148 SjÖman et al.: Host Trees for Wood-Boring Pests: An Urban Forest Perspective sional host tree,” “attack primarily” and “complete development” in the “Range and Life Cycle” section for ALB. Hérard et al. (2006) mentioned infested trees and host plants without specifying the mean- ing, and stated that certain species were preferred host plants, but not whether this meant feeding, ovi- position, or full development. FAO (2007) explained that the larvae injure the tree by tunnels under the bark and bore into wood, but when listing trees spe- cies it stated “the main genera of trees that it feeds on are. . .” This is confusing, as adult beetles feed on some trees but oviposition and larval development do not always occur on the same species as adult feeding. The Danish Natural Agency (Naturerh- vervstyrelsen 2012) concluded that one should dis- tinguish between host plants where the beetles can undergo full development, and host plants where the adults feed on the trees. There are several exam- ples of rating systems that include the possibility of a reproductive cycle for the species. For example, Yin and Lu (2005) used a scale from 1 to 5 to rate tree species, where grades 3 to 5 included the ALB being able to complete a life cycle. Ric et al. (2006) used a three-point rating system, where 1 was suit- able for the entire life cycle, 2 was where the bee- tles had laid eggs but there was no evidence that a whole cycle was possible, and 3 was for species with unknown suitability for beetle larval development. Among the studies concerning ALB, Lud- wig et al. (2002), Smith et al. (2002), MacLeod et al. (2002), Morewood et al. (2003; 2004a; 2004b; 2005), Auclair et al. (2005), and Hajek and Kalb (2007) had obtained their host-related informa- tion from greenhouse tests, while the remaining publications appeared to refer to cases and obser- vations in native communities or in public planta- tions. Only one of the 13 studies of CLB reviewed had obtained host-related information from greenhouse tests (Adachi 1994; Appendix 3). To develop more accurate species-related informa- tion concerning susceptibility to ALB and CLB, some authors point out that controlled laboratory tests are needed (MacLeod et al. 2002; Morewood et al. 2004a). However, when beetles are introduced to one or few species in a controlled environment, they may use less favorable species in the absence of more susceptible species. Therefore, host-related conclusions from controlled laboratory or green- house tests must be thoroughly analyzed. How- ©2014 International Society of Arboriculture ever, if a species in these tests shows resistance to the beetles, this could be important information. In the review by Yin and Lu (2005), a number of tree species native to China were classified as resistant or rarely affected by ALB (Appendix 1). In fact, the majority of the species/genera classified as resistant or rarely affected by ALB in Appendix 1 are native to China and Japan, where they have been living for generations, side-by-side with the beetle and might have developed natural strategies to avoid attacks. For example, there may be chemical substances in the wood making it unattractive for feeding or unsuitable to support complete development of the ALB (Morewood et al. 2004a). Once the bio- chemical basis of resistance against ALB and CLB is elucidated, researchers may have a greater under- standing of species that are superior to use, while any biocidal compounds produced could perhaps be manipulated to help protect more vulnerable trees from these pests. In the study by Morewood et al. (2004a), an evaluation of four tree species for ALB in controlled greenhouse conditions showed that the Chinese callery pear (Pyrus calleryana) was most likely to cause adult mortality of the beetle. No larvae survived, although eggs in callery pear hatched and the neonates began to feed and con- struct galleries in the wood. In work on ALB and CLB, it is interesting to know not only which spe- cies are resistant or rarely affected by these beetles, but also why they are resistant or rarely affected. Data on citations of host-related information within the publications reviewed here clearly revealed a large number of cross-references, especially among recent studies. For example, the paper by Lingafelter and Hoebeke (2002) was included in six other pub- lications as a host-related reference, but they in turn based their host-related information mainly on older Chinese and Japanese studies (Appendix 3). This pattern of much older host-related informa- tion originating from Chinese studies, especially for ALB, makes it difficult to analyze the methodology and approach used in the studies or to evaluate from where and how the conclusions were developed. The most frequently cited publication within this review (that by Nowak et al. 2001) used three Chinese stud- ies (in Chinese) and two unpublished sources from North America as the basis for a host-related evalu- ation. More recent studies from North America and Europe make a much more transparent presentation
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