BEAVER ISLAND, MI – Mature ash still fill the swamps and remain part of the tree canopy on Beaver Island, where the invasive emerald ash borer arrived years after the pest insect had already decimated the ash forests of mainland Michigan.
Island residents first detected the ash-killing beetles in 2017, a full 15 years after the species’ initial discovery in the United States. Beaver Island is about 30 miles offshore from the Lower Peninsula in the middle of northern Lake Michigan, a distance that helped protect its remote forests.
In recent years, environmental advocates on Beaver Island enlisted mercenary wasps for their war against the harmful, invasive species. They were determined not to lose their ash trees without putting up a fight.
“We like our ash forests. They’re part of the biodiversity here on the island,” said Beth Leuck, retired biologist and ecology professor who lives part-time on Beaver Island.
Emerald ash borers – frequently called by acronym EAB – were first discovered stateside in 2002 in Michigan. Scientists suspect they were unwittingly brought from Asia in shipping pallets.
This is an adult emerald ash borer beetle, an invasive pest insect known for destroying mature ash forests.
The invasive species kills practically every tree it attacks, from shade trees planted in suburban neighborhoods to tall stands of ash in the woods. The beetles have destroyed tens of millions of ash trees nationwide.
It’s presumed the invasive pest insect arrived aboard contaminated firewood, despite a longtime ban on mainland firewood brought to the Beaver Island Archipelago. It’s the only conclusion since the 30-mile offshore distance is too far for the tree-killing insect to have arrived on its own.
Now island residents and federal scientists are using tiny, stingless wasps to kill the emerald ash borers that threaten the remaining mature ash trees on Beaver Island, as well as those on uninhabited Garden Island just to the north. The first year of parasitoid wasp releases there was 2021.
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“The thing with EAB is they don’t like the little diameter trees,” said Pam Grassmick, a lifelong Beaver Islander and environmental advocate. “When the trees get bigger, they attack.”
Studies show the three types of parasitoid wasps released on Beaver Island and in other places across the nation are now killing the borers, resulting in fewer attacks on ash trees. The wasps are killing between 20% and 80% of emerald ash borers in ash trees up to eight inches in diameter.
Parasitoids are insects whose larvae live as parasites, eventually killing their hosts. Using a natural enemy of an invasive species like this is called a biocontrol.
“I’m happy to say the biocontrol results from the field have been encouraging, but we certainly have a lot more work ahead of us,” said Ben Slager, former USDA senior entomologist who worked with the emerald ash borer control program.
Federal agriculture officials said since 2007 they’ve released more than eight million of these parasitoid wasps across 30 states and recovered proof of them becoming established in 22 states. It’s a last-ditch effort to overcome the invasive species responsible for the nation’s widespread ash decline.
“They’re spreading, so it’s working. They are doing very good recoveries on the mainland. We have tried one recovery because it is very labor intensive,” Leuck said.
The three species of parasitoid wasps used in the effort don’t have common names and are instead only known by their scientific names: tetrastichus planipennisi, spathius galinae, and oobius agrili. The wasps are reared at a USDA laboratory in Brighton, Michigan.
How these three types of wasps from Asia kill emerald ash borer beetles is somewhat grim.
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Tetrastichus lay their eggs inside the beetle larvae, and the wasp larvae eat the beetles from the inside out. Spathius lay their eggs on the surface of the beetle larvae, and the wasp larvae eat the beetles from the outside in. And finally, oobius lay their eggs inside the beetle eggs, and the wasp larvae eat the beetle eggs before even hatching.
One wasp type is released into the woods as adults, while the other two are placed onto an ash tree in makeshift insect nurseries while still in larval stage. When they reach adulthood, they fly into the forest to find their shiny, green, ash-killing victims.
Beaver Island residents striving to save their mature ash stands release parasitoid wasps that attack invasive emerald ash borers from these old medicine bottles fashioned into an insect nursery. (Sheri McWhirter | MLive.com)
The parasitoid wasps are effective but are not expected to completely eradicate emerald ash borers in North America. They are just one weapon in the battle.
“I can say they keep them down to a dull roar,” Leuck said. “That’s exactly what’s happened in places that have been releasing these parasitoids for a longer time – the EAB are still there, but they’re at much lower density.”
It’s enough relief from the emerald ash borer attack that plenty of the ash trees on Beaver and Garden islands can continue to grow and potentially help restore forests elsewhere in the future.
“Someday they may be able to come back here, collect seeds, and reestablish some of the populations on the mainland that have been so decimated. So, it’s well worth it,” Grassmick said.
“We probably won’t live to see it, but someday, hopefully somebody will come back, and they’ll say, ‘Oh, this was a good effort.’”
Read more of our Beaver Island coverage here.
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