Population trends of the redbay ambrosia beetle (Coleoptera: Curculionidae: Scolytinae): does utilization of small diameter redbay trees allow populations to persist?
The redbay ambrosia beetle, Xyleborus glabratus Eichhoff, vectors laurel wilt, Raffaelea lauricola T.C. Harr., Fraedrich & Aghayeva, that quickly kills all large diam (> 2.5cm) redbay trees [Persea borbonia (L.) Sprengel] in an area but smaller diam trees (< 2.5cm) survive for years. We measured densities of X. glabratus attacks on hanging bolts of freshly cut mature redbay annually for 5 yr (2007-2011) at 7 locations varying in age of infestation from newly infested in 2007 to those that were among the oldest infested areas (infested in 2002 or earlier), to determine if populations persist after mature host trees are gone. Attack densities on redbay bolts at the field sites varied from 10-25 X. glabratus attacks/100 cm2 where large dying redbay trees were still present, to < 1 attack/100cm2 at sites where few or no trees > 2.5 cm diam near ground level were still living at the beginning of the study in 2007. Despite having no large trees available, populations of the beetle persisted at 2 of the 3 oldest infested sites throughout the 5-year survey period (2007-2011). In 2012 we studied X. glabratus utilization of small diam redbay wood as a possible explanation of how populations might survive in these areas in the absence of larger trees. In laboratory trials, X. glabratus produced 27.8 ± 6.63 adults/gallery (± SE) from 18 galleries constructed in portions of redbay trees that averaged 3.2 ± 0.02 cm diam at the point of attack. The smallest stem section to support a successful gallery was 1.7 cm diam but it produced only 2 adults. In field trials no attacks occurred in portions of stems < 1.6 cm diam. Upon stem dissection, more brood was found in stem sections near the ground (0-10 cm) than in those over 80 cm above ground. In addition, brood abundance in10 cm long tree sections exhibited a positive non-linear relationship (cubic polynomial; R2 = 0.21) with section diam. Our data suggest that X. glabratus can maintain low populations in areas devoid of large redbay by utilizing 2-3 cm diam portions of small trees primarily near ground level.
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