Count to know the rodent amount
By Dr. Stuart Mitchell
Ever get the questions "How many mice?" or "How many rats?" Any pest management professional (PMP) conducting rodent control services has received these questions – and some days, more than once. What are the answers? The basic answer is that it is not realistic to be able to determine exact mouse and rat populations. Rather, it’s about the "count to know the amount."
There are several methods to plausibly determine estimated mouse and rat populations. Let's take a look at a few examples:
COUNT THE PUPS
Fecundity, or the ability to produce an abundance of offspring, is a function of reproductive biology. Mice and rats possess amazing reproductive biology. Mice can birth up to 10 pups per litter. Rats can birth up to 12 pups per litter. For example, if there are at least 10 female mice, there is a probability of up to 120 pups being birthed. If there are at least 12 female rats, there is a probability of up to 144 pups being birthed.
USE THE INDEX
The Rodent Index Formula, from Dr. H. Michael Opitz, University of Maine, can be used to estimate a mouse population.1
Rodent Index Formula: Number of mice caught in all traps ÷ Number of traps ÷ Number of days traps are set x 12 x 7 = Number of mice for Index
Example:
25 mice caught ÷ 15 traps ÷ 2 days x 12 x 7 = 69.9 (Rodent Index 3 or high population)
| Number of mice for Index | Rodent Index | Description of Index |
| 0 - 10 | 1 | Low |
| 11 - 25 | 2 | Moderate |
| 26+ | 3 | High |
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COUNT BURROW ENTRANCES
Rat observation guidelines from the Pest Control Video Network can be used to estimate a rat population.2
Rat observation:
- 1 rat/night + 0 rats/day + evidence (scats and gnawing) = medium population
- 3 rats/night + 1 rat/day + evidence (scats and gnawing) = large population
- Rat burrow is 1.2 to 1.5 m (4 to 5 feet underground) with protective cover
- Average 3 entrances/burrow
- Average 8 rats/burrow
Example:
- Observe 15 burrow entrances
- 15 entrances ÷ 3 entrances/burrow = 5 burrows
- 5 burrows x 8 rats/burrow = 40 rats
VERIFIABLE RODENT DATA
Use verifiable rodent count data that is reliably provided by a monitoring system. Such a system provides PMPs with accurate catch counts by trap and location. Monitoring devices help PMPs immediately calculate a new rodent count from a distance and in real time. Devices in the system can confirm which ones have activity as well as which areas are hot spots.
Whether it’s counting pups, using the rodent index, counting burrow entrances, or using monitoring system data, understanding what to count will help PMPs better understand the amount. After all, it’s about count to know the amount!
References:
- Steinhart, Terry L. and Karsten, Arlin W., "Trapping to Estimate Rodent Populations: A Demonstration Project" (2002). Swine Research Report, 2001. 21.
- Pest Control Video Network, "The Pests of Your Life," Edition M6: "Rodents and IPM."
Stuart Mitchell, DO, 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. 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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