Redbay ambrosia beetle (RAB), Xyleborus glabratus Eichhoff (Coleoptera: Curculionidae: Scolytinae), a native of Asia, was first discovered in the USA near Savannah, Georgia in 2002 (Haack 2001; Rabaglia et al. 2006). RAB is an effective vector of Raffaelea lauricola T.C. Harr., Fraedrich & Aghayeva (Harrington et al. 2008) that causes laurel wilt (LW), a lethal disease of several trees in the Lauraceae in the southeastern USA (Crane et al. 2008; Mayfield et al. 2008). Ambrosia beetle adults bore through the bark and into the xylem (wood) where they lay eggs, then adults and larvae cultivate and feed on symbiotic ambrosia fungi that grow in the galleries. Native Persea (Laurales: Lauraceae) species appear to be preferred hosts. LW is responsible for high mortality of redbay [P. borbonia (L.) Spreng.], swampbay [P. palustris (Raf.) Sarg.], and sassafras [Sassafras albidum (Nuttall) Nees] in Alabama, Florida, Georgia, Mississippi, North Carolina and South Carolina (Fraedrich et al. 2008; Hanula et al. 2008; Gramling 2010). As LW encroaches upon the Lake Wales Ridge ecosystem in southcentral Florida, silkbay (P. humilis Nash) is also showing susceptibility to LW and is dying. Additional species affected by LW include avocado (P. americana Mill.), spicebush [Lindera benzoin (L.) Blume], and other woody Lauraceae (Fraedrich et al. 2008) (Table 1).
The susceptibility of 5 avocado cultivars of Mexican, Guatemalan or West Indian origin to RAB and LW was demonstrated by Mayfield et al. (2008). However, with more than 23 West Indian cultivars grown in Florida, it is necessary to determine their susceptibility. Moreover, as an adventive species to the North American continent, RAB might affect other valuable New World species. Most Persea species are of Mexican, Central American, or South American origin. These Persea may have significant value in germplasm collections, some have been discovered recently, and some have resistance to diseases that afflict their commercial relative, the avocado (Skutch et al. 1992; Scora & Bergh 1992; Zentmyer & Schieber 1992). Thus, their susceptibility to RAB and LW warrants evaluation. Another member of the Lauraceae of much concern is the California bay laurel [Umbellularia californica (Hook. & Arn.) Nutt.], a dominant hardwood species of the U.S. Pacific Coast. Through inoculation experiments, Fraedrich (2008) demonstrated that U. californica is susceptible to LW. With continued westward spread of LW, the host status of U. californica needs to be confirmed.
The 3 studies presented here evaluate susceptibility to RAB and LW in: 1) 13 West Indian avocado cultivars; 2) 10 non-commercial Persea spp., 1 Beilschmidia sp. (a genus related to Persea), and 3) U. californica. First, no-choice experiments were conducted to determine if RAB would bore into avocado cultivars not screened previously by Mayfield et al. (2008) and following similar methodology. Avocado cultivars ‘Bernecker’, ‘Beta’, ‘Brooks late’, ‘Choquette’, ‘Donnie’, ’Dupuis’, ‘Hall’, ‘Loretta’, ‘Lula’, ‘Monroe’, ‘Simmonds’, ‘Tower 2’ and ‘Waldin’ (4 plants each) were planted in 10-gallon pots in a screenhouse at the Plant Sciences Research and Education Unit, University of Florida (UF), Citra, Florida in VI2008. Two plants of each cultivar were infested by enclosing 4 newly emerged ♀ RAB (UF colonyreared) within a mesh sleeve on the lower trunk. Two plants per cultivar were uninfested controls. Entrance holes and perseitol (white exudate from wounds) were monitored for 4 wk. Severity of wilt symptoms was scored using the following LW index: 0 = no wilt; 1 = wilt, no leaf necrosis; 2 = wilt, 10% necrosis or defoliation; 3 = wilt, 30% necrosis/ defoliation; 4 = 50% necrosis/defoliation; 5 = 75% necrosis/defoliation; 6 = 100% necrosis/defoliation (Peña et al. 2011). After 2 wk, all cultivars had 1–2 entrance holes (df8,36; F = 2.18; Pr> F = 0.052) (Table 2). This is an important indication of successful beetle boring. The LW index assessed during the last wk of the experiment fluctuated between 0 – 1.8(df8,38; F = 1.85; Pr > F = 0.10), which corresponded to an average between wilt only and 10% leaf necrosis (Table 2). On 22-VIII-2008 all plants were harvested, and wood chips were collected, surface sterilized, and plated on medium selective for Raffaelea lauricola (Mayfield et al. 2008). Isolation frequency ranged from 0–50% for the different cultivars (Table 2). These results are in in agreement with the results of Mayfield et al (2008) who reported that when given no choice, RAB can bore into and transmit the pathogen R. lauricola into the xylem of avocado, which characteristically presents as dark discoloration of the outer sapwood
In the second study, RAB attraction to 10 Persea spp. and 1 Beilschmidia sp.was tested in the field and laboratory. Four replicate logs (33 cm long × ∼2.5 cm diam.) of P. caerulea Ruiz and Pav., P. borbonia, P. pachypoda Ehrenb [syn: Cinnamomum pachypodum (Nees) Kosterm., Phoebe pachypoda (Nees) Mez], P. floccosa Mez., P. skutchii C. K. Allen, P. nubigiena L. O. Williams, P. indica (L.) Spreng., P. tolimanensis Zentmyer & Schrieber (also called “aguacate de mico”, a Central American species), P. cinerascens S. F. Blake, P. tilarensis and Beilschmidia were hung in full sun ∼1.2 m high near infested redbay trees with an approx. distance of 10 m between adjacent treatments in Hastings, Florida from IX to X-2009. After 30 d, the logs were collected and the number of entry holes determined under a stereomicroscope. Logs were placed individually in cardboard containers for beetle emergence at 26 °C and 70–80% RH for 60 d. Bolts of P. skutchii, P. cinerascens and P. indica appeared to be preferred by ambrosia beetles over other Persea spp., including P. borbonia (Table 3). Unfortunately, no beetle emerged from these bolts, perhaps due to desiccation of bolts under field or storage conditions.
Another survey was set up at Hickory Hammock, a 4,000-acre (1,619 ha) natural preserve in Highlands County, Florida (27°25′35″N, 81°9′42″ W). This site was known since 2009 to have LW and RAB. Bolts (same species as above) were hung on 23-II-2010 on the sunny edge of a trail, removed 30 d later, and stored as described above. With the exception of P. floccosa, bolts of all species had entrance holes (df 10,10; F = 2.04; Pr > F = 0.13). RAB emerged from P. caerula and P. tilarensi; Ambrosiodmus lecontei Hopkins (Curculionidae: Scolytinae) from P. nubigena, P. pachypoda and P. tilarensis; and Xylosandrus crassiusculus (Motschulsky) (Curculionidae: Scolytinae) (df10,10; F = 1.61; Pr > F = 0.23) (Table 3). No beetle emerged from other Persea spp. However, lack of emergence from those species could be due to desiccation and not the result of plant resistance.
MEAN INFESTATION BY XYLEBORUS GLABRATUS AND SEVERITY OF LAUREL WILT OBSERVED AFTER 4 WK IN NO-CHOICE TESTS WITH 13 WEST INDIAN AVOCADO CULTIVARS. YOUNG POTTED TREES (2 PER CULTIVAR) WERE EXPOSED TO 4 X. GLABRATUS FEMALES ENCLOSED IN A SLEEVE AT THE BASE OF THE TRUNK.
On 24-II-2010, host boring bioassays were set up in the laboratory, using methods similar to Kendra et al. (2011). Bolts (10.9 × 2.7 cm diam) of the same species above (2 replicates per species) were cut and immediately placed individually in glass jars (0.95 liter) with 200 mL of water to prevent desiccation. Five ♀ newly emerged RAB were placed on top of each bolt and kept for 24 h at 22 ±2 °C and 12:12 h L:D. RAB boring was recorded at 1, 2, 3, 4 and 24 h. RAB bored into all species except P. floccosa, and infestation varied from 1.5 to 4 beetles boring per bolt (Table 3) (df, 10, 11; F = 3.79, Pr > F = 0.02) (GLM procedure, Tukey's Studentized Range (HSD) Test (SAS, 2008). P. floccosa is a Guatemalan-type species, which is believed to be the most ancient form of Persea (Scora & Bergh 1992).
SUSCEPTIBILITY OF PERSEA SPECIES TO ATTACK BY XYLEBORUS GLABRATUS AND OTHER SCOLYTINAE IN FLORIDA. BOLTS WERE HUNG FOR 30 DAYS IN FIELD TESTS. BOLTS WERE EXPOSED TO 5 FEMALE X. GLABRATUS IN NO-CHOICE LABORATORY BIOASSAYS.
In the third experiment, bolts of redbay, avocado and California bay laurel were hung at OrdwaySwisher Biological Station, University of Florida, Gainesville (N 29° 41.040, W 082° 22.109). Nine logs of each species were hung in an area where both diseased and healthy red bay were present, and left in the field for 1 mo (18 IX-19-X-2009). Logs were brought into the laboratory, bore holes were measured, and those of appropriate diameter for RAB (0.8 mm, Hanula et al. 2008) were counted and recorded. No dissection of the galleries was made. Surprisingly, no RAB entry holes were found on redbay logs, while entrance holes were recorded on avocado (0.55 ± 0.29) and California bay laurel (0.22 ± 0.14). However, no beetle emerged from these bolts. Field tests and lab bioassays conducted during 2011 have shown that female RAB are highly attracted to, and will bore into freshly-cut bolts of U. californica (P. E. Kendra, unpubl.; A. E. Mayfield, unpubl.).
These preliminary results indicate that there are numerous New World species of the Lauraceae potentially at risk of attack by X. glabratus. More research is needed to fully determine the susceptibility of Persea spp. and other genera within the Lauraceae to both the pathogen and vector. However, because of the difficulty in obtaining bolts of non-native Persea species and other genera, efforts should be directed particularly at those species where an indication of nonsusceptibility to X. glabratus has been observed.
We thank Drs. R. Giblin-Davis and R. E. Litz for suggestions to improve the manuscript. We thank Jose Alegria, Ana Vargas and the personnel of the Plant Science Research Station of the University of Florida, IFAS, Citra, Florida for their help. This research was partially funded by the Florida Avocado Committee.