98 Strom et al.: Evaluating Insecticides for Preventing Southern Pine Beetles tive. Contact toxicity has been observed but LC50s are about 10x lower for ingested compared to con- tact toxicity with the Asian longhorned beetle (ALB) (Anoplophora glabripennis) (Wang et al. 2005). Even with ingestion, mortality from imidacloprid can occur slowly in adult beetles. For example, with ALB feeding on Norway maple (Acer platanoides) a 1.3 ppm residue concentration required 21 days to kill 50% of the population (Ugine et al. 2012). In addition, even with stem injection, only 5.2% of treated trees eliminated egg laying (Ugine et al. 2012). Sub-lethal effects may help to increase effec- tiveness but they are as yet undetermined, and with bark beetles they are probably less useful because beetles are already attacking, producing gallery and attractants, and exposing the tree to phoretic organ- isms. Researchers do not know the tissue residue concentrations of imidacloprid required to impact bark beetles, but tree protection from these pests requires faster action resulting in less adult activity than indicated for ALB in the Ugine et al. (2012) study. Although feeding by adult ALB may be a tar- get for insecticides, bole protection requires larval poisoning. A similar situation exists with the emer- ald ash borer (EAB) (Agrilus planipennis); however, with bark beetles, adult activity kills the host tree so they must be impacted directly and quickly by phloem residues to provide adequate tree protection. The mode of activity of systemic insecticides creates at least three problems for tree protection against bark beetles. Current systemic products do not reduce the number of attacks, they do not impact associated organisms (e.g., fungi), and attacking beetles are still able to make galleries, even if abbreviated. Thus, the magnitude of the feral population of beetles becomes integral to treatment effectiveness because a high number of attacks is still likely to kill a host tree almost regard- less of beetle success. That is, attacking beetles con- struct galleries and produce frass, so even if gallery length is significantly reduced, an ample pool of feral beetles may still be sufficient to cause treat- ment failure through a higher attack density. On the other hand, quantifying ambient beetle popu- lations is notoriously difficult, as is estimating the number of beetles available to attack a particular tree in time and space. This can lead to a treatment appearing to be effective, particularly over short beetle exposure periods, so long as it is marginally ©2015 International Society of Arboriculture more effective than doing nothing. Successful mass attack requires synchronous events, which in turn require an ample pool of attackers during a limited moment in time (probably weeks or less, but this can happen in more than one episode). In addition, the impact of organisms like fungi, that are pho- retic on beetles and inoculated during the attack process, is uncertain in the unnatural within-tree environment created by an effective systemic; research into these factors is only beginning. The study authors do not know the ingested dosage of imidacloprid, or residue concentrations, that are required to impact SPB. However, residue values in this study were low for successful tree protection, although similar to those observed in other types of tissues researchers and others have examined (Xu et al. 2008; Fischer et al. 2009; Ugine et al. 2012). In results from published laboratory studies, SPB kept at 28°C averaged 2.55 cm of gal- lery before the first egg niche (Clarke et al. 1979). It is widely believed that under natural conditions, oviposition by SPB is indicative of a host that will succumb to attack. Thus, one method for develop- ing a threshold value to assess the potential utility of a systemic insecticide is to select a portion of the gallery length as a target reduction that must be achieved to indicate treatment success. Shea et al. (1984) employ a target of 90% (80% with the CI) protection (tree survival) as a basis for efficacy with standing trees, and Strom and Roton (2009; 2011) base their preliminary suggestions for screening effectiveness with host bolts on this approach. They suggest, as a starting point for assessing treat- ments, the number of attacks must be reduced by 80% relative to controls for a treatment to be considered effective. For gallery length, 20% of the overall control average for field-generated GLA in the present study would provide a gallery length value of about 1.3 cm, or half of the gallery length that is typical before the first egg. If adopted in this study, this criterion would agree with the other criteria and result in neither imidacloprid product in this study being considered successful. Another possible approach to predicting the utility of systemics is to use an experimentally determined toxicity of the AI for the target insect and estimate the tissue consumption needed for this amount to be ingested. This level of tissue consumption can be considered as likely to occur because toxic levels
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