Isabella Fassler

To Beat Rats, Follow the Science

Michael Parsons

March 23, 2023

Five research-informed ways to control rodents in an urban environment

Five research-informed ways to control rodents in an urban environment

I have what some might consider an odd credential: I am one of the world’s few Ph.D.-qualified urban rodentologists. My colleagues and I study the biology, behavior and control of Norway rats — that’s the type that you see scurrying around New York City, and cities everywhere.

In case you missed it, rats are plaguing the five boroughs, or at least that’s how the city’s human residents see it. Annual rat complaints hit 35,000 by November 2022, and earlier this year, a property owned by Mayor Eric Adams himself was cited for violations. Adams, who’s touted and explored new ideas for controlling rats for many years, has put forward policies he says will reduce the population. As City Hall closes in on a rat eradication czar — the mayor says the search is down to "one person that we're getting ready to announce" — the mayor tweeted a spirited video featuring the Department of Health’s man in charge of pest control.

Unfortunately, the administration’s agenda is underwhelming. Over the years, we in the rat-research community have watched in frustration as our city’s leaders have not only been ignoring science, but have actually worked against the best evidence as they invest money in methods that have long been debunked. 

The present moment is no exception. I have developed a five-step framework to help us start anew. 

Before I dive in, some humbling context. Rats have been present in New York City at least since Thomas Willett served as the first mayor in 1665. Two hundred years later, in 1865, the New York Times reported that rats had become so emboldened that they were traveling in the daytime alongside people and pets. Rats, of course, are primarily nocturnal animals; individuals come out in the daytime after the population size has reached its peak. More recently, in 2015, then-Mayor Bill de Blasio “declared war” on rats armed with a $32 million budget, to little avail. This is the war that, nearly a decade later, Adams has renewed with fervor.

The fact that rats keep getting the better of administration after administration isn’t just a problem for those who startle when they see one run across their path. In addition to transmitting disease, rats’ penchant for gnawing may be responsible for up to 25% of structural fires in America, as well as the disabling of countless motor vehicles. Rats cost our national economy $19 billion per annum nationwide, with much of the expense stemming from human-dense areas such as New York. Given these risks and costs, we can easily justify the extensive efforts needed to manage our rat populations.

But when we invest in rat control, we need to do it right.

Step 1: “Know thy enemy”

When we go to war with another nation, we need to understand the enemy’s capabilities. Similarly, science asserts that in order to control a pest, we need to first understand its biology.

Science has shown that rats are among nature’s most highly adapted organisms. As what we call commensal organisms, they depend on humans for food and shelter. Yet, because they are generalists, they can eat all types of human food sources. Thus, they breed prolifically and thrive when provided with a regular supply of human garbage.

When food is plentiful, there’s no check on growth. When the cycle of regular feeding has been broken, then rats will disperse, injure, kill and even consume one another.

At a press event last December, Adams appeared alongside a chart showing that, if garbage is plentiful, then two rats can lead to nearly 35 million descendants in two years. He mostly got that right. 

In my field, there’s an equation that best explains rat population size. Simplified, it states: Garbage in = rats out. When food is plentiful, there’s no check on growth. When the cycle of regular feeding has been broken, then rats will disperse, injure, kill and even consume one another. 

Put another way, when deprived of food and safety (harborage), rats will, in effect, control one another. This is the primary reason that rat control should be focused on the left side of the equation (input), which drives the right (output). 

Unfortunately, though Adams and other mayors seem to grasp that basic insight, they spend way too much time, effort and money trying to kill the critters. This results in a costly and even counterproductive game of whack-a-rat.

It’s folly to pour money into gimmicky rat-control devices.

Step 2: Allocate ample funding to the left side of the equation

We need to be strategic about where limited city funds are allocated. I specifically reference the recently created, and now infamous, “Rat Czar” job opening to head the city’s rodent control. A primary qualification for the new hire in the seemingly tongue-in-cheek posting was to be “bloodthirsty,” a voracious rat killer. This brought to mind Adams’ promotion, as Brooklyn borough president, of a drowning bucket to more efficiently exterminate the critters.

But it’s folly to pour money into gimmicky rat-control devices. (There are some exterminating products that work, as I describe later, but they all have their drawbacks.) Given the constant supply of garbage available to the creatures and their prolific reproductive rate, the act of killing rats becomes largely irrelevant to their control. It’s an approach that commercial and management companies relish, while the rest of us see no real abatement in their numbers.

The city must avoid getting caught in the trap. The selective pressure that pest management and the city is placing on killing a never-ending supply of rats could even lead to further resistance to rodenticides and an increase in their genetic fitness.

Step 3: Allocate funds to science-supported approaches and collect data on progress

A military leader institutes surveillance through any combination of covert spies, satellite images and guesswork to garner evidence as to whether the current strategy is working. Similarly, the next step of rat control is to make the commitment to continually assess the efficacy of our varied pest-control strategies.

We’re bad at this now. One high-profile example: In 2015, de Blasio’s Parks Department invested millions in mint-scented, supposedly rat-repellent garbage bags. To be clear, there has never been any peer-reviewed evidence showing that such a garbage bag would deter hungry rats from their primary food source. 

The current administration made a similar mistake when it claimed rat populations would be affected by moving back the time garbage sits on the curbside, from 4 p.m. to 8 p.m. Any expert can tell you that, because most rats are nocturnal, the overall population will be unaffected by the shift.

Adams erred again when he entertained the idea of using cats to control rats. Our research at Fordham and that of others has demonstrated that cats cannot control rats. That’s because felines prefer safer and easier meals, including ecologically-functional native wildlife. And when birds and reptiles that disperse seeds and pollen are eliminated by cats, then even our soil, air and drinking water are impacted. 

In Alberta, instead of privatizing rodent control, leaders utilized public support to treat all residential and commercial buildings along a 300-km-wide rat zone systematically. Alberta is now, for the most part, rat-free.

Adams risks another sizable mistake as he rolls out an expensive citywide composting program without a shred of evidence that rats were being controlled in the few neighborhoods where composting has been in place for years. The scientific literature shows how composting, if not carefully planned and controlled, can actually increase rat populations. Indeed, it is common knowledge among local rodentologists that composting has not had any beneficial impact on urban rat numbers. 

Step 4. Educate: It takes a village to get results

People are timid around rats because we are adapted to fear organisms that can make us sick. So most of us shy away from confronting the vermin until they are so numerous that they cannot be ignored.

Instead of ignoring the problem, we need to come together. 

Worldwide, areas that practice world-best rat control, such as Alberta, Canada and  New Zealand, have adopted the same unified tack. In Alberta, instead of privatizing rodent control, leaders utilized public support to systematically treat all residential and commercial buildings along a 300-km-wide rat zone. Subsequently, Alberta’s administration launched a public education campaign and appointed numerous officers to patrol the treated zone. Alberta is now, for the most part, rat-free. Nor are Albertans passively waiting for rats to reinvade; instead, they have allocated ample funds for surveillance and maintenance.

New Zealand has also educated the general public as an important part of their $17-million-dollar investment into a predator-free country, with rats perceived as one of the most serious predators. Conservation, leaders say, “begins in the homeowner's backyard.” New Zealand now boasts more than 110 islands that are rat-free, with the mainland exceeding expectations for reducing their rat populations. 

Additional justification for investing in, and recruiting, public support is embodied by the mayor’s own experience. He recently received that $300 fine because, even though his property was purportedly rat-free, rodents migrated to his property from neighbors who had not controlled their rats. 

This lesson also helps explain the main reason a voluntary citywide compost program is unlikely to curb rat populations. Motivated individuals who compost are doing so independently, while neighbors might not be participating. Only when folks citywide become personally committed to the conditions that cause rats can we collectively turn the tide. 

Step 5. Implement humane, sustainable and integrated pest management plans that include smart rat killing

Though it’s crucial to realize we can't exterminate our way out of the problem, we should smartly invest in extermination and related tools. We researchers call this Integrated Pest Management, or IPM. 

Again, exclusion is much more important than killing. Well-designed rat-proof trash cans separate rats from the food that drives their growth. Solar-powered crushing trash cans, for instance, seemed like a good move. But in this as in all things, there has to be follow-through. For instance, many times, while walking along Broadway and Riverside, I saw solar-powered cans overflowing with food. 

Composting has similar risks. Without timely pick-ups, and with transfer of wastes into a place where they can break down, it could make rats worse. 

Finally, let’s turn back to those shiny objects policymakers seem to love, killing as many rats as possible. Dry ice (frozen carbon dioxide) works remarkably well, and is humane. But due to the risks carbon dioxide conveys to people, it can only be used away from residential dwellings. Rat contraceptives have been used to slow population growth, but given that rats must voluntarily consume the toxin, there will be animals that can detect and refuse to drink, even at remarkably low concentrations. 

The evolution of traps has proceeded nicely, with some traps even humanely killing and disposing of the bodies. Restaurants, though, cannot deploy such traps in view of patrons. 

Rodenticides are widely considered the best rat killer. However, rodenticides lead to secondary poisoning of wildlife, and they are taken by non-target animals, including pets. And they are terrible for the environment, especially at the rate they are being used. Sadly, rodenticides cause extensive suffering to rats, sometimes taking days to slowly kill. 

But it only makes sense to deploy these weapons when we’re taking sufficient, responsible, and scientifically sound steps to rein in the population. Otherwise, rats will keep beating us for the next hundred years.