for colonizing suitable breeding substrate. The much higher number of T. piniperda caught on the pine logs than on the healthy unattacked pine trees can be ascribed to the large amounts of host volatiles released from the initiated egg galleries (cf., Schroeder, 1987) and from the end surfaces of the logs cut just before the start of the beetle flight period. T. piniperda, H. palliatus, and some other scolytids were caught in low numbers on the plastic tubes, demonstrating that when searching for breeding substrate these species may also land on objects lacking characteristics specific to their host-tree species. The decreased attraction of R. depressus to ethanol in the presence of aspen and birch wood demonstrates that beetles associated with bark- and woodboring species also may respond to volatiles from nonhost tree species. R. depressus

R E C O G N I T I O N O F N O N H O S T S BY B A R K B E E T L E S

1591

is a predatory species inhabiting the galleries of bark beetles such as T. piniperda and H. palliatus in Scots pine and Norway spruce (Nuorteva, 1956). R. depressus was earlier demonstrated to be attracted by the conifer monoterpene c~-pinene and by ethanol (Schroeder and Lindelrw, 1989). Substances causing insects to make oriented movements away from its source are termed repellents according to designations of Dethier et al. (1960). The response of beetles to wood alone was not tested in the present study. Hence, it was not possible to determine if wood of birch and aspen has a true repellent effect. Moreover, since no unbaited traps were included in the experiments, it was not possible to establish whether the traps baited with wood and ethanol caught lower numbers of bark beetles than what would have been caught in unbaited traps. Thus, "decreased attraction" is used to account for the observed results. Only a few reports of release rates of ethanol from cut wood were found in the literature. No information on ethanol release rates from aspen or birch is available. Ikeda et al. (1980) reported a daily average ethanol release of 0.7 #l/kg from bolts of red pine, Pinus densiflora, during a 14-day period following cutting. This suggests that if any ethanol were formed in the wood used in the present study, the release rates should have been negligible compared with the amounts released from the dispensers. The somewhat higher release rate of the ethanol dispensers in traps baited with wood was probably due to the heat-storing capacity of the wood. The rate of release of ethanol from the type of dispenser used in the present study increases with temperature. Thus, as temperatures fell in the evenings, heat stored in the wood could have acted as a temperature buffer, thereby preventing release rates from dropping as fast as in the dispensers in the treatments without wood. Nevertheless, the somewhat higher release of ethanol from wood-baited treatments could not have caused the lower catches of the two bark beetles and R. depressus. In previous dose-response field experiments with ethanol, the beetle attraction increased as release rates of ethanol were increased from 0.002 to 50.4 mg/24 hr for H. palliatus and from 0.002 to 3.1 mg/24 hr for R. depressus (Schroeder, 1988; Schroeder and Lindelrw, 1989). About the same numbers of T. piniperda were attracted by all release rates covered by the latter dose range. The combination of birch or aspen volatiles and ethanol was more attractive than ethanol alone to the nitidulid beetles G. quadripunctatus, E. bickhardti, and E. unicolor and to the rhizophagid beetle R. parvulus. These species inhabit fungi-infected bark and galleries of bark- and woodboring insects (Nuorteva, 1956; Palm, 1959). R. parvulus and E. unicolor are found almost exclusively on broad-leaved trees, and G. quadripunctatus is common on both conifer and broad-leaved trees. G. quadripunctatus is also attracted by ethanol and c~-pinene (Schroeder and Lindelrw, 1989). In contrast to the other species, E. bickhardti

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has b e e n r e c o r d e d m a i n l y f r o m S c o t s p i n e and N o r w a y spruce in S c a n d i n a v i a , a n d only a f e w reports o f this s p e c i e s f r o m b r o a d - l e a v e d trees exist (Saalas, 1917; N u o r t e v a , 1956). Acknowledgments--I wish to thank R. Axelsson, H. Nordenhem, and B. Viklund for technical assistance; and H.H. Eidmann for comments on the manuscript. Financial support from the SJFR Research Council is gratefully acknowledged.

REFERENCES BORDEN,LH. 1982. Aggregation pheromones, pp. 74-139, in J.B. Mitton and K.B. Sturgeon (eds.). Bark beetles in North American Conifers. University of Texas Press, Austin, Texas. BYERS, J.A., LANNE, B.S., LOFQVlST, J., SCHLYTER, F., and BERGSTRt3M, G. 1985. Olfactory recognition of host-tree susceptibility by pine shoot beetles. Naturwissenschaften 72:324-326. CADE, S.C., IrlRUTFIORD, B.F., and GARA, R.I. 1970. Identification of a primary attractant for Gnathotrichus sulcatus isolated from western hemlock logs. J. Econ. Entomol. 63:1014-1015. CHAPMAN, LA., and KINGHORN, J.M. 1955. Window flight traps for insects. Can. Entomol. 87:46-47. DETHIER, V . G , BROWNE, L.B., and SMITH, C.N. 1960. The designation of chemicals in terms of the responses they elicit from insects. J. Econ. Entomol. 53:134-136. ELKINTON, J.S., and WOOD, D.L. 1980. Feeding and boring behavior of the bark beetle lps paraconfusus (Coleoptera: Scolytidae) on the bark of a host and non-host tree species. Can. Entomol. 112:797-809. ELLIOTT, H.J., MADDEN,J.L., and BASHFORD, R. 1983. The association of ethanol in the attack behaviour of the mountain pinhole borer Platypus subgranosus Schedl (Coleoptera: Curculion~dae: Platypodinae). J. Aust. Entomol. Soc. 22:299-302. GILBERT, B.L., and NORRIS, D.M. 1968. A chemical basis for bark beetle (Scolytus) distinction between host and non-host trees. J. Insect Physiol. 14:1063-1068. HARBORNE, J.B., and TURNER, B.L. 1984. Plant Chemosystematics. Academic Press, New York. IKEDA, T., ENDA, N., YAMANE,A., ODA, K., and TOYODA, T. 1980. Attractants for the Japanese pine sawyer~ Monochamus alternatus Hope (Coleoptera: Cerambycidae). Appl. Entomol. Zool. 15:358-361. KHMETZEK,D., KOHLER,J., VIT~, J.P., and KOHNLE,U. 1986. Dosage response to ethanol mediates host selection by "secondary" bark beetles. Naturwissenschafien 73:270-272. KOHNLE,U. 1985. Untersuchungen fiber die Pheromonsysteme sekund~irerBorkenk/ifer (Col., Scolytidae). Inaugural dissertation, Forstwissenschaftliche Fakult/it, Albert-Ludwigs-Universi~t zu Freiburg i Br. LANNE, B.S., SCNLYTER,F., BYERS,J.A., LOFQVlST,J., LEUFVEN,A., BERGSTROM,G., VAN DER PERS, J.N.C., UNEHUS, R., BAECKSTROM,P., and NORIN, T. 1987. Differences in attraction to semiochemicals present in sympatric pine shoot beetles Tomicus minor and T. piniperda. J. Chem. Ecol. 13:1045-1067. MILLAR, J.G., ZHAO, C.-H, LANIER, G.N., O'CALLAGHAN,D.P., GRrGGS, M., WEST, J.R., and SILVERSTHN, R.M. 1986. Components of moribund American elm trees as attractants to elm bark beetles, Hylurgopinus rufipes and Scolytus muhistriatus. J. Chem. Ecol. 12:583-608. MOECK,H.A. 1970. Ethanol as the primary attractant for the ambrosia beetle Trypodendron lineatum (Coleoptera: Scolytidae), Can. Entomol. 102:985-995. MONTGOMERY, M.E., and WARGO, P.M. 1983. Ethanol and other host-derived volatiles as attracrants to beetles that bore into hardwoods. J. Chem. Ecol. 9:181-190.

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NIJHOLT, W.W., and SCHONHERR, J. 1976. Chemical response behavior of scolytids in West Germany and western Canada. Can. For. Serv. Bi-Month. Res. Notes 32:31-32. NUORTEVA, M. 1956. Uber den Fichtenstamm-Bastk~ifer, Hylurgops paUiatus Gyll., und seine Insektenfeinde. Acta Entomol. Fenn. 13:188 pp. PALM, T. 1959. Die Holz- und Rinden-Kfifer der sfid- und mittelschwedischen Laubb/iume. Opusc. Entomol. Suppl. 16:374 pp. PERTTUNEN, V., OKSANEN, H., and KANGAS, E. 1970. Aspects of the external and internal factors affecting the olfactory orientation of Blastophagus piniperda (Coleoptera: Scolytidae). Contrib. Boyce Thompson Inst. 24:293-297. ROLING, M.P., and KEARBY, W.H. 1975. Seasonal flight and vertical distribution of Scotytidae attracted to ethanol in an oak-hickory forest in Missouri. Can. Entomol. 107:1315- ~320. SAALAS, U. 1917. Die Fichtenk/ifer Finnlands. I. Ann. Acad. Sci. Fenn. Ser. A 8(1):547 pp. SCHgOEDER, L.M. 1987. Attraction of the bark beetle Tomicus piniperda to Scots pine trees in relation to tree vigor and attack density. Entomol. Exp. Appl. 44:53-58. SCHROEDER, L.M. 1988. Attraction of the bark beetle Tomicus piniperda and some other bark- and wood-living beetles to the host volatiles u-pinene and ethanol. Entomol. Exp. Appl. 46:203210. SCHROEDER, L.M., and LINDELOW, ,~. 1989. Attraction of scolytids and associated beetles by different absolute amounts and proportions of u-pinene and ethanol. J. Chem. Ecol. 15:807817. SJi~DIN, K., SCHROEDER, L.M., EIDMANN, H.H., NOR1N, T., and WOLD, S. 1989. Attack rates of scolytids and composition of volatile wood constituents in healthy and mechanically weakened pine trees. Scand. J. For. Res. 4:379-391. STROMVALL, A.-M., and PETTERSSON, G. 1991. Conifer monoterpenes emitted to air by logging operations. Scand. J. For. Res. 6:253-258. TCiMMER~,S, B.A. 1989. Host selection by odorous compounds from host and non-host trees in bark beetles. Fauna Norv. Ser. B 36:75-79. T~MMER~S, B.A., and MOSTAPARTA,H. 1989. Single cell responses to pheromones, host and nonhost volatiles in the ambrosia beetle Trypodendron lineatum. Entomol. Exp. Appl. 52: 141148. VITE, J.P., VOLZ, H.A., PAIVA, M.R., and BAKKE, A. 1986. Semiochemieals in host selection and colonization of pine trees by the pine shoot beetle Tomicus piniperda. Naturwissenschaften 73:39-40.