As emerald ash borer spreads throughout North America, researchers remain optimistic that native and introduced natural enemies will help suppress EAB densities below a damage threshold for the long-term survival and reproduction of EAB-tolerant ash genotypes.
As part of National Invasive Species Awareness Week, Jian J. Duan, research entomologist and lead scientist with USDA Agricultural Research Service’s Beneficial Insects Introduction Research Unit, led a webinar examining the use of biological control agents currently being employed as control for emerald ash borer. The March 1 webinar began by explaining the premise of biological control of EAB.
Because the borer is originally from northeast Asia, U.S. and Chinese scientists have been searching for EAB and its natural enemies in that region since 2003. In Asia, EAB population densities are relatively low due to the combined effects of EAB resistance in Asian ash species, scarcity and patchiness of forests, and the EAB natural enemy complex.
Duan says that while North American natural enemies like woodpeckers cannot keep the pest under control by themselves, reconnecting EAB with its co-evolved Asiatic natural enemies would keep the borer in check.
“The hope is that reconnecting with natural enemies would result in permanent management of this invasive pest,” Duan says.
From 2003 to 2007, researchers explored China looking for those natural enemies. The work yielded several hymenopteran parasitoids. Three of these parasitoids have been approved by USDA-APHIS for release as biological control agents of EAB in the U.S.: an egg parasitoid, Oobius agrili (Encyrtidae) and two larval parasitoids, Tetrastichus planipennisi (Eulophidae), Spathius agrili (Braconidae).
Work continued in northeast Asia from 2008 to 2012, and turned up another EAB enemy. In 2015, Spathius galinae, from Russia, became the next EAB larval parasitoid approved for release.
Oobius is the smallest of three parasitoid wasp species used as EAB biocontrols, and the only one that targets EAB eggs. When the wasp locates an EAB egg in tree bark crevices, it inserts its own egg inside the host egg, where it will hatch, grow and kill the EAB larva before it can emerge. The other three parasitoids are also stingless wasps, but they attack the EAB larvae as it develops under the bark of ash trees.
Due to the large number of ash species in North America and the long life cycle of trees, Duan and his collaborators Juli S. Gould, entomologist at USDA-APHIS and Leah Bauer, research entomologist, USDA Forest Service, respectively, believe it will be many years before it is known if biocontrol can protect ash species against EAB. But there are some encouraging research results from study sites in Michigan, where parasitoid releases began in 2007. These results show the establishment and spread of T. planipennisi and O. agrili attacking EAB in surviving green, white, and black ash saplings and trees. Also, reductions in EAB densities following parasitoid release were correlated with increased parasitism, first by two native larval parasitoids when EAB densities were high, then by the introduced parasitoid T. planipennisi when EAB densities are low. The combined mortality of EAB caused by woodpeckers, native and introduced parasitoids, intraspecific competition, disease, innate tree defenses, and reduced ash abundance contributed to the collapse of EAB populations. Tree inventories at Michigan study sites found low numbers of large ash trees survived, while numbers of ash sprouts, saplings, and small to medium trees increased.
Duan’s presentation showed that T. planipennisi has caused about a 30 percent larval parasitism on ash trees. However, the parasitism rate of EAB larvae in ash saplings is much higher, ranging from 36 to 85 percent.
Growth, reproduction, and survival of large-diameter ash, however, will require EAB parasitoids capable of parasitizing EAB larvae feeding under thick bark. Although there are two such parasitoid species approved for release in North America, S. agrili and S. galinae, their sustained establishment in North America has yet to be confirmed.
Many EAB sites have not yet been sampled for two-plus years, but sites with longer track records have improved establishment. Eight sites in six states have been treated and monitored for with O. agrili for more than two years. Of those sites, 53 percent have confirmed establishment. For T. planipennisi, 12 sites in eight states have been sampled for more than two years. Of those 12 sites, 63 percent have confirmed establishment.
The parasitoids themselves begin life at the APHIS EAB Biocontrol-Rearing Facility, where they are mass-reared, stored, packaged, and shipped for release at approved sites.
For more information, USDA’s EAB Biocontrol Release and Recovery Guidelines (written by Gould, Bauer and Duan) are available at this link: bit.ly/EABguidelines.
USDA also maintains www.mapbiocontrol.org, the online database for the release and recovery of biocontrol agents. In collaboration with research partners, ongoing studies include continued foreign exploration for new natural enemies; evaluations of EAB population dynamics in response to parasitoid release; parasitoid taxonomy, genetics, biology, establishment, prevalence, and interactions; impacts of EAB biocontrol on the long-term survival, growth, and regeneration of ash.