Behavior and activity of commensal roof rats around rodenticide bait stations in southern California, USA

The roof rat (Rattus rattus) is a highly invasive rat that poses a threat to humans and native species. In urban settings they are typically managed with bait stations containing chemical rodenticides, however, their behavior around bait stations is not well understood. We conducted field studies in 36 residential yards in Orange County, California, to determine whether different bait station designs or baiting approaches influence roof rat behavior around stations. Specifically, we tested whether the bait station’s design, the presence of supplemental bait, and the addition of a scent lure in the station influenced rat behavior over three separate trials. Using digital game cameras, we monitored stations containing non-toxic bait for three weeks during each trial to estimate the time to discovery, entry, bait consumption, and nightly activity of rats around stations. We also determined whether landscape characteristics associated with each yard (presence of rodent management, pets/livestock, or fruits/vegetables) influenced rat behavior. Rats were detected in most yards (75 – 91 % of yards), and they discovered stations in most of these yards (59 – 89 % stations discovered). However, they did not enter many stations (24 – 63 % stations entered). Neither the station’s design nor the addition of a scent lure in the station affected any of the measured response variables. Supplemental bait around stations decreased the time to entry and increased the nightly activity of rats at two types of stations, and increased bait consumption in all station designs. The presence of fruits and vegetables in the yard decreased the time to discovery of stations, but did not affect any other response variables. Rats in yards that were currently or recently (within last six months) managed for rodents were just as active as in unmanaged yards but were less likely to consume bait, indicating that neophobia is not the only factor contributing to bait avoidance -- previous exposure to management may also lead to bait avoidance by commensal rats. This underscores that new approaches may be needed to effectively control commensal rats with the tools currently available.