New research could help scientists control the emerald ash borer, an invasive pest that is devastating native ash trees in North America.
In the study published in Environmental Entomology, researchers found that the emerald ash borer completes its life cycle in North Carolina in one year, as opposed to the sometimes two-year life cycle found in northern areas of the United States. Researchers say the findings will help researchers know when to release parasitoid wasps — a key strategy experts have been using to try to control the insect.
“This pest started in Michigan in the United States and spread from there, so most of what we know about it in the US comes from a cooler climate,” said the study’s lead author Ryan Bohannon, a former graduate student at North Carolina State University. “Prior to this study, we didn’t have a good understanding of what the life cycle looks like in a warmer climate in the southern United States.”
The emerald ash borer, or Agrilus planipennis, was first discovered in the United States in 2002. It is now confirmed in 36 US states and five Canadian provinces. The insect has been documented in more than 60 of North Carolina’s 100 counties, said study co-author Kelly Oten, assistant professor and consulting specialist in forestry and environmental resources at NC State.
The emerald ash borer feeds on all species of ash trees native to the United States, which in North Carolina include the white ash, green ash, Carolina ash, and pumpkin ash. Adult beetles lay their eggs on the tree bark, and then the larvae burrow into the tree and feed on the tissues within.
Researchers have attempted to release tiny wasps – some of them comparable to a mosquito – to attack and feed on the emerald ash borer in its natural range. But so far, Oten says, they haven’t been able to get the wasps to establish themselves in the warm, southern climate. The timing of the Emerald Ash Drill’s life cycle might be key.
“If we can reduce the emerald ash borer population, you might get more of your typical pest-natural enemy relationship,” Oten said.
In the study, researchers collected specimens of the emerald ash borer from under the bark of infested ash trees in central North Carolina, brought them back to the lab, and measured them to determine their life stage. They repeated this every two weeks for 26 months. In addition, they captured adult insects to learn more about when they are actively flying and laying eggs.
“In the ash stands we studied, more than 90% of the trees were killed,” said Bohannon, now Ph.D. Student at Clemson University. “There are few survivors. These trees are just not adapted to this insect and the devastation is quite noticeable.”
They found that the larvae that would be susceptible to the parasitoid wasps would be available in a specific window during summer and early fall, from late June to October. This window is critical in understanding the best time to release the parasitoid wasps.
“This is quite different from what we’re seeing in the northern United States, where colder temperatures and shorter growing seasons can take the insect two years to complete its life cycle,” Bohannon said.
In future work, the researchers plan to adjust the timing of wasp release to match larvae availability. They are also investigating whether to release a parasitoid wasp that lives in the southern part of the borer’s native range.
“We’ve done a lot of work to document a detailed life cycle of this pest so it can go directly into plans that focus on biological control,” Oten said. “This will likely help those efforts, especially as the pest spreads into warmer areas that have climates like ours.”
The researchers are also studying whether cutting down infested trees to allow them to sprout again can extend the life of the trees and give wasps more time to establish themselves. They also looked for trees that might have natural genetic resistance to the pest.
“In terms of our research on the tree side, our efforts to save the tree are currently focused on survivors to see if any of them have natural resistances,” Oten said. “But if we can get parasitoid wasps to colonize North Carolina, maybe we could start looking at reforestation for forest restoration.”
Note to the editor: The study summary follows.
“Phenology and Voltinism of Emerald Ash Borer (Coleoptera: Buprestidae) in Central North Carolina”
Authors: G. Ryan Bohannon, Courtney L. Johnson, Robert M. Jetton and Kelly LF Oten
Published: November 5, 2022, Environmental Entomology
abstract: The emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), has killed millions of ash trees (Fraxinus spp.) across North America. Classic biological control through the introduction of parasitoid wasps can provide a sustainable approach to controlling this invasive insect. However, establishing parasitoids in the southern United States has been difficult. The phenology of the emerald ash borer was studied in central North Carolina to inform biological control efforts that are more attuned to the seasonal availability of susceptible life stages of the emerald ash borer in that region’s warm climate. Biweekly life stage assessments of the Emerald Ash Borer were conducted in stands with infested green ash (Fraxinus pennsylvanica Marshall, Lamiales: Oleaceae) for 26 consecutive months (June 2019 to August 2021). Adults were also caught in these stocks in the spring and summer of 2019, 2020 and 2021. Based on these collections, the emerald ash borer has a univoltine (1 year) life cycle. Parasite-susceptible larvae (third and fourth instars in galleries) are present from late June to October (~1,100–3,000 degree days base 10 °C) and are mostly absent during the rest of the year. Parasite release timings and life histories of selected parasitoid species should be matched with this window of host availability to be effective. This characterization of the emerald ashborer phenology and voltinism will help improve the timing and effectiveness of management efforts as this forest pest continues to spread in southern North America.