Last week’s bitter cold was potentially deadly for animals and humans. But it wasn’t enough to kill off a pest destroying thousands of Hamilton’s ash trees.

The emerald ash borer survived last week’s cold snap, said forestry manager Mike McNamara. And crews are moving ahead on a 10-year plan to chop down all of Hamilton’s city-owned ash trees.

Research shows that temperatures of -30 C would kill off some of the larvae — enough to slow but not stop the insect’s spread. Last week's temperatures dropped as low as -24 C.

“We haven’t had cold enough temperatures yet to kill larvae,” he said.

The ash borer is a deadly pest that has destroyed thousands of ash trees across southern Ontario. The city first spotted it in Hamilton in 2009, with the Mountain hit particularly hard. The invasive species burrows under the bark of the trees, effectively girdling them and killing them.

Over the next decade, crews will chop down 22,738 city-owned ash trees, accounting for about 10 per cent of Hamilton’s public tree cover. It will cost taxpayers $26.2 million over 10 years. For each ash tree crews chop down, they will plant another tree in its place.

Ash borer by the numbers

$26.2 million — Amount the city will spend over 10 years to cut down its ash trees

22,738 — Number of city-owned ash trees that will be chopped down in Hamilton. There are thousands more that are privately owned

216 — Number of trees treated so far with TreeAzin

​1,297 — Number of trees cut down so far

456 — Number of trees planted so far to replace them

So far, the city has cut down 1,297 trees and 777 stumps. Crews have planted 456 trees in their place.

Eight hundred trees will be injected with TreeAzin, an expensive chemical that increases the tree’s survival. In 2013, 216 trees were treated. The city will identify the rest of them early this year and treat them in 2014.

The federal government is currently testing a wasp that kills the borer. But not enough is known about it yet to distribute it to municipalities, McNamara said.

As for cold, the larvae live under the tree bark, which means wind chill isn’t a factor, McNamara said. And in Moscow, which is colder than Hamilton, the borer continues to spread.

“They’re finding it may slow it down, but it’s not definitive,” he said.

The ash borer is just one problem forestry crews face. They’re also scrambling to clean up branches after the Dec. 21 ice storm.

Since the storm, the city has received 3,675 calls about trees, and had 1,750 work orders, McNamara said.

Twenty crews are working five days a week to clean up the debris. Brush crews are also working Saturdays.

It’ll be late March or early April before all the debris is collected, McNamara said.

There have been more than 8,000 trees damaged in Hamilton in 2013, more than the years of 2007 to 2012 combined, McNamara said.

Two storms — the Dec. 21 and an earlier wind storm — were the culprit.