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Plant secondary metabolites and extracts

There is scientific interest in applying Plant Secondary Metabolite (PSM) odour mixtures to deter rodents from inflicting feed loss and damaging agricultural infrastructure (Hansen et al., 2016). If successful, PSM-based repellents could be a cost-effective and ethical alternative to poison or trapping.

There are several different classes of PSMs: plants and plant materials; essential oils and terpenoids; alkaloids and alkylamides; di-carboxylic acids, glucosinolates, phenolics and flavonoids and tannins (Hansen et al., 2016). Laboratory studies of plant materials from the common sunflower (Helianthus annuus), peanut (Arachis hypogaea) and walnut plants (Jugulans regia) have observed repellent effects in Norway rats (Grant-Hoffman and Barboza, 2010). In laboratory studies, repellent effects in house mice have been observed from various essential oils, including bergamot oil, fennel oil and neem oil (Hansen et al., 2015). Studies on alkaloids from the Japanese pepper shrub (Zanthoxylum piperitum) found a strong post-ingestive repellent effect in Norway rats in laboratory studies (Epple et al., 2001). Research on various di-carboxylic acids, glucosinolates, phenolics and tannins have observed repellent effects against several rodent species, although not against the commensal rodent species of concern to the Australian poultry industry (Hansen et al., 2016).

Despite extensive literature on the rodent-repellent effects of various PSMs, only a handful of products are available commercially. In Australia, two plant-based products are registered; the first is a mixture of white pepper and garlic oil; and the second is a mixture of corn mint oil, camphor oil, eucalyptus oil and methyl salicylate applied to garbage bags (APVMA PUBCRIS database, accessed June 2019). There is little published research on the repellent effects of these products, specifically against commensal rodent species.

Plant extracts have also been observed to inhibit rodent fertility by limiting a range of reproductive processes, such as gonadal function and development to gestation. Extracts from at least 40 plant species have been observed to disrupt reproductive effects at the ovarian level when orally administered to rats and mice. Unfortunately, researchers have been unable to identify specific active compounds responsible for these reproductive effects. Furthermore, the effects of these plant extracts are short term and reversible when treatment stops (Tran and Hinds, 2013).

In summary, some plant metabolites and extracts appear to have potential to be used as humane and environmentally friendly tools for rodent management. However, inconsistent results of laboratory and field studies have ultimately led to a small number of commercially viable products that cannot be compared to the wide range of traditional rodenticide compounds.

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