[Rodent Control] Of mice and mousetraps

Editor’s note: The following is an excerpt from “Rodent Control: A Practical Guide for Pest Management Professionals.” To order, visit www.pctonline.com/store or call 800/456-0707.

Although the ordinary mousetrap is one of the most common devices used against mice for more than 100 years, there is relatively little science behind the use of these traps. For example, what is the capture and miss rates of snap traps? How often can mice steal bait from each of the different styles of traps? How often are traps ignored by mice?

One comprehensive field study conducted at Purdue University in 1990 attempted to address some of these questions — at least to a cursory level. Tests were conducted in enclosed and heated livestock houses (60 feet by 300 feet/18 meters by 91 meters) containing severe house mouse infestations.
Two tests each containing 57 traps of the two most commonly used snap trap models (the VictorTM standard 4-way trigger [Model MI], and the Victor professional [Model M7] expanded trigger model) were conducted. For Test One, the traps were set and placed every 1.5 meters in a pattern alternating each model. Traps were baited with one gram of smooth peanut butter placed directly on the trigger pans of both models. Each trap was positioned perpendicular to the inside perimeter wall, with the bait pedal positioned farthest away from the wall. For Test Two, the same experiment was repeated one day later using all new traps, and in areas not previously trapped. The position of the traps was reversed so as to place the bait pedal closest to the wall as is done in standard positioning of a snap trap. All traps in both tests were run 24 hours after setting. The study, while 18 years old, helps answer some commonly asked questions about snap traps.

How are mice captured by mouse traps? It is probable that mice interact with snap traps for a couple of reasons: 1) they approach a trap to feed or to investigate the bait odor; 2) they attempt to investigate the new object in their territory; or 3) they unwittingly attempt to walk, run, or jump on or over the object in the paths they have been habitually or kinesthetically traveling each night and day.

The data from this study demonstrated that the expanded trigger traps produced higher capture rates than the non-expanded trigger models. Most of the mice (79 percent) were caught in the head or neck region of the body, indicating that most mice approached the traps head-first. But whether or not the mice were captured when they were attempting to feed from the trap or investigate the bait odor; or they were captured when they attempted to simply travel across the trap when the trap kill bar came down on their heads, is difficult to determine.

Only 15 percent of the captured mice were trapped in the mid-torso region of the body and a few (3 percent), were captured by only their hind feet and tail. Some of the mice in these last two groups were likely attempting to jump over the traps in their path. A few mice (3 percent) were found dead at short varying distances from setoff traps indicating “near escapes.”

How do mice ‘escape’ lethal encounters with traps? The escape rates between the two models were not statistically different in these trials. However, when the triggers of the traps were positioned closest to the wall, a significant decrease in the number of escapes among both models can be seen. It is likely that the mice approaching the traps next to a wall have less room to maneuver an escape response.

It was noted that some mice “escaped” capture by either jumping over the object (trap) in their path, by “bumping” the traps from a side or corner, or by stepping on or otherwise interacting with the non-trigger half of the trap. Such encounters are not true “escapes” (i.e., the mouse is not captured in the trap and then works free). It may be sheer luck that a mouse bumps a trap and is not within the target zone of the trap. Or the mouse jumps high, long and fast enough to escape the ensuing kill bar. Perhaps the adults are better physically equipped and able to escape such dangerous objects than are the younger, weaker, less agile and less experienced juveniles. It’s also possible that some mice may be inherently more adept at escaping interactions with traps than others.

How often do mice steal baits without getting caught?
Anyone who has ever used a mouse snap trap is aware of mice stealing the baits off the traps (e.g., licking the peanut butter off the trigger or lifting candy out of the trigger well). The results of these field tests indicated a significant difference in the ability of mice to steal bait between the standard triggers and expanded-trigger trap models. (Bait was determined stolen from a trap when a trap was found without bait, yet still in a set position.) The mice were able to steal bait successfully 12 percent and 16 percent of the time from the non-expanded trigger traps, but were able to steal the bait only once from the professional model trap.

Do some mice ignore traps?
Most mice tend to be investigative by nature. And thus it is often said that mice are “curious” about new objects in their territories. But research over the years has demonstrated that there is considerable individual variation in the behavior of mice toward traps. Some mice may take several hours or days to interact with new objects (e.g., traps). Some may never interact with a trap at all.

Previous research on mouse behavior indicates the possible reasons for mice bypassing traps in their territories. First, the bait on the trap is not attractive even when detected by the mouse; second, the odor of the bait may not be evident to the mouse until it is within a few inches of the trap; and third, some mice, like some rats, are neophobic and thus avoid new or strange objects in their environments. So, it seems in any trapping program (live or lethal) some traps are apt to remain unvisited.

Does trigger-wall position affect capture or escape rates?
In these tests, no difference was seen in the capture rate when the bait trigger was positioned either closest or farthest from the wall when the traps are located in a perpendicular setting to a wall. Apparently, when mice investigate the trap/bait, they spend enough time to become committed in the target zone in either position.

However, significantly fewer “escapes” occurred when the triggers were set closest to the wall surfaces. Possibly, the mice have less room to maneuver an escape when they themselves are positioned closest to the wall. Also, a mouse stepping on a trap with the trigger closest to the wall obviously has less space in which to move away (or be catapulted away) from the kill bar compared to a mouse moving or positioned along the open space side of the trap (i.e., the trigger is positioned farthest from the wall).