Early in May, Toronto resident Anne Lennie went to her shed and ran into a nemesis: carpenter ants.

Fire ants, already well established in the southern United States, are a recent arrival to Canada. Their nasty bite and swarming tactics make them one of the most feared ants. (USDA Agricultural Research Service)

The ants first appeared last spring when city workers finally removed part of a dying tree in front of her house in East York. The tree, as it turns out, was infested with the big, black ants, and it wasn't long before she had to call pest control companies to douse the tree and her home to control the outbreak.

Now the ants are back, residing in her shed's roof.

"I've been here 10 years and never had a problem with these carpenter ants until last year," the 41-year-old homeowner told CBC News online.

Lennie's not the only one to notice ants in Toronto this spring. CBC News Online spoke to a number of people who've spotted all kinds of ants — but mostly the bigger carpenter ants — roaming their kitchen or bathroom foraging for food.

Ant experts say there's no evidence the insects are out in greater force this year over any other year. Instead, they suggest the marching of the ants across hardwood and linoleum could best be considered a rite of spring.

"In any given spring, the ants are active," said Gary Umphrey, a University of Guelph mathematics professor who is the university's resident ant collector, having helped contribute ants from thousands of different colonies to the university specimen collection.

"They've spent all winter in hiding, and when the weather warms up they start searching for food, and people notice it more."

Ants, like all insects, are cold-blooded, so in the winter they tend to huddle together in their colonies and wait for warmer, sunnier weather. They tend to be less active in cold weather or when it rains, often giving homeowners a false impression that they've moved on.

Colonies can contain as many as 10,000 ants, and because most live for several years, their populations have more continuity than other insects, said Umphrey.

"It's very unlikely you would have a population explosion of ants," he said. "They aren't like soybean aphids that have a lifespan of a week. They are perennial. If an area had ants last year, it's likely to have them again this year. It's just different people notice them each time."

Dean Stanbridge, director of the Association of Pest Management Professionals of Ontario, said ants are most often seen from April to June, searching for food. Ants are omnivorous, but love a good carbohydrate early in the season, said Stanbridge, and so often enter homes in search of sweets, juice or grains.

Ants are usually less active in the summer once the weather get hotter, and by autumn, when the insects are no longer urgently trying to feed their brood, activity also drops off. People also are more likely to notice ants in spring because it's the time winged ants are flying about during the mating season.

But knowing the ants might disappear in a couple of months might provide little comfort to homeowners, depending on the kind of ants they have.

Ant problems are relative

One of the more common kinds of ants seen outside or in the home in Eastern Canada are pavement ants or sand ants, said Stanbridge. There are a number of these species, with Tetramorium caespitum (the European pavement ant) being particularly common in eastern and southern North America.

Pavement ants are typically dark brown or black and very small — about 3 mm in length — and tend to nest under sidewalks, pavement and in the crevices of housing structures. They rarely travel more than 30 metres from their colony and typically only pose a problem to homeowners when they go foraging for food.

Other species of ants are more problematic, particularly the pharaoh ant. Pharaoh ants (Monomorium pharaonis) are smaller than pavement ants — with workers typically 1.5 mm long — and are yellow to red. They have an unfortunate ability to multiply quickly and start new colonies, making them more difficult to contain, and because of their small size, pharaoh ants can get into places other insects can't, making them a health risk in hospitals.

Another kind of ant relatively new to Canada is the fire ant. There are over 200 species of fire ants, and on a glance seem similar to pharaoh ants — they are about the same size and typically yellow. But they all have an unpleasant habit of stinging en masse when they are attacked.

"When you step on an anthill, with most ants, they will scatter," said Stanbridge. "With fire ants, they'll all climb onto you and bite all at once."

Carpenter ants scourge of homeowners

Far more common a pest than pharaoh or fire ants are carpenter ants, and their unwelcome renovations in houses can be a headache for homeowners.

Unlike termites, carpenter ants don't eat wood. But they do nest in it and can be a pest for homeowners. (Clemson University/USDA)

The most common of these in Canada are the red and black carpenter ant (Camponotus herculeanus) and the black carpenter ant (Camponotus pennsylvanicus). Carpenter ants are typically much larger than other ants, varying in length from 6 to 25 mm. They can easily be distinguished from other ants because of their size and because their body is constricted between their abdomen and thorax, making their parts appear bulbous or exaggerated.

While carpenter ants don't eat wood, they do nest in it and will burrow through as much as they need to find a home, leaving a telltale trail of sawdust behind. They prefer moist, decaying wood such as that found in dead trees, but if a house has a moisture problem or has wood structures suffering from decay, it could provide a welcome habitat for the insects.

They usually get into a house through holes, where trees branches contact the house or from wooden structures attached to the house like a porch or deck.

The federal government's Pest Management Regulatory Agency has a number of recommendations for dealing with carpenter ants, including:

• Clear out decaying or infested wood from buildings.
• Remove any firewood that may also have transported the pests into the house.
• Correct humidity problems that may be affecting wood structures.
• Remove possible food sources such as meat products, grains and sweets.
• Get a full chemical treatment if the colony is well established in the home.

Stanbridge said most carpenter ants found in buildings aren't living there, but are simply passing by in search of food.

"These ants can forage hundreds of metres from their colony. So when they appear, the instinct is to say, 'Oh my God, I've got ants,'" said Stanbridge, whose company, the Steritech Group, deals with commercial and not residential clients.

"But 90 per cent of the colonies of carpenter ants are outside, likely living in a dead tree," he said.

Umphrey, a self-described ant lover who has been studying the insects since 1974, also thinks carpenter ants get an unduly bad reputation.

"If I saw black carpenter ants in my home, I'd be inclined to put up a sign saying 'ant sanctuary,'" said Umphrey. "But others may decide they are a problem and have other ideas."

Count Lennie among those who would like to see the end of the critters in her neighbourhood. She's still waiting for the city to remove the last remains of the tree stump that housed the original population of the insects. On a cold rainy Thursday, she looked up at her shed and couldn't see any, but she knows they haven't gone away.

"I saw four or five big ones with wings the other day," she said. "I know they are going to come out again in full force."

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