The Dry Ice Challenge: What Happens Next?

The media lit up with stories about the promise of dry ice for rodent control last fall, as Boston, New York, Chicago and other U.S. cities experimented with this humane treatment protocol in controlling rat populations. City workers and PMPs achieved positive results from placing dry ice pellets into rat burrows where this frozen carbon dioxide (CO₂) would sublime (turn from solid to gas) and suffocate its target populations.

According to a recent white paper co- authored by urban rodentologist Bobby Corrigan and PMP John Stellberger of Environmental Health Services, these pilot trials uncovered the potential of dry ice as a weapon in the battle to eliminate the public health threat of city rat infestations. Corrigan and Stellberger detailed five benefits of treating exterior Norway rat infestations occurring at least 3 meters away from buildings, stating dry ice is:

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• Nontoxic to humans and pets when applied in the prescribed manner
• A green approach, posing no secondary threats to non-target animals
• A humane asphyxiate (not a fumigant) that allows animals to die in their sleep
• Effective in killing the ectoparasites — fleas, lice, mites and ticks, for example — associated with rodents
• Effective in controlling even bait-aversive and trap-cautious rats

There’s one major sticking point, however: While CO₂ has proven to be safe in a variety of applications, including the euthanization of wildlife and lab rats, it is not registered as a rodenticide under the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA). Thus, in October, the U.S. EPA determined that the practice of using dry ice for rodent control was in violation of federal law. NPMA and state pesticide regulatory officials have since warned PMPs to cease and desist, at least until EPA has had an opportunity to fully review the matter and issue a final ruling.

“Jim Fredericks and I are working with EPA on this issue,” says Andrew Bray, director of public policy at NPMA. “NPMA is maintaining a dialogue with EPA to make sure the agency understands the industry perspective on the merits of CO₂ in rodent control applications. In the meantime, we will continue to provide updates and guidance on permissible and impermissible uses of products, like dry ice, to assist our members in FIFRA compliance.”

Who will take on responsibility for registering dry ice then? Manufacturers don’t stand to profit from “owning” it. But maybe there’s another answer.

Boston’s City Inspectional Services Commissioner William “Buddy” Christopher, a proponent of dry ice treatments, has partnered with EPA at the state and local levels. He told PCT, “EPA is working with us closely and collaboratively to explore whether it makes sense to apply this potentially outdated regulation to CO₂. We’ve explained to them that the results we’ve achieved in controlling our rat populations through experimentation with dry ice have been outstanding, and that it’s also cheaper, safer and more humane than many other treatment methods.”

Christopher says EPA is researching options to identify the most appropriate means of addressing this issue. “We’re sharing all of the information we can to make it easy for them to find a way to allow us to use dry ice again,” he adds.

RESURGENCE OF DRY ICE. According to the paper by Corrigan and Stellberger, dry ice was used in the 1940s and ’50s, but abandoned or reserved for special circumstances with the advent of ready-to-use rodent baits. And even though direct burrow baiting is a relatively small portion of a city’s rodent control efforts, some PMPs decided it was time to try this important tool again. Stellberger was instrumental in bringing dry ice back into use in 2012.

“I heard about this humane treatment method and thought it was worth testing,” he says. “We found that, next to our preferred method of permanent landscape exclusion, it was by far the most effective method available. We consistently achieved a 96 percent success rate after two applications.”

When Stellberger received written notification from the Massachusetts Department of Agricultural Resources that his firm needed to stop using dry ice, customers were disappointed and surprised that a naturally occurring gas would be off- limits. Stellberger was prepared with other tools in his arsenal, but none, he says, as safe, inexpensive and easy to use as CO₂.

“We’ve defaulted to mechanical trapping for existing infestations and installation of anti-rodent burrowing landscape fabric to eliminate burrowing and prevent re-infestation,” he said. “Barriers work well, but their installation is much more involved than dry ice treatments, as we need to remove the soil and install the barrier, and then apply mulch and crushed stone. As far as mechanical traps, while they can be effective, they aren’t as effective as dry ice — first, because many rats avoid the traps entirely, and second, because they don’t control ectoparasites as CO₂ does.”

Another PMP, who previously used dry ice in a popular New York City park with large rat populations (and asked to remain anonymous), also points to the value of dry ice in controlling ectoparasites — and the rats themselves. “When rodents die deep in their burrows, you don’t subject the public to fleas and other parasites that grow on them when they die on the grass or sidewalks,” he says, adding, “dry ice applications culled rodent populations so effectively that people who came to the park said they were amazed to not be seeing rats. Rats had been heavily entrenched there for a very long time.”

An equally important benefit of dry ice, he continues, is that it’s not a danger to hawks and other raptors. “When FIFRA was drafted in 1972, we had many more options in terms of rodenticides — we could apply tracking powder around a park, for example. We wouldn’t use those methods today as we focus on protecting the environment and non-target species. Dry ice is a smart solution. Since the cease and desist order, we’ve been watching rat populations steadily return, even in the cold winter weather,” he says.

ROAD TO RESOLUTION. Corrigan and Stellberger have recommended, with the agreement of at least one EPA administrator, that a scientific advisory board be convened to weigh in on the dry ice issue, believing such a board might facilitate resolution. They call for three experts: a toxicologist, to address the safety of dry ice when applied as directed at recommended concentrations and locations; a veterinarian, to speak to the humane aspects of treating rodents with dry ice; and an environmental lawyer, to recommend alternate workarounds, pending the determination that dry ice is indeed a safe/humane treatment option. They also address the important issue of public health in the white paper: It’s important for all parties to keep in mind that recent research continues to show that urban rats and their parasites remain important potential health risks to city residents…This risk…should be compared to the risk (if any) associated with the use of very low concentrations of the naturally occurring carbon dioxide gas inserted into rat burrows, which in turn sinks further downward into the earth below a rat burrow and completely dissipates (i.e., it does not rise up and out of a burrow as would a fumigant).

Christopher remains optimistic. “As EPA examines the data and recognizes that dry ice is a natural tool posing no adverse threats to the environment or to people, I’m confident the agency will lift the cease and desist order. Dry ice could become a very important tool for us in protecting the public.”

The author is a frequent contributor to PCT.