Finding Ways to Reduce the Risk of Non-Target Rodenticide Exposure in Roof Rat Control

Earlier this month, a research paper was published in the journal Pest Management Science titled “Use of Rodenticide Bait Stations by Commensal Rodents at the Urban-Wildland Interface: Insights for Management to Reduce Non-Target Exposure”. This research was partially funded by a Pest Management Foundation grant made to Dr. Niamh Quinn’s research program at University of California Agriculture and Natural Resources, South Coast Research and Extension Center in Irvine California. The Pest Management Foundation is an independent non-profit charity that provides scholarships for outstanding urban entomology students and funds structural pest control research at universities across the United States. All of the Foundation’s work is made possible by donations from pest control companies and individuals interested in advancing the science of structural pest control.

In recent years, anti-pesticide groups have placed pressure on pest management professionals regarding non-target exposures of wildlife to anticoagulant rodenticides – especially in California mountain lions, birds of prey in New England, and most recently in bobcat populations on Kiawah Island, South Carolina. The knee jerk reaction by many of these activist groups has been to ban the use of these products based on residue data that may show sub-lethal amounts of anticoagulants in non-targets. In California, a measure was successfully passed in December 2020 to ban most SGAR uses across the state, even by licensed professionals. Unfortunately, outright bans like these don’t take into consideration the important beneficial role that rodenticides play in managing these important public health pests and that instead of banning the products altogether, it’s important to first understand how non-targets are becoming exposed, then try to limit those exposures through common sense risk mitigation.

Although it is not known exactly how these non-target exposures are occurring, it is thought that native rodents like deer mice, wood rats, kangaroo rats and ground squirrels are likely entering stations and consuming rodenticide bait, then raptors, coyotes, pumas or other non-target predators are eating the non-target, non-pest, native rodents.

To better understand which native rodents are entering bait stations, Dr. Quinn and her colleagues, positioned two bait stations in more than 90 backyards in southern California.  One station was placed at ground level and one was positioned 3-5 feet off the ground.  Non-toxic commercial bait was placed inside each station, then digital cameras were focused on the two bait stations and every animal that interacted or entered the bait station was photographed.  More than half a million photos were captured during the study and here’s what was learned: 

• Roof rats were present at more than 80% of the sites. House mice and Norway rats were observed much less commonly.
• Native rodents (deer mice, wood rats, kangaroo rats and ground squirrels) were relatively rare, visiting or entering bait stations at only 13% of the sites. 
• Native rodents were five times less likely to enter the stations that were positioned off the ground.
• Roof rats, on the other hand, were equally likely to find, enter and interact with bait stations when positioned off the ground.

The authors made four specific pest management recommendations based on this research:

1. PMPs should monitor bait consumption closely during the first few weeks to ensure that adequate bait is available to control populations.  High populations of rats can deplete baits quickly.
2. On average it took roof rats 7-8 days to enter a station.  SGAR baits typically take 3-5 days to work, so control effects may not be seen for as long as two weeks after application.  This should be communicated to clients so that expectations can be managed.
3. In areas where predator species like coyotes are common (that is, homes closer to natural areas and parks) care should be taken to clean up any rat carcasses that are found in the open.
4. When roof rats are the target pest, positioning bait stations 3-5 feet off the ground can limit non target entry into the stations. 

Jim Fredericks, PhD, BCE

To read the full research paper visit: https://onlinelibrary.wiley.com/doi/abs/10.1002/ps.6345

To learn more about the other research projects taking place in Dr. Quinn’s lab visit http://ceorange.ucanr.edu/humanwildlifeinteractions/

To learn more about the kinds of research that the Pest Management Foundation funds, or to make a donation, visit www.NPMAFoundation.org .