An interesting read:
Here is some of an extract of research being done with Eucalyptus oil deterring rodents. Full story here.
Rodent repellents are chemicals which by taste or odour or possibly by both will prevent animal from feeding or gnawing. Such substances may be used in protecting an area from rodent infestation or in protecting packaged food, packing materials, electric cables, and other important vulnerable materials. Mature and healthy house rat, Rattus rattus of both sexes, was exposed to 5, 10, and 20% eucalyptus oil applied as spray in laboratory pens in bichoice tests. Each concentration was applied through three different modes of application, that is, daily, once, and alternatively in a week. Repellent effect of the oil was assessed based on food consumption from treated and untreated sides for four days. In overall, food consumption was significantly () low from treatment side compared to the untreated side indicating significant repellent effect of the oil at all the three concentrations tested. Repellent effect of the oil was, however, not found to differ significantly between the two sexes. Percent repellency in both male and female rats was apparently more with daily application of 5 and 10% eucalyptus oil. Present studies reveal the potential of eucalyptus oil in repelling away R. rattus; however, further studies may be conducted to enhance the persistence of repellent effect for longer period of time.
During the present studies, significant differences were found in mean daily food consumption from treatment and untreated sides at different modes of application and at different concentrations of the oil between male and female rats. This may be due to the sex specific variation in response to R. rattus towards the oil. Similar sex specific differences in response to R. rattus of two sexes towards toxic baits were reported by Kaur et al. . During the present studies, higher standard deviation values than mean values of food consumption were observed in some cases which may be due to individual variation in response shown by rats of different species .
In overall, no significant difference in repellent effects of oil applied as spray was found between the two sexes and among the three concentrations of the oil tested during the present studies. All the three concentrations were equally effective and the repellency was highest when the oil was applied daily. The cost of spraying 5% eucalyptus oil (the minimum effective concentration) per 1 m2 area comes out to be Indian Rs 10 (US $ 0.17), which can be considered cost effective if we keep in view the extent of loss caused by R. rattus through damage and contamination of food. Among the various components of eucalyptus oil, 1, 8-cineole is the most important one largely responsible for a variety of its pesticidal properties . The presence of this essential oil also provides defense advantage to eucalyptus leaves against herbivory and attack by harmful insects . The present study is the first of its kind evaluating repellent potential of eucalyptus oil against vertebrate pests. Previous to this, repellency of eucalyptus oil has been recorded against the tick, Ixodes ricinus  and against acaricide-resistant mites . Application of 1.0% concentration of 1, 8 cineole reduced oviposition rate of Thrips tabaci by 30–50% as compared to untreated controls . Eucalyptus oil (1%) added to sugar syrup, repelled honey bees . Eucalyptus oil (2%) on filter paper and wood floor repelled termites . Eucalyptus oil can also protect plants against rice weevils, pine processionary moths, and mushroom flies . Essential oils of eucalyptus appear particularly potent as mosquito repellents .
Since eucalyptus oil possesses a wide spectrum of biological activity and is regarded as safer compounds, there have been attempts to commercialize and market the insecticides/repellent products containing eucalyptus oil as such or based upon them. Quwenling is a eucalyptus-based product that has been successfully marketed as an insect-repellent in China . It provides protection against Anopheles mosquitoes parallel to DEET and has, in fact, replaced the widely used synthetic repellent, dimethyl phthalate.
The present studies reveal the potential of eucalyptus oil applied as spray in repelling away R. rattus of both sexes. Percent repellency was more when the oil was applied daily and alternatively as compared to when applied once a week indicating low persistence of the repellent effect due to volatile nature of the oil. Further studies may be conducted to prepare formulations of the oil leading to slow release of the oil so as to increase its persistence for a longer period of time.
Mice are generally looking for food or a place to nest, so make sure you don’t have anything in your garage or home to tempt them. Keep your woodpiles outside of the garage as well as your garbage cans. Garbage cans in the house should be covered and obviously not kept around long.
Block any holes around pipes or other places bigger than one-quarter inch across, since mice will sneak in through the tiniest of spaces. An easy way to block access is by stuffing cracks and holes full of steel wool.