EVALUATION OF CROSS-ATTRACTION BETWEEN SYMPATRIC CHORISTONEURA OCCIDENTALZS AND C. RETZNZANA (LEPIDOPTERA: TORTRICIDAE) POPULATIONS IN SOUTH-CENTRAL OREGON ANDREWM. LIEBHOLD,W. JANA. VOLNEY,and WILLIAME. WATERS Division of Entomology and Parasitology, University of California, Berkeley 94720 Abstract Can. Ent. 1 16: 827-840 (1984) The pheromone specificity of female Choristoneura occcidentalis Freeman and C. re- tiniana (Walsingham) from several locales, laboratory colonies, and interspecific mat- ings was determined by observing the numbers and wing maculations of maies attracted at several sites in southern Oregon. Pheromone s~ecificitvof females reared from field- collected brown (typical of c.-occidentalis) and green kypical of C. retiniana) larval morphs differed considerably but differed little among sites of origin. Field-collected females attracted more males than conspecific individuals from laboratory colonies. F, and F, interspecific hybrids most closely resembled C. occidentalis in the numbers and types of males attracted. As a group, progeny of backcrosses to C. retiniana appeared intermediate between pure lines of the species in their pheromone specificity. Females reared from intermediate-colored field-collected larvae varied considerably in attrac- tiveness. Most attracted groups of males similar to those attracted to female C. reti- niana, but others attracted males most similar to those attracted to female progeny of C. retiniana hybrid backcrosses. These findings support the conclusion that hybrid matings occur between these species at a low frequency in nature. Resume La spCcificitC de la phtromone chez des femelles de Choristoneura occidentalis Free- man et de C. retiniana (Walsingham) provenant de plusieurs sites, de colonies de laboratoire et de croisements interspecifiques a ete CtudiCe en determinant les nombres et les taches alaires des mdles attires a plusieurs endroits en Oregon. La spCcificitC de la phtromone differe considkrablement entre des femelles provenant de larves brunes (couleur typique chez C. occidentalis) et vertes (C. retiniana) prClevCes sur le terrain, mais trks peu entre les sites d'origine. Les femelles prklevCes sur le terrain ont attirC plus de miles que des conspCcifiques provenant de colonies de laboratoire. Des hy- brides intersptcifique de F, et F, ressemblaient plus a C. occidentalis quant aux nombres et aux types de miles attirks. En tant que groupe, la progCniture des rCtrocroisements avec C. retiniana est apparue intermediaire entre les lignCes pures quanta sa specificitk. Des femelles ClevCes a partir de larves de couleur intermkdiaire provenant du terrain ont montrC une variabilitC considCrable. La plupart ont attire des groupes de m2les similaires a ceux attires par les femelles de C. retiniana, mais certaines ont attire des mdles ressemblant plus ceux attires aux femelles provenant des rktro-croisements d'hybrides avec C. retiniana. Ces observations appuient la conclusion voulant que des croisements entre ces deux espbces se produisent avec une frkquence rCduite en nature. Despite several taxonomic treatments, the status of conifer-feeding Choristoneura species is uncertain (Freeman 1953, 1967; Powell 1964, 1980). Much of this uncertainty is due to the great intraspecific variation in, and interspecific overlap of, morphological characters (Harvey and Stehr 1967; Volney et al. 1983, 1984). In addition, the degree of reproductive isolation among various populations is uncertain. Postmating incompatibility is not a major factor in the reproductive isolation of conifer-feeding Choristoneura species. When interspecific pairs are confined in small cages, mating takes place and little inviability exists in their progeny (Smith 1953; Sanders et al. 1977; Volney et al. 1984). Thus, premating factors must be the major barriers to gene flow between these species. Considerable overlap exists in the seasonal and die1 periods of sexual activity between sympatric species (Smith 1954; Sanders 1971a; Lieb- hold and Volney 1984~).Therefore, temporal factors are not important isolating mecha- nisms. Males are attracted to female-released pheromones whether females are in host or 828 THE CANADIAN ENTOMOl.O(ilST June 1984 non-host trees (Liebhold and Volney 1984b). Consequently, host specificity is not a major isolating mechanism either. There are no major differences in the close range mating behavior of C. occidentalis Freeman and C. retiniana (Walsingham) (Liebhold and Volney unpub.). Thus, it appears that reproductive isolation between sympatric conifer-feeding Choristoneura is due largely to differences in female-produced attractant pheromones and responses of males to these compounds. Smith (1953, 1954) concluded that reproductive isolation between sympatric C. fu- miferana (Clemens) and C. pinus Freeman in Ontario was complete and was due mainly to ecological isolation (host selection), temporal isolation, and "sexual selection" (adult behavior and the role of pheromones were still not fully understood). Subsequent evalu- ations of cross-attraction between conifer-feeding ~horistoneuraspecies largely compared allopatric populations of these species (Sanders 1971a,b; Sanders et al. 1977). Thus, the degree of reproductive isolation between sympatric conifer-feeding Choristoneura, es- pecially western species, remains uncertain. Our observations on the relative abundance, distributions, host associations, and phenotypic variation of C. occidentalis and C. retiniana in southern Oregon have indicated that these two species are sympatric over much of this area (Volney et al. 1984). We also have found larvae of intermediate phenotypes in addition to the characteristic green larvae of C. retiniana and brown larvae of C. occidentalis. We therefore undertook this study to determine the attractive abilities of females from sympatric and allopatric populations of C. occidentalis and C. retiniana in this area. Specifically, we wished to determine if geographically separated populations of the same larval morph were similar in their abil- ities to attract males, the degree to which the two different morphs were cross-attractive in sympatric populations, and what sort of attractive abilities females reared from inter- mediate larvae possessed. With this knowledge we hoped to better evaluate whether hy- bridization between sympatric populations of these species occurs and what consequences this hybridization would have on the genetic structure of these populations. Methods Descriptions and locations of trapping sites are given in Volney et al. (1984). An additional trapping test was conducted in a predominantly young growth Douglas-fir, Pseu- dotsuga menziesii (Mirb.) Franco, stand located in the Mt. Hood National Forest near Bear Springs, Wasco Co., Oregon (BS). This area has a history of C. occidentalis out- breaks (Dolph 1980) and C. retiniana is not known to occur there (V. M. Carolin Jr., Ders. comm.: Powell 1980). All insects were collected as larvae either by pruning and searching branches from the entire crown of host trees (Douglas-fir and whte fir, Abies concolor (Gord. & Glend.) Lindl.), or by searching foliage of host trees at eye level. Larvae were returned to a field laboratory and in the sixth instar they were classified as either brown, green, or inter- mediate larval morphs using the method of Volney et al. (1984). All field-collected larvae were reared at ambient laboratory temperature in 90 x 23 mm shell vials containing foliage of the host tree species on which they were found. Upon pupation, foliage was removed, pupae were sexed, and moths were allowed to emerge. For convenience, females reared from field-collected green larval morphs, characteristic of C. retiniana, will be referred to as 'green', females reared from field-collected brown larval phenotypes, characteristic of C. occidentalis, will be referred to as 'brown', and females reared from field-collected larvae of intermediate phenotype will be referred to as 'intermediate'. One- to two-day-old virgin females were placed singly in 12 x 6 x 4 cm aluminum screen cages.u To Drevent desiccation of moths. a half dram water-filled vial fitted with a cotton wick was added to each cage. Females were exposed to at least 24 h of natural photoperiod before being taken to the field. Each cage was attached under the roof of a Pherocon 1C trap (Zoecon Corp., Palo Alto, CA). Control traps contained a cage with Volume 116 THE CANADIAN ENTOMOLOGIST 829 only a water vial. Traps were placed 20 m apart in a grid and hung from tree foliage approximately 1.5 m above the ground. Treatments (i.e. female types and control) were assigned at random to the trap locations within a grid. In 1979, green larval morphs were collected at Dutch Oven Flat (DOF), Riverbed Butte (RB), and Dry Lakes Flat (DLF). Females from these sites were deployed in traps at all sites except that DLF females were not deployed at RB, and RB females were not deployed at DLF. Control traps were deployed at all sites. Traps were left up for 2 nights after which trapped males were counted. If the caged female had died, data from the trap were excluded from the analysis. In 1980, green females from Mt. Ashland (MA) and RB, and brown females from Hyatt Lake (HL) and Tolman Creek (TC) were deployed at MA. At RB, green females from MA and RB, and brown females from HL were deployed. At DLF, green females from MA and DLF were deployed. At TC, green females from MA and RB, and brown females from TC were deployed. At HL, green females from MA and RB, and brown females from