The granulate ambrosia beetle, Xylosandrus crassiusculus (Motschulsky) (Coleoptera: Curculionidae: Scolytinae), is an ambrosia beetle of Asian origin that was first detected in the continental United States near Charleston, South Carolina (Anderson 1974). While X. crassiusculus is considered only a nuisance pest in some areas, this insect has caused significant losses (in excess of US $5,000 per nursery) in several nurseries across the southeastern United States and in Texas (Ree & Hunter 1995). The damage potential of this pest is further amplified by X. crassiusculus having in excess of 120 potential tree host species, ranging from hardwoods to pine in both the nursery setting and forest (Weber & McPherson 1983; Solomon 1995; Hudson & Mizell 1999; Oliver & Mannion 2001). Scolytinae pests also include bark beetles, which are the most significant pest of coniferous forests, but can also attack deciduous trees.

Adult ambrosia beetles cause mechanical damage to trees as they tunnel into the heartwood of their hosts. Spores of a symbiotic fungus, which serves as a source of nutrition for the larvae and adult ambrosia beetles, are found within a pouch-like structure called a mycangium. As ambrosia beetles tunnel, the tunnel walls are coated with spores of the symbiotic fungus giving rise to colonial proliferation (Stone et al. 2007). The symbiotic fungus can block parenchyma cells within the vascular tissue of the trees, but Dute et al. (2002) found ambrosia beetle fungi were not particularly pathogenic. However, secondary pathogens such as Fusarium spp. can also be introduced to the host during the process of tunneling, or due to the open wounds created in trees. Secondary pathogens are suspected of playing a role in the eventual dieback and the death of infested hosts (Reding et al. 2010).

The physiological condition of a potential host tree plays an important role in the selection behavior of X. crassiusculus and other ambrosia beetles (Ranger et al. 2012a). When a tree is stressed, it can produce and emit acetaldehyde, acetic acid, acetone, ethane, ethanol, ethylene, and/or methanol (Kimmerer & Kozlowski 1982; Kimmerer & MacDonald 1987; Holzinger et al. 2000; Ranger et al. 2012a). Water damage, improper planting, drought stress, pollutants, low temperature, impaired root function, and diseases are some of the factors that can lead to the emission of ethanol and other stress-related volatile cues (Kelsey 2001; Ranger et al. 2012). This gaseous ethanol is important for attracting ethanol-responsive ambrosia beetles to invade host trees (Ranger et al. 2010; Ranger et al. 2012a,b). Ethanol is an attractant for a number of ambrosia beetles (Ranger et al. 2010, 2011, 2012b). Mixtures of ethanol and methanol have also increased trap captures of the bark beetle Hypothenemus hampei (Ferrari) compared to ethanol alone (Da Silva et al. 2006), indicating there is potential for synergistic relationships among volatile compounds.

In addition to olfactory cues, visual cues can also play a role during host selection by the Scolytinae. For instance, Da Silva et al. (2006) found green plastic bottle traps increased captures of H. hampei and other Scolytinae species compared to clear traps or transparent red traps. Similarly, Strom et al. (1999) demonstrated that both the southern pine beetle Dendroctonus frontalis (Zimmerman) and Thanasimus dubius (Fabricius) are affected by altering visual silhouettes on traps, finding that white paint and white trap paneling caught significantly fewer southern pine beetles than black or transparent coloration. In order to improve the detection and monitoring of various bark and ambrosia beetles, olfactory and visual cues were examined for their ability to enhance the attractiveness of ethanol-baited traps. Current management of ambrosia beetles in nurseries relies primarily on preventive treatments of insecticides applied to the trunks of trees, which can be improved with a greater knowledge of their seasonal activity and ecological interactions (Reding et al. 2010; Hudson & Mizell 1999).

MATERIALS AND METHODS

RESULTS

Experiment 1: Influence of Olfactory Cues

No significant difference was detected in captures of all Scolytinae for traps baited with varying ratios of ethanol to methanol (100:0; 75:25; 50:50; 25:75; 0:100) (F_4,30 = 0.99, P = 0.43) (Fig. 1). No significant differences in mean (± SE) captures of X. crassiusculus were detected for traps baited with ethanol alone (0.57 ±0.30), methanol alone (1.0 ±0.38), or varying ratios of ethanol to methanol at 75:25 (1.0 ±0.53), 50:50 (1.57 ±0.53), 25:75 (0.86 ±0.59) (F_4,30 = 0.54, P = 0.71).

Fig. 1.

Number (mean ±SE) of Scolytinae adults collected with bottle traps baited with various ratios of ethanol to methanol (100:0, 75:25, 50:50, 25:75, 0:100) deployed from Jun to Jul 2011 in Tuskegee National Forest, Alabama, USA. Means with different letters indicate significant differences (one-way ANOVA; P < 0.05).

DISCUSSION

Montgomery & Wargo (1983) did not find a mixture of 50% ethanol, 5% methanol, and 5% acetaldehyde to be more attractive than ethanol alone to various Scolytinae. Similarly, Ranger et al. (2010) found methanol-baited traps were more attractive to Xylosandrus germanus (Blandford) than acetaldehyde-and acetone-baited traps, but less attractive than ethanol-baited traps. Injecting sweetbay magnolia, Magnolia virginiana L. (Magnoliales: Magnoliaceae), with methanol did not induce more ambrosia beetle attacks than a non-injected control, but over 120 attacks per plant were induced following ethanol-injection (Ranger et al. 2010). We found methanol alone was just as attractive as ethanol alone (Fig. 1), which differs from findings of other studies involving Scolytinae (Montgomery & Wargo 1983; Ranger et al. 2010). The lack of significant differences among any of the treatments involving various ratios of ethanol to methanol could be due to low trap counts from deploying traps after peak flight, which typically takes place in Mar to Apr in Macon County, Alabama (Gorzlancyk, pers. obs.) The relatively high release rate of ethanol and methanol from our lure apparatus may also have resulted in low trap counts. A positive concentration response to ethanol has been demonstrated for a few ambrosia beetles (Klimetzek et al. 1986; Ranger et al. 2011, 2012b), but too high of a release rate could have a repellent effect on ambrosia beetle orientation. Montgomery & Wargo (1983) found ethanol released at 2g/day attracted more Scolytinae than higher release rates. Thus, a mixture of ethanol and methanol cannot be recommended as an alternative or replacement of ethanol alone as an attractant for X. crassiusculus and other Scolytinae.

Fig. 2.

Number (mean ±SE) of Scolytinae adults collected with ethanol-baited traps supplemented with various LED lights during Jun to Jul 2011 in Tuskegee National Forest, Alabama, USA. Means with different letters indicate significant differences (one-way ANOVA; P < 0.05).

UV and household floodlights are currently used in general insect traps (Syms & Goodman 1987; Ferreira et al. 2012). Results from our current study demonstrated Scolytinae showed a higher attraction to ethanol-baited traps supplemented with green (525 nm) and UV (395 nm) LED lights compared to ethanol-baited traps alone. Attraction to green wavelengths has also been noted to occur in other members of the Curculionidae. For instance, Nakamoto & Kuba (2004) found evidence that the weevil Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) preferred green LEDs to the UV light traps.

Our results suggest ethanol-baited traps supplemented with green or UV LEDs would be useful for detecting various Scolytinae in a nursery or forest setting. The integration of specific LED wavelengths into other trap configurations could potentially yield species specific responses. A more detailed examination is therefore warranted to elucidate the physiological mechanisms underlying the role of visual cues in behavioral responses by X. crassiusculus and other Scolytinae.