Arboriculture & Urban Forestry 41(2): March 2015 would not be ingested until it is reached; the result- ing plant injury can then be evaluated for its conse- quences on achieving management goals. For SPB, the toxicity of imidacloprid is unknown, preclud- ing a direct estimate. However, an example of the approach can be generated by assuming imidaclo- prid toxicity is similar between SPB and EAB, which has an estimated oral LD50 of 7.1 ng/adult beetle (Cregg et al. 2005). With respect to SPB this is prob- ably conservative because EAB is considered to be relatively susceptible to imidacloprid, while imida- cloprid treatments have been ineffective against SPB and other bark beetles (DeGomez et al. 2006; Gros- man and Upton 2006). Windell and Kautz (2007) report the average mass of adult EAB to be 25.6 mg for males and 43.5 mg for females. Adult SPB mass also varies by sex, but a mean of about 1 mg is rea- sonable (Wagner et al. 1981; Ayres et al. 2000), so at the same susceptibility level an estimated LD50 would be about 30 times lower, or 0.24 ng imida- cloprid, per adult beetle. At an average imidacloprid tissue concentration of 1 µg/g phloem dry weight, as observed in the current study, a consumption of 240 µg dry weight of phloem tissue would be required to reach the projected LD50 level. The specific mass of loblolly pine phloem has been estimated as 29.8 mg/cm2 dry weight (Ayres et al. 2000). South- ern pine beetles are about 1.25 mm wide (Wagner et al. 1981) and are assumed cylindrical. Gallery width should be slightly larger but similar. A 2.54 cm long gallery should encompass the movement of about 31.2 mm2 phloem tissue, or 0.312 cm2 , lead- ing to a total phloem displacement of 9.3 mg. This amount of phloem would be estimated to contain 0.0093 µg imidacloprid or 9.3 ng or about 39x the necessary tissue for the estimated LD50. Although contact toxicity may affect the overall toxicity of the treatment (e.g., Wang et al. 2005), the observed gallery lengths suggest the LD50 assumptions esti- mated from EAB are indeed inaccurate and that SPB is less susceptible to imidacloprid than is EAB, perhaps being closer to 10 ng/adult beetle. Additional experiments, conducted in situ, are necessary to clarify the efficacy of bole spray treat- ments against SPB in New Jersey. Field studies are expensive and have the primary difficulty of requir- ing that a threshold of untreated trees be killed before inference can be made regarding treatment efficacy (Shea et al. 1984). Also, established methods for in 99 situ studies are more difficult to employ success- fully with SPB than with related western species (for which they were initially developed) because of the nature of SPB’s spot infestations. The level of aggre- gation in SPB is very high, causing heterogeneity in beetle recruitment across long transects, and alternatively, with the management of “spillover” attacks that occur on trees nearby to study trees. This is particularly difficult in areas where spillover trees cannot be quickly and efficiently cut or har- vested. However, if determination of a single, best treatment for tree survival is considered impor- tant, including factors like treatment durability and the probability of tree mortality under various field conditions, then Astro and Onyx should be compared on standing trees in a well-replicated experiment under sufficiently high beetle pressures. Researchers of the current work interpret the results of this study as support for the effectiveness of permethrin and bifenthrin products applied as bole sprays to uninfested pines for the prevention of attack by SPB. In addition, SPB from New Jersey responded similarly to those from Mississippi to the treated bolts, suggesting that extrapolation from previous studies with SPB in the southeastern U.S. may be useful for predictions with New Jersey SPB and their response to insecticide treatments. How- ever, differing environmental conditions between New Jersey and those areas more oſten infested with SPB, especially winter duration and temperatures, dictate that the durability or persistence of bole sprays be studied in New Jersey if this knowledge is considered sufficiently important. To start, the study authors suggest that one season of protection is likely from either bole spray product tested here when properly applied. Imidacloprid products, on the other hand, did not reduce SPB attack number or gallery length per attack. Thus, little or no support- ing evidence were seen for the application of either imidacloprid product against SPB. When combined with the ineffective results generated in past studies (Grosman and Upton 2006), researchers would not recommend products based upon this AI for pro- phylactic treatment of trees for management of SPB. ©2015 International Society of Arboriculture
March 2015
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