Appendix D
Rodent Control Strategies
A comprehensive list of rodent control strategies that are in use or in development are listed in Table D-1. As noted in Chapter 3 of this report, many of the strategies in use are labor-intensive, expensive, and have limited effectiveness.
Rodenticides
First-generation compounds, such as warfarin, must be administered in high concentrations over multiple doses, and thus have now been replaced by second-generation compounds, such as the odorless and tasteless toxicant Brodifucoum (Thomas and Taylor, 2002; Mensching and Volmer, 2008). If the terrain affects the ability to successfully apply the chemicals, then rodents in these areas may not be treated. Mechanical methods such as trapping are not considered feasible but can be used in conjunction with other methods.
Traps
Mechanical traps are considered by some to be more humane than rodenticides. Collectively, these mechanical methods cannot discriminate between target and non-target organisms (Lorvelec and Pascal, 2005), and so similar issues are raised to the use of chemical toxicants.
Biological Controls
Biological controls of invasive rodents include predators, parasites, or other disease-causing agents that act by recapitulating the factors that would normally limit the population. One of the considerations in using this method is whether the introduction of such an agent would itself become invasive given its placement in an environment that is not its own. Several unsuccessful examples of the deployment of this method can be found in the literature, such as the introduction of rabbits into Australia in the late 1800s (Garden, 2005), means to control their subsequent substantive, and unexpected, population growth (Saunders et al., 2010), or the introduction of the cane toad to control agricultural pests of Australian sugar cane (Weber, 2012). The cost of this type of intervention will vary depending upon the organism of interest and the biological control agent being introduced.
Genetic Engineering Strategies in Development
One method being explored takes advantage of the process of RNA interference (RNAi), in which double-stranded RNAs that target endogenous RNAs essential for the life of the rodent would be introduced to the rodent in an analogous fashion to that observed currently for agricultural pests (Xue et al., 2012). Technical issues associated with this technique include delivery of double-stranded RNAs, their inherent stability and thus persistence of inhibition, the concentration required to effect species eradication, mechanism of spread, and potential biosafety risks. Proof-of-concept using RNAi as a toxicant has been demonstrated, however, with sea lampreys (Heath et al., 2014), and delivery of small interfering RNAs has been shown to be possible in mice (He et al., 2013). Another approach is autoimmune infertility, in which a virus is used to
Summary of Current Technology for Rodent Control (adapted from NCSU website)1
TABLE D-1 Some Rodent Control Strategies in Use or in Development
| Name | Primary Outcome(s) | Key Advantage(s) | Primary Challenge(s) | Select References |
| Strategies in Use | ||||
| Toxicants (coagulants such as Brodifucoum) | Species elimination |
Very effective for use in rats but not so for mice
Are odorless and tasteless so rodents can’t evade them |
Low number of feedings required in order to prevent avoidance of them
Animal welfare issue (leads to painful death) Secondary, non-target effects (ecological and animal welfare concerns) lead to question of feasibility |
Mensching and Volmer, 2008; |
| Mechanical (kill and live traps) | Species elimination or translocation | Little to no risk to human health or environment, no toxins released to ecosystem |
Inability to discriminate between target and non-target species
Animal welfare issues |
Lorvelec and Pascal, 2005; |
| Biological controls | Species elimination | Easy to identify, potential decreased risk to humans Sometimes species-specific in their efficacy | Biological controls | Garden, 2005; |
| No action (i.e., species remains in the environment) | N/A | No cost | Damage to biodiversity; other (ecological outcomes) | |
| Strategies in Development | ||||
| RNAi, immunocontraception | Species elimination or reduction | Species-specific, lowers reproductive capacity (autoimmune infertility) | Technical challenges associated with the design and delivery of treatment, target population at correct time and in large numbers. Anti-fertility technique may not be effective if these animals attempt to mate with wildtype animals. | Chambers et al., 1999; |
| Transgenic approaches | Species elimination or reduction | Species-specific; induces sex lethality or sex reversal | Would require multiple releases of modified males; may not be scalable | McLaren and Burgoyne, 1983; |
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1See https://research.ncsu.edu/islandmice/what-has-been-done/history-of-rodent-eradications.
express proteins that elicit an immune response targeting the fertilization process, thus preventing formation of the zygote (Chambers et al., 1999). This technique would achieve population reduction, but challenges still remain with respect to administration of the virus at the appropriate life cycle time of the rodent, the number of rodents that would be required to be infected (Jacob et al., 2008), and the need to ensure that infected rodents mate with one another as opposed to untreated rodents.
Another line of research involves a genetic approach in which rodents could carry transgenes that, upon mating to the invasive population, do not produce any progeny (e.g., lethality) or cause the female offspring to develop as males (sex-reversal) (McLaren and Burgoyne, 1983; Bax and Thresher, 2009; Gemmell et al., 2013). This method, however, will likely require multiple releases of transgenic males and may not be scalable (Campbell et al., 2015). Finally, in some instances it may not be possible to eradicate an invasive rodent population, due to the high cost involved, the location and topography of the land area under investigation, the presence of humans, or risks posed to the ecosystem.
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