Posted by Frank van Steenbergen, Cecilia Borgia, Getachew Engdyaheu, Berhanu Amni, Taddesse Tilahun
July 05, 2016
Rodents are arguably our oldest and most pernicious pest problem, causing several global disease pandemics, attacking all crops in the field as well as after harvest where food is stored, and even during transportation and processing. It is well-known that rodents affect the poor disproportionately, through transmission of more than 60 types of diseases, damage to crops and infrastructure.
The impacts of rodent pests on food security in Asia have been well-documented. Annual pre-harvest losses have been chronic, ranging from 5 to 10%. During outbreaks of spike in rodent populations, they shoot up to 30% on average. These averages hide extreme impacts: some farmers are known to have experienced nearly 100% losses, leading to food insecurity and indebted families. In Asia, a loss of 5% amounts to 35 million tonnes of rice, i.e. enough rice to feed 280 million people in developing countries for a year. If we focus only on countries where people are undernourished, then a 5% reduction in losses from rodents is enough to feed 34% of the undernourished people in the world.
Soil and water conservation and rat abundance
In Ethiopia, studies report pre-harvest losses of cereal production due to rodent attacks in the range 9-44%. Not only have reforestation and soil and water conservation efforts been counteracted by damages caused by rodents, they have thrived amid stone and earthen bunds constructed as part of the efforts. Having vascular tissues of young tree seedlings for rodents to gnaw on, these structures make for a perfect hideout. Watershed areas under rehabilitation are often closed to grazing livestock which would otherwise chase away rats and mice. In their absence, these areas turn into perfect shelters and breeding grounds for these vertebrate pests.
Rodent pest infestation in and around households, food stores and villages have largely been ignored due to a combination of lack of awareness of impact, scale of the problem and the failure to manage the problem using traditional poisoning methods. Despite the urgency to find effective methods to control rodent populations and damage, remedies available are still limited mostly to anticoagulant rodenticides.
However, the use of anticoagulants is increasingly being challenged because of their damage to non-target species and to the environment at large. Besides the persistence of synthetic poisons in the environment, anticoagulant rodenticides are becoming less and less effective as rat populations develop physiological resistance to them. Their safe and effective use requires good training and, unfortunately, such knowledge is often not provided to or found among the many rural and urban users of rodenticides in Asia. Furthermore, small-holder farmers – owing on average 1 acre land – often report that anticoagulants available are ineffective, unaffordable or unavailable. Ultimately, many families resort to highly toxic and illegal substances for rat control, leading to accidental poisoning of family members and domestic animals.
Biological rodent control: an untapped potential
Against this backdrop, local knowledge of biological rodent control based on medicinal plants can prove to be an untapped source of innovation. Farmers in Ethiopia for instance use the leaves of Croton macrostachyus to plug mice holes. When the rodents try to get out they eat some of the leaves, which contain a toxic alkaloid, and die within days. Another rodenticide plant is Crinum abyssinicum. The tuber is sliced or reduced to powder and used to prepare poisoned baits. Chrysopogon zizainoides and Tephrosia vogelii act as repellents for rodents because of their itchy parts.
In Bangladesh and India, the leaves of Dendrocnide sinuata, locally known as ‘Gilmat’, are put in boiled rice for 24 hours. The rice gets poisoned and is then placed along rat pathways in fields and around houses. Gilmat is a woody shrub with stinging hairs that are irritating upon contact. It is used in several parts of South Asia as a traditional method to repel rodents. It is also called ‘elephant nettle’ as its irrigating effect keeps even elephants at bay. The stinging hairs, particularly present on the leaves, are glandular trichomes that contain various compounds such as formic acid, serotonin, moriodin and histamine.
Still much remains to be discovered and validated
The science behind Gilmat is still poorly understood, and it is not yet clear whether the plant is repellent and/or toxic, inducing a physiological response in rodents or whether it is acting as a behavioural deterrent through its physical and/or chemical properties.
Moreover, many indigenous methods are based on anecdotes and have not been scientifically validated. Much remains to be discovered in terms of best practices. It is important to evaluate the efficacy and sustainability of indigenous, plant based rodent control methods in order to use them on a wider scale.