Sexing live mountain pine beetles D endroctonus ponderosae : refinement of a behavioral method for D endroctonus spp.

Members of the genus Dendroctonus (Coleoptera: Curculionidae, Scolytinae) are some of the most aggressive tree-killing bark beetles in the world. As such, much research on this genus has been undertaken to understand the factors that affect the population dynamics of these insects (Six & Bracewell, 2015; Aukema et al., 2016). Despite biome-level ecological impacts of the most aggressive members of this genus when at outbreak levels, the flight periods of many temperate species are constrained to just a few weeks of peak emergence during which beetles locate and procure hosts via pheromone-mediated mass attacks (Rudinsky, 1962; Raffa, 2001; Bentz et al., 2014). Females initiate boring into a host. Thus, for many manipulative laboratory and field experiments assessing reproduction or host selection, the ability to quickly and accurately determine the sex of live insects is required. Sexual dimorphism on the frons and pronotum is present in some species ofDendroctonus, most prominently in those species closely related to the southern pine beetle (Dendroctonus frontalis Zimmermann) (Wood, 1982), and provide varying degrees of accuracy in determination (Osgood & Clark, 1963; Tate & Bedard, 1967). The only consistently 100% accurate method of sex determination via secondary characters for Dendroctonus spp. requires examination of adults for the presence of a highly sclerotized plectrum on the seventh abdominal tergite (Lyon, 1958; Safranyik & Carroll, 2006). This plectrum is used for stridulation by males but absent in females. This morphological character is highly accurate (Lyon, 1958; Jantz & Johnsey, 1964; Godbee & Franklin, 1978), but can pose challenges when working with live insects (Tate & Bedard, 1967). For example, female mountain pine beetles (Dendroctonus ponderosae Hopkins) tend to draw their abdomens tight against the elytra when prodded. Squeezing the abdomen or manipulating its position with a metal probe under a dissecting microscope (McCambridge, 1962) can extend handling times and result in harm to the insect (Godbee & Franklin, 1978). Morphological examinations remain a popular technique, however, and work reliably when executed properly. Several authors have tested the efficacy of stridulatory behavior as a potential method for sex determination of live Dendroctonus beetles (Table 1). When disturbed, males will use stridulation to produce predominantly simple ‘stress’ chirps that are characterized by rapid short bursts (McCambridge, 1962; Michael & Rudinsky, 1972; Fleming et al., 2013). Chirps are produced as males move the plectrum against the pars stridens on the underside of the elytra (Hopkins, 1909; Michael & Rudinsky, 1972). Female Dendroctonus spp. beetles are also able to stridulate, using a different stridulatory apparatus, but their short, simple chirps, characterized by a low sound pulse rate, are easily differentiated from the rapid chirping and higher sound pulse rates of males (Barr, 1969; Rudinsky & Michael, 1973; Yturralde & Hofstetter, 2015). Sonic emissions of female Dendroctonus spp. are typically restricted to courtship behaviors, though a stress response has been detected in the red turpentine beetle (Dendroctonus valens LeConte) and the larger Mexican pine beetle (Dendroctonus approximatus Dietz) (Ryker & Rudinsky, 1976a; Yturralde & Hofstetter, 2015). The ability to chirp likely confers reproductive advantage to the joining sex, as this trait has been independently gained in both Dendroctonus and Ips (Barr, 1969; Lewis & Cane, 1990), and is conserved amongmales acrossDendroctonus spp. (Ryker, 1988). Though audible observations of stridulation can be useful for sexing adults of Dendroctonus spp., some error is *Correspondence: DerekW. Rosenberger, Department of Entomology, University ofMinnesota, 1980 Folwell Avenue, St. Paul, MN 55108, USA. E-mail: rose0675@umn.edu