Economic and scientific importance. Bark beetles (Scolytinae) pose a threat to forests worldwide. More than 5,800 species of bark beetles (Wood and Bright 1992) utilize all plant parts, from root to fruit, as a food resource for larvae and adults (Wood 1982). (read more...)

Economic and scientific importance. Bark beetles (Scolytinae) pose a threat to forests worldwide. More than 5,800 species of bark beetles (Wood and Bright 1992) utilize all plant parts, from root to fruit, as a food resource for larvae and adults (Wood 1982). Many species play an ecologically important role as primary decomposers (Wood and Bright 1992). The beetles create cavities in sections of dying or unhealthy trees that are used as an entrance for other decomposers (e.g., fungi, bacteria, etc.). However, some species introduce fungi and diseases into otherwise healthy trees as a consequence of feeding and reproductive activities. These groups cause great destruction of economically valuable tree species, and give bark beetles their nefarious reputation as an ecological and economically destructive pest (Carolin and Furniss 1977). Even generally benign species of Ipina can become potential killers of healthy trees, and in fact, Ips species are second in destructiveness only to Dendroctonus (Carolin and Furniss 1977). Forest damage caused by Ips is often sporadic and short term (Carolin and Furniss 1977), although some outbreaks affect as many as 25,000 acres a year (Werner 1988, Anonymous 1996). Damage caused by Ips is most severe in times of environmental stress, which is mostly associated with drought and other insect infestations (Knizek and Zahradnik 1998, Schmutzenhofer 1988). For example, after years of drought, forests in Bhutan suffered an epidemic of Ips schmutzenhoferi that killed approximately 2 square kilometers of spruce and pine (Schmutzenhofer 1988). The economic impact of bark beetle damage is staggering and their control can also be costly. For example, Ips grandicollis is an exotic pest in Australia, and more than $500,000 US has been spent on its biological control (Anonymous 1991). Pityokteines species have also been reported as pests of fir trees (Abies spp.) (Wood 1982), although losses associated with their damage have not been quantified. In North America, the introduction of exotic species of bark beetles continuously threatens native forests (Cavey et al. 1994). Exotic species, including species of Ipina, have been intercepted from shipping dunnage and packing crates (Marchant and Borden 1976). Despite the vigilance of port authorities, some exotic species have established resident populations in the U.S. (Wood and Bright 1992, Haack et al. 1993). The ability to detect and control both native and exotic species relies on a solid understanding of systematics, phylogenetics and taxonomy. This trinity has not been achieved for most groups of bark beetles on a worldwide scale.

Although bark beetles were initially studied because of their destructive behavior, they are also intrinsically interesting organisms. Their varied biologies (e.g., mating systems, host use, etc.) provide the intellectual fodder for investigation of general evolutionary hypotheses (e.g., Cognato et al. unpublished, Normark et al. 1999, Kelley and Farrell 1998, Reid and Roitberg 1994). Furthermore, the complex con- and hetero-specific semio-chemical communication among Ipina groups are ideal systems for investigating hypotheses in many areas of biology, including biochemistry, community ecology, and evolution (e.g.Tittiger et al. 1999, Seybold et al. 1995, Raffa and Klepzip 1989, Wood 1982). In addition, detailed knowledge of their plant host preferences, coupled with phylogeny, can provide insight to their evolutionary ecology (Mirsky and Ris 1951). Once again, these studies are hampered by poor taxonomy and limited phylogenetic analysis. For example, Ipina genera and species groups are assumed to be evolutionary units in ecological and behavioral studies (Vite et al. 1972, Seybold et al. 1995), yet species-level classification to date has been based mostly on phenetic analysis of morphological characters (Wood 1982, Hopping 1963). Therefore, generalizations of species' biology based on these earlier results are open to reinterpretation. Many Ipina species are well known to North American and Eurasian foresters, experimental as well as theoretical biologists, but their systematics and taxonomy rarely have been investigated in a phylogenetic context. The importance of phylogeny for taxonomic classification and evolutionary inference has been discussed at length (e.g., Wiley 1981). In short, a phylogenetic analysis will help clarify existing taxonomic controversies and provide a template to test evolutionary hypotheses. The turbulent taxonomic history and biological complexities are detailed below. (collapse)

Reproductive Biology. In general, the holarctic Ipina are pholeophagus and bore under the outer bark of coniferous host trees and feed within the phloem and cambium bark layers. Males select a suitable host, and while he feeds, he creates a nuptial chamber and produces pheromones that attract other males and females to the host. (read more...)

Reproductive Biology. In general, the holarctic Ipina are pholeophagus and bore under the outer bark of coniferous host trees and feed within the phloem and cambium bark layers. Males select a suitable host, and while he feeds, he creates a nuptial chamber and produces pheromones that attract other males and females to the host. Mating is usually polygamous. One to eight females mate with the male in the nuptial chamber, tunnel an egg gallery, and lay eggs in niches in the side of the gallery wall. The larvae feed and complete their development under the bark. Many holarctic species carry fungal spores (e.g. Ceratocystis spp.) which inoculate the host during adult feeding. Healthy and unhealthy hosts are ultimately killed by dehydration caused by the growth of fungal hyphae in the sapwood (Whitney 1982).

Semio-chemical mediated insect interactions contribute to the complexity of bark beetle population ecology (Wood 1982). These pheromones often inhibit or enhance attraction of other bark beetles, insect predators and/or parasitoids (e.g., Poland TM, Borden JH, 1997, Allison et al. 2001). Several monoterpene pheromone components are found in all holarctic Ipina genera (Wood 1982), but unique combinations of these components, including enantiomeric structures, can be species-specific. For example, the compounds ipsenol, ipsdienol, and cis-verbanol have been identified as important aggregation pheromones among species of Ips and are biologically active in some species of other Ipina genera (Pureswaran et al. 2000). The production of, and response to, chalcogran, a chemical similar to a bicyclic ketal, appears to be unique to Pityogenes species (Francke et al. 1977). Despite the association of taxa with specific pheromone chemical composition, the evolutionary patterns of pheromone use among Ips species indicate that the production and response to pheromones are not phylogenetically constrained (Cognato et al. 1997, Cognato unpublished), and appear to be influenced by ecological variables. For example, the selection pressure of parasitoid response to bark beetle pheromones has been observed to cause yearly changes in beetle pheromone composition (Raffa and Klepzip 1989). Also, competitive interaction among bark beetle species for food resources may influence local use of certain pheromone components (Savoie et al. 1998). The use of pheromones among the known tropical species has not been reported. (collapse)

Classification History and Phylogenetics. Taxonomic controversies have nettled Ipina since its recognition. Ipina was first described by Bedel ( 1888) and included Ips and Pityogenes species. The species of this sub-tribe are distinguished by several characters, the most obvious being a sulcate elytral declivity with spines on the lateral margin and face (Bedel 1888, Wood 1986). (read more...)

Classification History and Phylogenetics. Taxonomic controversies have nettled Ipina since its recognition. Ipina was first described by Bedel ( 1888) and included Ips and Pityogenes species. The species of this sub-tribe are distinguished by several characters, the most obvious being a sulcate elytral declivity with spines on the lateral margin and face (Bedel 1888, Wood 1986). Since then, four tropical (Acanthotomicus, Dendrochilus, Isophthorus Schedl, Mimips Eggers) and five holarctic (Ips, Orthotomicus, Orthotomides, Pityogenes, and Pityokteines) genera have been added to this group based on these morphological attributes (Wood and Bright 1992). Curiously, and with little justification, Dendrochilus, which has a convex elytral declivity, is included in Ipina (Wood 1986, Wood and Bright 1992). The “convex” character state contradicts the diagnostic sulcate elytral declivity of the remaining Ipina species. Re-evaluation of these characters suggests that Dendrochilus belongs to the sub-tribe Dryocoetina (Cognato, in prep.). In addition, recent studies suggest that Premnobius clavipennis is a member of the Ipina (Normark et al. 1999, Farrell et al. 2001 ), which questions the morphological definition of the sub-tribe. Despite the questionable inclusion of these taxa in Ipina, morphological characters and phylogenetic studies suggest the sub-tribe is monophyletic (Wood 1986, Normark et al. 1999, Farrell et al. 2001). At present, the majority of taxonomic uncertainty lies among the genera. Acanthotomicus species have been the least studied. Mimips and Isophthorus have been synoymized under Acanthotomicus because of morphological diagnostic characters shared with members of Acanthotomicus (Wood 1972, Farrell et al. 2001). Mimips was most likely created to reflect their African distribution, however monophyly of the included species remains untested. The holarctic genera have received more taxonomic attention. Most Pityogenes species are morphologically distinguishable from other genera by the presence of a fossa on the frons in the female. Its generic status has remained stable. However, the boundaries of some morphological characters among the remaining genera, including Acanthotomicus, are obscure (Wood 1982). Hopping (1963) detailed antennal, elytral, and male genetalic characters that are diagnostic for Orthotomicus, Orthotomides, Pityokteines and Ips, but he mostly considered only North American species. Schedl (1964) proposed a classification that synonymized the above genera, including Acanthotomicus, with Ips. This classification was based on the opinion that the antennal morphology was an insufficient taxonomic character for defining Ipina genera Schedl (1964). Other characters such as eye and elytral shape weakly support these groups, and their recognition was continued in the interest of preserving taxonomic tradition (Wood 1972, Wood and Bright 1992, Pfeffer 1995) except for Orthotomides, which was synonymized with Pityokteines (Wood 1971). Yet this classification remained problematic because of the gradation of generic-level characters among Ipina species (Wood 1982). The paucity of morphological variation hindered taxonomic resolution of these genera. Recently a combination of morphological and molecular data has provided some resolution of Ipina classification. Phylogenetic reconstruction of Ipina species (less tropical species) with molecular characters suggested that a revision of Ips was necessary to preserve its monophyly (Cognato and Sperling 2000). Thus a new genus, Pseudips, was described for the Ips concinnus species group (Cognato 2000), and with additional morphological and molecular characters, Ips was redefined as a monophyletic group to the exclusion of I. latidens, I. mannsfeldi, I. nobilis and I. spinifer (Cognato and Vogler 2001). The relationships of other genera remain equivocal, although the holarctic genera are monophyletic with the exclusion of Pseudips. (collapse)

Species Diversity, Distribution, and Host Use. There are approximately 200 Ipina species that are grouped into 6-7 genera (96 Acanthotomicus spp., 25 Premnobius spp., 44 Ips spp., 13 Orthotomicus spp., 24 Pityogenes spp., 10 Pityokteines spp., 3 Pseudips spp.) (Wood and Bright 1992, 24). Recent phylogenetic analyses suggest that Premnobius is related to Acanthotomicus (Normark et al. 1999, Farrell et al. 2001). (read more...)

Species Diversity, Distribution, and Host Use. There are approximately 200 Ipina species that are grouped into 6-7 genera (96 Acanthotomicus spp., 25 Premnobius spp., 44 Ips spp., 13 Orthotomicus spp., 24 Pityogenes spp., 10 Pityokteines spp., 3 Pseudips spp.) (Wood and Bright 1992, 24). Recent phylogenetic analyses suggest that Premnobius is related to Acanthotomicus (Normark et al. 1999, Farrell et al. 2001). Acanthotomicus and Premnobius are distributed in the tropics, while the bulk of the generic diversity of the Ipina is distributed in the holarctic. However, since Acanthotomicus is the most speciose genus, more than half of Ipina species consequently are found in the tropics, whereas only 94 species are found in the holarctic biogeographic region (Wood and Bright 1992). Geographic distribution of Acanthotomicus species diversity occurs mostly in tropical Africa (53%), Asia (33%) and Central and South America (11%) regions where beetle biodiversity remains poorly assessed. All Premnobius occur in Africa and P. clavipennis has been introduced to the Neotropics. Therefore, discrepancy in species diversity between the tropics and holarctic regions is likely underestimated due to a lack of understanding of tropical fauna taxonomy. For example, based on the assessment of tropical Ipina specialists, potentially twice as many Acanthotomicus species await description (R.A. Beaver, pers. comm.).

Host use of Ipina species is generally region-specific; tropical species use angiosperms and holarctic species use conifers. Despite this trend, the pattern of host use within each biogeographical region is complex. Most Acanthotomicus and Premnobius species have been recorded from one host species, but a few species have been recorded from several different tree families (Wood and Bright 1992, Schedl 1962). For example, tropical Acanthotomicus and Premnobius species use boles and broken branches of angiosperm species from a diversity of families. Conversely, the holarctic species are monophagous, and use only members of the Pinaceae (Pinus, Picea, Larix, Abies and Cedrus) as hosts. Pinus is most widely used host by all North American and European genera except Pityokteines, which uses Abies almost exclusively. Picea and Larix are less commonly used, but one group of Ips spp. appears to have diversified on North American spruce species (Schedl 1962). Bark beetles in the Himalayas are reported to use Cedrus as a host (Wood and Bright 1992). (collapse)