Few trees left standing for beetles
After eight years of feeding on ash trees the beetle has run out of food, experts say
Like a freeloading relative who cleans out the fridge then skips town, Ottawa’s emerald ash borers are moving on.
After eight years and 50,000 tree removals, the voracious treekilling beetle infestation appears to be on the decline. The reason? There’s just not that much left for it to eat.
“We are, in a sense, over some of the more significant and difficult years of large numbers of tree removals,” said Jason Pollard, a forester with the City of Ottawa.
The beetle arrived in the city in 2009 with a heavy infestation in the area around St. Laurent Boulevard. Since then, the beetles have spread east, south and west, and the city has aggressively cut ash trees, not so much to control the beetle, but because of the safety risk posed by towering dead trees. The infestation and tree cutting peaked in 2015, Pollard said.
“Since that time, we’re still removing incredible numbers of ash trees, but we’re in decline. We’re seeing fewer issues with dead trees on the landscape.”
The city, in conjunction with the City of Gatineau, the National Capital Commission, Ontario’s Ministry of Natural Resources and Forestry, and various conservation authorities — all part of the Regional Forest Health Network — has placed traps throughout the area to catch and monitor the EAB population. Pollard said the traps show the number of adult beetles is also in decline.
That’s good news for landowners who paid to have their trees inoculated against the beetle. Though cheaper than it once was, the treatment still costs close to $200 per tree and needed to be applied annually when the trees were under intense pressure from the insect.
“I think that pressure is starting to wane,” Pollard said. “Our hope is that with declining EAB populations the requirement for treatment might be every two years or perhaps even less frequently.”
Even so, the treatment will have to be kept up long term — “for decades,” Pollard said — because even though emerald ash borer numbers are falling, the pest is here to stay.
The glossy green beetle, about 8 mm to 15 mm in length, is an invasive species first found in Michigan in 2002. It lays its eggs under the bark of ash trees where the larva hatch and burrow in tunnels that starve the tree of nutrients from its roots. The pest spread across Ontario and Quebec, mainly in shipments of wood and nursery stock, and has killed millions of trees in the province. Before the beetle arrived, ash trees made up about a quarter of all Ottawa’s trees, with the percentage even higher in some areas, Pollard said.
“On Smyth Road and Alta Vista Drive, at one time there was a continuous canopy of ash trees.”
Since 2009, the city has cut 30,000 trees from parks and roadways and another 20,000 from woodlots and natural areas such as ravines.
The city plants about 100,000 trees a year, mostly seedlings, but also about 4,000 to 5,000 of more substantial “calibre trees” with trunks about 50 millimetres diameter and two or three metres high.
The city is replanting with a variety of species, to give the urban forest more diversity, he said. In Alta Vista, for example, crews are planting red and sugar maples, oak, service berry and honey locusts.
“We’re making sure there is diversity and hopefully avoiding these catastrophic insect or pest or disease issues that have historically rolled through Ottawa (like) Dutch elm disease and emerald ash borer,” Pollard said.
And city foresters are watching as the insect spreads from the urban area out to the countryside.
“As we’ve seen emerald ash borer move from urban areas out to rural roads and rural areas, we know we have more work to do.”
One glimmer of hope is a parasitic wasp introduced into the region last summer that feeds on emerald ash borer larvae. Researchers with Natural Resources Canada announced this spring that the wasp, Tetrastichus planipennisi, has established populations which means it is feeding on the EAB. The wasp, which doesn’t sting and is harmless to humans, comes too late to save Ontario’s forests, but may play a role in controlling the spread of EAB westward onto the Prairies, Pollard said.