HAGATNA, Guam (AP) — Christmas trees arriving by ship to the U.S. territory of Guam are getting decked out with a special decoration: Doses of methyl bromide gas to prevent them from becoming nesting grounds for invasive species.

Guam’s Customs and Quarantine’s BioSecurity Task Force is filling containers of imported Christmas trees with odorless and colorless methyl bromide gas to kill potentially invasive species, The Pacific Daily News reports.

Customs agents have treated six containers of more than 2,500 imported trees, wreaths and garlands and expect to treat another seven containers in coming weeks, officials said.

The Guam Invasive Species Council approved the Department of Agriculture’s fumigation policy in 2016 to prevent unwanted pests from impacting Guam’s agriculture, natural resources or the homes of Christmas celebrants.

Each container will be released after a customs inspection to ensure the treatment works, officials said.

The Environmental Protection Agency characterizes methyl bromide as a toxic fumigant to control a wide variety of agriculture and shipping pests including fungi, weeds, insects, roundworms, and rodents.