Joyce Hostyn

Ecological approaches to controlling rats

“Our idea of what a park or public space should look like mirrors its native environment, which… was almost certainly the grassy Asian steppe. We mow grass, plant a few shrubs and low bushes, a line of trees. Then we improve on nature by adding a constant source of food, our trash… If we’d like fewer of them around, we might start thinking about how to make the city more attractive to other animals.”
~Jason Munshi-South

Understanding brown rats

Illustration: N. Burgess/ Science

Brown rats (Rattus norvegicus) co-evolved with humans over hundreds, perhaps thousands, of years likely because cities offer everything brown rats need to thrive: plenty of food, shelter and water.

Sarah Gunawan, Synanthropic suburbia

Brown rats are nocturnal, sub-terrestrial, social creatures who prefer to build their nests in easy to dig soil protected by a hard roof like a sidewalk or a deck and in close proximity to a food source. While they have poor eyesight, they have excellent smell, taste, touch and hearing. They dislike open areas, preferring to travel along walls, fences, buildings and subterranean infrastructure. Since they avoid crossing streets, their range is often limited to a city block. They can gnaw through cinder blocks, lead sheeting, pipes and aluminum siding. They’re naturally neophobic—disliking anything new—and if their colony experiences trapping or poisoning they may become even more neophobic. As a result, permanent changes in the environment, such reducing food availability, is more effective at reducing rat populations than trapping or poisoning.

Getting creative in controlling Rats

Since the early 1900s, the favoured approach to control has been to declare a war on rats, despite research demonstrating that this approach doesn’t work. Without habitat modification, colonies quickly recover from short-term disturbances such as trapping and poisoning. More recently, researchers such as the Vancouver Rat project and architects like Sara Gunawan, Ned Dodington and Mackenzie Wilson are exploring how we can think more creatively and holistically about controlling urban rat populations.

Think holistically with an ecological approach

Reducing carrying capacity, Dana Sanchez/Oregon State University

Rats are here to stay. As a result, the goal of an ecological approach isn’t to eliminate them, but instead to reduce the carrying capacity of a neighbourhood through habitat modification.

Identify ratty places and ratty atmospheres

“The common characteristics of rats’ life worlds that all student groups employed when thinking of good places for rats were that rats are drawn to food sources from human origin and that rats avoid places that are very open or crowded with humans and tend to seek cover from and move close to the walls and buildings. The students discussed and looked for places where they stated there should be a ‘lot of trash’ but ‘not too many people’ or ‘not too much noise from the kids’ and where rats can move ‘along the walls’ and ‘stay hidden’.”
~Anttoni Kervinen et al., Ratty places – unsettling human-centeredness in ecological inquiry with young people

Helsinki’s urban rat project is exploring how learning to see from a rat’s point of view helps people identify ratty places and ratty atmospheres. In synanthropic suburbia, Sarah Gunawan compiles observations about evolving human perceptions of Brown Rats, their perceptual abilities and biological facts about their habitat, behaviours and environment.

Illustration: Sarah Gunawan

Rats communicate and attract each other through their urine, droppings and greasy track marks. Go out at night with a UV 365nm LED flashlight and check for urine trails along walls, decks and fences. If you find evidence of rats, disrupt these perceptual markers by cleaning up dark greasy track marks with water and a mild bleach solution (one part bleach, 10 parts water). Sweep up droppings.

Eliminate food sources

“Is it really worth feeding the birds if you’re going to have a rat problem and then you’re going to put out poison, which in turn is going to kill other birds like hawks and owls?”
~Lisa Owens Viani, Rodenticides – A modern day DDT

When food is plentiful, rat populations explode. When food is scarce, rats disperse, injure or kill each other. As a result, eliminating food sources is one of the best ways to deter rats. Preferred food sources in residential yards includes:

  • Garbage: New York’s rat explosion was traced to the switch to plastic garbage bags. Use rodent proof garbage bins or put your garbage out in the morning.
  • Bird feeders: Birdseed is a favourite high protein food for rats. If your bird feeders drop seed, there’s a good chance you’re feeding rats. Consider creating bird habitat instead by birdscaping with pocket forests and pocket meadows. If you want to use bird feeders, install a baffle on the pole, bring in the bird feeder at night, clean up spilled birdseed and don’t ground feed.
  • Green bins. City of Vancouver shares some good tips for preventing rats feasting on the food in green bins.
  • Chicken coops. Rats are attracted to chicken feed and freshly laid eggs. Utah State University has some good suggestions on how to rodent proof your chicken coop.
  • Pet food. Store pet food properly and, if you feed outdoors, bring in bowls at night.

Rat proof your compost

Image: Institute for Local Self-Reliance (ILSR)

Food scraps in compost (especially meat and dairy) can attract rats. Compost piles are also cozy warm places to build a den. Fortunately, you don’t have to give up composting if you follow the practices recommended in Oh, rats! How to avoid rodents at community composting sites and community composting done right: a guide to best management practices.

Attract natural predators

“If there are too few trees and shrubs where owls or hawks, say, can shelter and protect their young, the rodent population is going to stay high, because there are not enough baby birds making it to adulthood or living long enough to keep the rodent population in check. I would look into getting more owl and bird boxes built around the area, as well as planting more shade trees and native shrubs that are known to support local wildlife.”
~Pierre Mineau, Retired Ecotoxicoloist

Natural predators—coyotes, foxes, snakes and birds of prey—help keep rat populations in check. In 2025, Lakeside Community Garden is going to experiment with attracting birds of prey for rodent control by installing nest boxes for American Kestrels and Screech Owls.

What NOT to do

“There are high levels of exposure in every species we’ve looked at. Not just in the rodent eaters but in the accipiters [which eat mostly birds]. I wouldn’t have expected that. It’s still a mystery how this stuff is moving through terrestrial food chains. Insects may be picking it up and passing it to the songbirds that eat them. That might account for the accipiter [poisoning] connection.”
~Pierre Mineau, Retired Ecotoxicologist Environment Canada

N. Burgess/Science

Rodenticides have become the tool of choice for attempting to manage rat populations, something Vancouver Rat Project calls a whack-a-mole approach as populations quickly recover. Rodenticides harm other wildlife, may inadvertently exacerbate public health risks by making rats more susceptible to leptospirosis and evidence is mounting that rats are developing resistance to rodenticides. Instead of rodenticide, use rat traps set along their travel paths or dry ice in their burrows.

In 2023 Pickering became the first Ontario city to act against rodenticides and pass an animal poisoning prevention policy. In July 2024 Clarington banned the use of rodenticides at all municipal recreation facilities after a successful pilot project which demonstrated that identifying and addressing potential access points and using traps to keep mice and rats out of the facility was just as effective as rodenticides.

It takes a neighbourhood

To have a real impact on controlling rat populations, work with your neighbours on a neighbourhood-wide approach.

References