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1 September 2015 Seasonal and Spatial Dispersal Patterns of Select Ambrosia Beetles (Coleoptera: Curculionidae) from Forest Habitats into Production Nurseries
Christopher T. Werle, Juang-Horng Chong, Blair J. Sampson, Michael E. Reding, John J. Adamczyk
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Exotic ambrosia beetles (Coleoptera: Curculionidae) are important pests of ornamental tree nurseries. Although these beetles reportedly disperse in early spring from peripheral forested areas into nurseries, few studies have determined how far they fly to infest new host trees, or whether a masstrapping strategy can adequately protect a nursery crop. Field monitoring with ethanol baits in South Carolina (2011–2012), Mississippi (2013–2014), and Louisiana (2013–2014), USA, determined the timing of peak ambrosia beetle flights, dispersal distance, and optimal trap location. In addition to the well-documented spring flight peak, southeastern nursery managers may need to be aware of a second, late-summer flight. Captures from traps placed in a nursery at various distances (−25 to 200 m) from the forest—nursery interface showed a significant linear and quadratic trend in decreasing numbers of beetles captured with increasing distance from the forest in South Carolina, whereas significant linear, quadratic, and cubic trends were detected in Louisiana and Mississippi. Although captures at the nursery edge were lower than within the forest, traps placed at the nursery edge may still represent the optimal tool for both monitoring and mass-trapping programs because of easier access for personnel. Susceptible tree cultivars may gain added protection when placed deeper within nursery interiors and when baited traps line adjacent nursery edges.

Exotic ambrosia beetles, particularly Xylosandrus crassiusculus (Motschulsky), Xylosandrus germanus (Blandford), Xylosandrus compactus (Eichhoff), and Cnestus mutilatus (Blandford) (Coleoptera: Curculionidae), have been important tree pests in the southeastern United States at nurseries and in landscapes for decades. These species have gained prominence in recent years due to their wide host range, frequency of attacks, and difficulty of control (Mizell et al. 1994; Oliver & Mannion 2001; Fulcher et al. 2012). Foundress beetles tunnel into trees and inoculate their brood gallery with a symbiotic fungus, which is then consumed by adults and larvae (Biedermann & Taborsky 2011). These primary fungal symbionts, as well as secondary fungal pathogens, contribute to host plant mortality (Weber & McPherson 1984; Kuhnholz et al. 2001). Larval development of ambrosia beetles is completed within the gallery, and newly-eclosed, mated females disperse to new tree hosts (Weber & McPherson 1984). Although ambrosia beetles invading ornamental nurseries were presumed to originate from peripheral forested areas, few studies have fully investigated invasion source.

Standard management recommendations for ambrosia beetles include using ethanol lures to monitor adult flight in early spring, followed by applications of pyrethroid insecticides every 3 to 4 wk after the first beetle flights are detected (Hudson & Mizell 1999; Ranger et al. 2010, 2012; Reding et al. 2010, 2011). Prior studies describe an early spring population peak followed by a summer decline, and possibly a second, late summer peak for southern populations (Hudson & Mizell 1999; Oliver & Mannion 2001; Reding et al. 2010; Werle et al. 2012).

In natural environments, the directed flight of ambrosia beetles occurs within forests where wind speed is relatively low, particularly close to the ground where most beetle flight occurs (Browne 1961; Reding et al. 2011). However, within large open nurseries, where fewer windbreaks exist, higher wind speeds make directed flight significantly more difficult for small beetles (Pasek 1988). In a mark—recapture study, the striped ambrosia beetle, Trypodendron lineatum (Olivier) (Coleoptera: Curculionidae), which is a coniferous forest tree pest in the western United States, only exhibited non-directed flight for distances of 100 m or more, whereas recaptures at 500 m were primarily downwind of the release point (Salom & McLean 1989). Mean dispersal distances of marked lesser grain borers, Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae), were significantly longer in wooded sites as compared with open sites (Mahroof et al. 2010). Similarly, T. lineatum was recaptured in significantly higher numbers from baited traps in forested as opposed to open settings, likely due to wind speeds roughly 4 times higher in the open settings (Salom & McLean 1991). Further knowledge of ambrosia beetle dispersal patterns may augment available cultural measures; for example, there may be a distance from the forest edge beyond which ambrosia beetles are unlikely to fly and attack trees. In large nurseries encompassing at least 50 hectares, we hypothesize that locating susceptible cultivars at the interior may provide added protection from ambrosia beetle attack.

Mass trapping is a technique that has been used successfully to suppress or even eradicate incipient populations of invasive insects at their advancing front (Brockerhoff et al. 2010). Even for established populations, mass trapping can offer cost-effective control when an attractant is perceived by a high proportion of the target insects and has a stronger pull than its ambient source (i.e., stressed trees), when traps collect insects throughout the dispersal period, and when traps, lures, and labor are cost effective (El-Sayed et al. 2006). Traps used in conventional ambrosia beetle monitoring programs meet all of these criteria; therefore, by capturing and killing a large proportion of dispersing females, mass trapping could be used as a population ma