Denning is the practice of killing animals in their burrows or dens—usually with poisons, or removing them from the den and dispatching them above ground. Sodium and potassium nitrates, poisons commonly used for mammalian carnivore pups, are combined with sulfur and carbon in canisters that are ignited and used as rodenticides, predacides, or insecticides in burrows or dens in a practice that Wildlife Services calls “denning.” Denning also consists of removing pups from the den with mechanical devices and then dispatching them above-ground in various horrific ways.
Target species include rodents (moles, ground squirrels, woodchucks, prairie dogs, and pocket gophers), skunks, coyotes, red foxes, and ground-nesting wasps. The EPA considers gas cartridges as a Category II toxicant – the second highest degree of toxicity on a scale of four. Because this pesticide is used in burrows and dens, many non-target species, such as desert tortoises, black-footed ferrets, and burrowing owls are susceptible to unintentional poisoning.
Sodium nitrate explodes when heated to 1,000 degrees and produces the toxic fumes of nitrous oxide and sodium oxide. The gas released is carbon monoxide. Nitrite converts the blood’s hemoglobin to methemoglobin, which does not carry oxygen.
Wildlife Services uses large gas cartridges, which agents bury in dens to asphyxiate pups of skunks, foxes, and coyotes. Large gas cartridges are comprised of a mixture of sodium nitrate (53 percent), charcoal (29 percent), and inert ingredients (19 percent). The EPA’s label for the cartridges warns that it will burn with vigor until empty and could cause severe burns and start a fire. It further warns against inhaling, and if symptoms occur (headache, nausea, dizziness) to lie down, stay warm, and breathe pure oxygen if available. This label requires specific considerations for black-footed ferrets, Fresno kangaroo rats in southern Utah, Hualapai Mexican voles, and blunt-nosed leopard lizards. It also requires that Wildlife Services not harm red and gray wolves.
Small gas cartridges are used for the following rodent species: woodchucks, yellow-bellied marmots, ground squirrels, black-tailed prairie dogs, white-tailed prairie dogs, and Gunnison’s prairie dogs (a species considered for federal listing). (Wildlife Services also uses other toxicants to kill prairie dogs in the burrow.) This label specifically warns about harm to several non-target species including burrowing owls, rodents, and amphibians.
Wildlife Services has admitted, “because it may be difficult for users to determine the presence of some non-targets within an underground burrow, the label may not be stringently followed in all situations, and some non-target individuals may be killed” (emphasis added, 1997 PEIS, Appendix P, p. 250).