DTY Rodent Management
Always Leaving Something Behind


Within forensic science there is an investigative concept known as Locard’s Exchange Principle. This principle simply states that a perpetrator of a crime will bring something into the crime scene, and leave with something from the crime scene. The longer the perpetrator remains within the crime scene, the more evidence will be left behind.

Mouse and rat perpetrators leave considerable evidence within pestiferous crime scenes. As part of an active monitoring program, the savvy pest investigator can use state of the art visual inspection tools such as a blue light flashlight with orange goggles or a UVA lamp to detect low levels of rat and mouse fluorescing evidence. Always leaving something behind, mice and rats readily shed bodily evidence.

  • Blood
  • Bone Fragments
  • Hair
  • Saliva
  • Scats
  • Sebum (oily substance)
  • Semen
  • Urine
The pest management professional is a pestiferous crime scene investigator. In addition to active monitoring or visual inspections, passive monitoring can establish the presence of mice and rat perpetrators within a pestiferous crime scene. As active monitoring can provide a crime scene snapshot in time, passive monitoring can provide a crime scene movie over time.

During the times the pest investigator is not present, and based upon environmental history, passive monitoring uses strategically deployed devices, gathering pest evidence over long time intervals. Mechanical, inert and/or active baiting and remote sensing rodent control devices collect evidence of mice and rat perpetrators 24/7/365. From evidence left behind, the pest investigator can take curative action and remove root causes.

For the pest investigator, discovering and understanding the best evidence of a pestiferous crime scene involves best practices in rodent control device placement. When mice and rats are caught, determine the approximate age and sex of each, which edifies pursuant to harborage locations, routes of travel and population growth.

The following attributes will help you better understand the pest you’re up against:
  • A mouse newborn possesses visible pink skin and no pelage (fur)
  • A mouse is at least 13 days old when its eyes are open
  • A mouse covered in pelage is older than a week
  • A mouse a week or more older covers the length of a quarter coin and a half (not including tail)
  • A mouse’s head is the length of a quarter coin at 28 days or more
  • A mouse possesses full pelage, eyes open and clear, all teeth visible and a female's nipples fully developed at 28 days or more
  • A young rat’s incisors are pale yellow (darkening during aging)
  • An adult rat’s incisors are dark yellow or orange
  • An older rat’s incisors are dark orange
  • Sex mice and rats by examining the distance of the anus-urinary/genital openings (distance is longer in males than females)
  • Female mice and rats possess visible nipples at 10 days of age
  • Male mice and rats possess no nipples
  • Male adult mice and rats possess testicles at the base of the tail (undescended or descended depending upon mating season)
Presence of pregnant female and young male mice and rats suggests searching for new spaces and a growing rodent population. A 1:1 ratio of females to males is characteristic. More of one sex than another may suggest recent litters by females and nest restriction to care for young.

Be a forensic pest expert and apply Locard’s Exchange Principle. After all, mice and rats are always leaving something behind!

Stuart Mitchell, D.O., DVM, PsyD, BCE, is an entomologist, veterinarian, observing physician and consulting clinical psychologist, and a regular contributor to Pest Management Professional's Direct to You series.

PMP’s Direct To You provides pest management professionals with educational refreshers on timely and critical topics essential to operational success. This content is not be used as a substitute for obtaining legal advice from an attorney licensed to practice where you live. Look for the content-rich PMP Direct To You archives at mypmp.net/direct-to-you-archive.

This newsletter was produced by North Coast Media’s content marketing staff in collaboration with Bell Laboratories.

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