ISSN 1911-2173

Canadian Journal of Arthropod Identification

Dichotomous and matrix-based keys to the Ips bark beetles of the World (Coleoptera: Curculionidae: Scolytinae)

CJAI 38 — June 27, 2019


Hume B. Douglas, Anthony I. Cognato, Vasily Grebennikov, and Karine Savard




Ips bark beetles (Coleoptera: Curculionidae: Scolytinae: Ipini), are sub-cortical phloem feeders in Pinaceae (conifer trees), especially Pinus (pine), Picea (spruce) and Larix (larch or tamarack) species (Cognato, 2015). In non-outbreak times, Ips beetles mainly inhabit weakened or dead trees (Cognato, 2015). Adults and larvae kill healthy trees during outbreaks (Cognato, 2015) by destroying the phloem and cambium in tree trunks and limbs when feeding and tunnelling (Furniss and Carolin, 1977). Outbreaks can destroy thousands of hectares of healthy trees (Cognato, 2015). Some or all Ips bark beetles also transmit pathogenic fungi (Krokene and Solheim, 1998; Meng et al., 2015), in particular blue stain fungi (genera Grosmannia and Ceratocystis, Ascomycota: Sordariomycetes). Ceratocystis fungifrom Ips beetlesalso interfere with biological control of the conifer pest Sirex noctilio Fabricius (Hymenoptera: Siricidae) (Yousuf et al., 2014). Certain climatic conditions (e.g. droughts) may promote Ips outbreaks (Wermelinger, 2004; Breshears et al., 2005; Marini et al., 2017). Trees injured in outbreaks are sometimes later killed by Dendroctonus Erichson, 1836 bark beetles (Furniss and Carolin, 1977).

Native Ips species are present in all countries where Pinus and Picea occur naturally (Cognato, 2015). Five Ips species (I. apache (full names presented in Table 1), I. calligraphus, I. grandicollis, I. subelongatus, and I. typographus) also occur as introduced species, especially in non-native plantations of Picea and Pinus. Some Ips species use Larix as a primary host tree genus (Table 1). A few species use Abies (fir) and Cedrus (true cedar) as hosts during outbreaks (Wood and Bright, 1992). Pseudostuga may be attacked occasionally outside its natural range (e.g. by Ips acuminatus).

There are 37 valid Ips species worldwide (Table 1). Phylogenetic analyses of the Ipini prompted transfer of several species to the genera Pseudips (Cognato, 2000) and Orthotomicus (Cognato and Vogler, 2001). Cognato (2015) reviewed literature on phylogeny, taxonomy, diagnosis and biology of all Ips species.

The genus Ips can be recognizedand identified to species by adult external morphology. Descriptions and regional keys to the species of Ips based on morphology are available (Balachowsky, 1949; Kurentsov and Kononov, 1966; Bright 1976; Grüne, 1979; Schedl, 1981; Wood, 1982; Holzschuh, 1988; Lanier etal., 1991; Pfeffer, 1995; Cognato and Sun, 2007). Cognato (2015) provides diagnoses for all world species. A generic key to Scolytinae larvae of eastern Canada is available (Thomas, 1957) but juvenile stages cannot be used for reliable identification on a global scale. Although Ips species have been discovered and identified using DNA sequence data (Cognato and Sun, 2007), validated protocols for universal DNA identification of Ips species have not yet been developed (Chang et al., 2012). Additional work is needed to demonstrate that DNA sequence records provide accurate identifications.

The following tribal-level diagnostic characters for Ipini are modified from Wood (1986): compound eye sinuate (narrowed at mid-height), ventral half narrower than dorsal part; antennal scape (basal segment) slender elongate, funicle 5-segmented, club either obliquely truncate or sutures on posterior face strongly displaced toward apex; pronotum strongly declivous on anterior half (posterior half approximately horizontal, anterior half descends abruptly), with large asperities (broad spines); procoxae contiguous, intercoxal piece deeply notched or absent; protibia with three or four socketed denticles; scutellar shield visible in dorsal view; elytral declivity moderately sulcate to strongly excavated, sides with tubercles or spines in most; vestiture hair-like (not scale-like or wider at midlength than at base), except for branched hairs at anterior opening of prothorax.

Ips can be separated from other genera of Ipini by the combined features of the antennal club and elytral declivity. The following diagnostic characters are modified from Wood (1986), as done by Cognato (2000) and Cognato and Vogler (2001):

  • Body length 2.1–8.0 mm (most are larger than 3 mm). Other Ipini are 1.0–4.3 mm long.
  • Antennal club flattened (thickness less than one-third maximum width) and marked by sutures (all illustrations included in key and glossary). Sutures nearly straight to strongly bisinuate (not procurved, or curved toward apex).
  • Elytral declivity broadly and deeply excavated, with sides acutely elevated and armed by three or more pairs of spines. Apices of spines aligned with edge of declivity (declivital margin). Spine 2 (counting from above) acute in lateral view. Lower edge of concavity with an acutely elevated, explanate transverse ridge separating declivital excavation from apical edge. Apex of declivity is not visible in dorsal view.

Ips is most similar in appearance to two other Ipini genera that also develop in Pinaceae: Orthotomicus Ferrari, 1867 and Pseudips Cognato, 2000. Ips can be distinguished from Orthotomicus by the acute spine 2 of its elytral declivity (right-angled in many Orthotomicus) and the broader explanate edge of its elytral declivity. Ips can be distinguished from Pseudips by its straight, bisinuate or acutely angulate antennal club sutures. These sutures are broadly procurved (curved away from the antennal base at the midline of the club) in Pseudips, and also in the tropical, angiosperm feeding Acanthotomicus Blandford, 1894 and the warm-climate, ambrosia-feeding Premnobius Eichhoff, 1878. Pityogenes Bedel, 1888 and Pityokteines Fuchs, 1911 are conifer-feeding Ipini, recognized by their small size (1.8–3.7 mm) and the rounded edges of their elytral declivity. The tropical, ambrosia fungus feeding Premnophilus Brown, 1962 lacks visible antennal sutures.

Most Ips species are grouped into subgenera, based on phylogenetic results by Cognato and Vogler (2001) and Cognato and Sun (2007). Diagnostic characteristics (external morphology only) of subgenera are as follows: Cumatotomicus Ferrari, body length >5 mm, spines on first and second elytral interstriae on declivity; Bonips Cognato, elytral declivity with four spines per side, elytral disc without punctures on interstriae; Granips Cognato, elytral declivity with five to six spines per side; Ips DeGeer, elytral declivity with four spines per side, elytral disc with punctures on interstriae; Incertae sedis, several Ips species are outside any named subgenus.

Diagnostic characters of Ips spp. adults are based on Cognato (2015) and characters developed here. The closely-related (Cognato and Sun, 2007) species: I. confusus and I. paraconfusus, and also I. cembrae and I. subelongatus, and some individuals or one sex of several other species are not fully distinguished from each other in the key to species. This missing information may be important as these species may differ in their biology and distribution and whether they are a regulated pest or not (Stauffer et al., 2001). Additional examination by Ips specialists with appropriate reference collections is required to identify these specimens to species level using morphology (Cognato, 2015). DNA-based studies have been published to support identification of I. confusus and I. paraconfusus (Cognato et al., 1995; Cognato and Sun, 2007) and I. cembrae and I. subelongatus (Stauffer et al., 2001; Cognato and Sun, 2007) but these results have not yet been developed into identification tools.

Ips species are distinguished primarily by characters of the elytra and frons. Identifiers usually begin by counting spines of the elytral declivity (i.e., along one edge of the apical elytral concavity). The following elytral declivity characters are useful: the number of spines on the declivity (not including small denticles on the first elytral interstria); the distance from spine 1 to the elytral suture relative to its height or to its distance from spine 2; and the shininess of the declivity compared to the elytral dorsal surface. Several characters come from the third declivital spine: its shape (acute, right-angled, and obtuse or rounded) and its profile (simple triangular); straight sided with acute apex; petiolate (narrower near base than near apex); hooked (with second point on ventral side); double pointed (appearing like two basally fused spines)). On the elytral disc (the horizontal part of the elytra), the presence or absence of punctures on the interstriae (elevated smooth surfaces between striae) are important, especially on the second and third interstriae midway between the anterior edge of the elytra and the declivity.

On the frons the following presence or absence characters are used: of a median tubercle; of a median carina (between median tubercle and labrum if both present); of a median fossa or pit (above median tubercle if present); of scattered circular tubercles; of setae; of dense setal brushes obscuring integument; or of setal punctures. A few species pairs can only be distinguished by the number of ridges on the pars stridens (Lanier et al., 1991), a stridulatory organ at the vertex of the head capsule. However, this technique is not included here because it requires removal of the head.