Anulewicz et al. (2008) carried out field experiments to examine the potential for Emerald Ash Borers in North America to expand their host range to include species other than ashes (Fraxinus spp.). In Asia, the Emerald Ash Borer seems not to be a major pest, generally occurring at low densities and attacking severely stressed or declining trees. In North America, however, where ash species have no co-evolutionary history with this insect, the Emerald Ash Borer has killed healthy, as well as stressed, Green Ash (F. pennsylvanica), White Ash (F. americana), Black Ash (F. nigra), and Blue Ash (F. quadrangulata). In Asia, Emerald Ash Borer has been reported to attack other hosts in addition to ashes, but attacks on non-ashes have not been reported for North America (although tens of millions of ash trees have been attacked). In field tests using several North American relatives of reported non-ash hosts from Asia, Anulewicz et al. found that, although Emerald Ash Borer adults would occasionally land on and oviposit on logs and trees of non-ash species, larvae did not successfully develop on anything other than ashes. (Anulewicz et al. 2008 and references therein)
Rebek et al. (2008) tested the resistance to Emerald Ash Borer of an Asian ash species, Manchurian Ash (Fraxinus mandshurica) and found that it was significantly less susceptible to Emerald Ash Borer attack than were tested North American ashes, suggesting the existence of targeted defenses resulting from a shared evolutionary history in their native Asia. Liu et al. (2007) reported that in China exotic North American species such as Green Ash are more susceptible to Emerald Ash Borer attack than are native Chinese ash species when planted at the same site.
Liu et al. (2007) studied two natural enemies of Emerald Ash Borer, Oobius agrili (Encyrtidae) and Tetrastichus planipennisi (Eulophidae), and found that both contribute significantly to Emerald Ash Borer population suppression on Green Ash in northeastern China. Previous studies showed that T. planipennisi was also an important mortality factor for Emerald Ash Borer on Manchurian Ash in China. Oobius agrili is a newly described solitary egg parasitoid of Emerald Ash Borer from China with no other known hosts. Although host resistance to Emerald Ash Borer differs between native Chinese ash species and species introduced from North America, the ability of these parasitoids to locate and attack Emerald Ash Borers apparently does not differ between Chinese and North American ashes. Based on the high observed parasitism rates, short generation times, high reproduction rates, and life-cycle synchronizations with their respective host stages, the authors suggest that these parasitoids may prove useful for biological control of Emerald Ash Borer in North America. (Liu et al. 2007 and references therein)
Yang et al. (2008) carried out no-choice tests to examine the potential host range of Spathius agrili (Hymenoptera: Braconidae), a braconid species described in 2005 that paralyzes Emerald Ash Borer larvae when it lays eggs on them, arresting their development, with newly hatched wasp larvae consuming the host beetle larva in 7 to 10 days (Yang et al. 2005). They found that although S. agrili can parasitize some other Agrilus larvae, observed attack rates were significantly lower than for its natural host, the Emerald Ash Borer.
Ulyshen et al. (2010) studied competitive interactions betwee two of the three hymenopteran parasitoids native to China that are being released in the United States as biological control agents for the Emerald Ash Borer, the larval ectoparasitoid Spathius agrili (Braconidae) and the larval endoparasitoid Tetrastichus planipennisi (Eulophidae) (the third species being released, Oobius agrili [Encyrtidae], is an egg parasitoid and therefore not expected to compete directly with the other two). Female S. agrili permanently paralyze their hosts by envenomation during oviposition and produce 1 to 18 offspring per host (mean 8.4); in China, they complete up to four generations a year and levels of parasitism range from 30% to 90%, with 1 to 35 eggs associated with a single host individual (Yang et al. 2005). Between 4 and 172 T. planipennisi offspring are produced per host (Uyshen et al. 2010). In contrast to larvae parasitized by S. agrili, host larvae parasitized by T. planipennisi remain active and continue to feed for about a week. After consuming the host larva, parasitoid larvae exit from the integument and pupate within the beetle gallery. Adult wasps eclose approximately 15 days after pupation and exit the tree through one or more holes chewed through the bark. In China, four or more generations are produced each year and observed levels of parasitism range from 0% to 65%, with 56 to 92 individuals developing in a single host larva (Liu et al. 2007; Yang et al. 2006). In competition trials, Ulyshen et al. (2010) found that S. agrili tended to excluded T. planipennisi, an observation they attribute to S. agrili being much more efficient at locating hosts. They also found that S. agrili parasitized larvae previously parasitized by T. planipennisi, although the reverse was not observed. However, S. agrili offspring failed to complete development on hosts that were previously parasitized by T. planipennisi. Ulyshen et al. (2010) suggested releasing these two species separately in time or space to avoid the antagonistic interactions observed in their study.
Although investigations of biocontrol options are focused on the use of parasitoids imported from China, Duan et al. (2009) surveyed the parasitoids currently attacking Emerald Ash Borer in western Pennsylvania. Five parasitoids were identified: Balcha indica (Eupelmidae), Eupelmus pini (Eupelmidae), Dolichomitus vitticrus (Ichneumonidae), and 2 ichneumonids identified only to genus, Orthizema sp. and Cubocephalus sp.. Together, these parasitoids parasitized 3.6% of the sampled Emerald Ash Borers (1,091 larvae, prepupae, and/or pupae). Balcha indica accounted for 82% of the parasitoids recovered. The association with Emerald Ash Borer of the two eupelmids, although not the ichneumonids, was confirmed in laboratory assays The authors suggest that these two eupelmid species may be complementary to the ongoing Emerald Ash Borer biological control efforts in the U.S., which include 1 egg and 2 larval parasitoids that attack Emerald Ash Borer in China (Liu et al. 2007; see above). Another native ectoparasitoid found to attack Emerald Ash Borer larvae, Atanycolus hicoriae (Braconidae) is being evaluated as a potential biocontrol agent as well. (Duan et al. 2009 and references therein)
Gandhi and Herms (2010) investigated the question of whether the large scale destruction of ashes (Fraxinus spp.) by Emerald Ash Borers in North America could result in the decline or extinction of other ash-associated arthropods.Their literature survey revealed that 43 native arthropod species in six taxonomic groups (Arachnida: Acari; Hexapoda: Coleoptera, Diptera, Hemiptera, Hymenoptera, and Lepidoptera) are known to be associated only with ash trees for either feeding or breeding purposes and are therefore at high risk. Most of these species are gall-formers, followed by folivores, subcortical phloem/xylem feeders, sap feeders, and seed predators. Another 30 arthropod species are associated with 1 to 2 host plants in addition to ash; herbivory on these hosts may increase as these arthropods shift from declining ash trees.
The Emerald Ash Borer (Agrilus planipennis) is an Asian wood-boring beetle in the family Buprestidae. This beetle was accidentally introduced to North America around the beginning of the 21st century (first noted in both Michigan, U.S.A., and Ontario, Canada, in 2002). Since then, it has killed millions of ash trees (Fraxinus spp.) in (at least) Michigan, Indiana, Illinois, Ohio, Pennsylvania, Maryland, West Virginia, and Wisconsin (U.S.A.) and Ontario and Quebec (Canada). Emerald ash borers colonize trees that range in size from saplings to fully mature trees. Larvae feed under the bark on phloem and outer xylem, girdling and killing trees within 1 to 4 years of colonization. Efforts to find one or more effective biocontrol agents are ongoing, but the potential ecological and economic costs of this pest are clearly enormous. (Poland and McCullough 2006; Duan et al. 2009; Gandhi and Herms 2010; Kovacs et al. 2010)
The native range of the Emerald Ash Borer includes China, Japan, Korea, Mongolia, the Russian Far East, and Taiwan (Anulewicz et al. 2008 and references therein).
In southeast Michigan, adult beetles emerge from host trees from late May through early September. Eggs are deposited singly in crevices and furrows on the outer bark of host trees. Upon eclosion, first instars immediately tunnel through the bark and begin feeding on phloem and outer xylem as they create serpentine, frass-packed galleries that impede translocation of water, nutrients, and photosynthate through the tree. Most individuals complete their life cycle in 1 year; however, a proportion of the population takes 2 years to complete development. (Rebek et al. 2008 and references therein)
Kovacs et al. (2010) modeled Emerald Ash Borer spread and infestation over the period 2009 to 2019. They estimated the discounted cost of the treatment, removal, and replacement of ashes infested by Emerald Ash Borer on developed land within communities in a 25-state study area centered on Detroit. An estimated 38 million ash trees occur in this area. Their simulations predicted an expanding Emerald Ash Borer infestation that will likely encompass most of the 25 states and warrant treatment, removal, and replacement of more than 17 million ash trees with a mean discounted cost of $10.7 billion. They note that expanding the land base to include developed land outside, as well as inside, communities nearly doubles the estimates of the number of ash trees treated or removed and replaced, and hence the associated cost. The authors argue that estimates of discounted cost suggest that a substantial investment might be efficiently spent to slow the expansion of isolated Emerald Ash Borer infestations to postpone the ultimate costs of ash treatment, removal, and replacement. Although many uncertainties could change assumptions underlying the predictions of this analysis, it nevetheless provides some sense of the scale of the economic threat of the Emerald Ash Borer.
