Crops: All crops, but especially maize, rice and potato
Why is it a problem? Wild boar are the main vertebrate pests of agricultural crops in Bhutan. Crop losses due to wild boar are one of the main constraints to alleviating poverty and increasing food security in the country. Even rural-urban migration and fallowing of land is promoted by the wild boar problem. The estimated cost of maize and paddy losses due to wildlife, primarily wild boar, is in the order of USD 4 -5 million per year. Lost output from fallow land has been estimated at USD 13 million for wetland (paddy equivalent) and USD 44 million for dryland (maize equivalent) (Maetz et al., 2012). Wildlife damage, especially by wild boar, and cost of protection, is regarded as one of the most important constraints to potato production. Guarding the field accounts for 20 – 50% of total labor inputs. In 2005, an average household spent 73 nights guarding the potato field. The conservative estimates of income lost annually are in the range of Nu. 20 – 50,000 per household. Wild boar are also a source of diseases that can affect both humans and domestic animals.
Where and when is it a problem? Serious agricultural losses to wild boar in Bhutan were first reported in 1991, although almost certainly were occurring much earlier than that. Since then the issue has been deliberated and discussed at all levels; from the local government to the highest national assembly. Serious wild boar problems have been reported from East, West and Central regions across Bhutan. They are also present in the south but serious problems have not been reported there, perhaps because of control activities.
It is not known when pigs were first introduced to Bhutan. There are several types being bred in captivity across Bhutan including the so-called indigenous pig that has been there the longest (see Nidup et al. 2011). Wild boar are thought to include hybrids of that and more recent types, based on dentition records from the Project on Wild Pig Conservation and Management that was operated from July 2002 to December 2004 (Ministry of Agriculture, Bhutan).
Wild boar damage is characterised by signs of rooting, wallowing, nesting, rubbing, and walking trails.
Confusion with other pests: Damage symptoms caused by wild boar could potentially be confused by that of other animals, or even by high winds. Always look for direct evidence, such as sightings, hoof prints or faeces.
Lifecycle: Wild boar biology in Bhutan has not been studied. However, informal interactions with farmers during the Rapid Assessment of Safe System Approach of human wildlife conflict management, learned that wild boar reproduce twice in a year. Further, farmers believed there to be two groups of wild boar; one that lives in groups of 6 to 50 animals and another that lives alone. These solitary individuals may be male boars.
Dispersal: Wild boar in Bhutan have not yet been successfully tracked using radio collars. However, studies in other countries suggest that where topography and climate allow they can easily move several kilometres to source food, water and shelter. It is not known where wild boar move to when they are not attacking crops, or whether the same or different wild boar are attacking different fields within a locality.
When can damage be expected? Greatest wild boar damage typically occurs around mid-August, but can extend through the growing season. Peak damage occurs mostly in the first maize crop which is then maturing. Mid-August is also when time spent in guarding maize fields is diverted to guarding paddy fields. This corresponds with when paddy fields become more attractive and vulnerable to wild boar.
Wild boar attack maize at two growth stages. First, they attack the young vegetative stage before flowering, aiming mainly at the tender sweet growth points at the tip of plants. Second, they go for cobs at the maturing and ripening stage. Unlike paddy, maize hardly recovers, lacking the ability to compensate through the formation of extra tillers or shoots.
Wild boar attack paddy fields at two stages. First, from transplantation around July through to mid-October when the water level in the paddy is lowered. Boars also damage the bunds surrounding terraces. Second, from the dough stage (around September) onwards they feed on the panicles. Indirect damage, through rolling, wallowing and trampling, is often much bigger than the actual feeding damage.
Wild boar damage potato crops from the moment of planting, and continue through the season. They will continue to attack replanted potatoes. Hollows made by the hooves of wild boar in the potato fields also collected rain water during the rainy season, making potatoes prone to rot.
Habitat and diet: Wild boar are opportunistic omnivores. In Bhutan wild boar are predominantly a problem in maize, paddy and potato but can affect most crops. Recent analysis of faecal samples found grains of maize, rice, oak nuts, and tubers to be a common part of their diet. In other countries they are known to eat mostly plant matter; but also invertebrates such as worms and insects, small mammals, newborns of larger mammals, and eggs and young of ground-nesting birds and reptiles.
Successful management of wild boar requires on-going effort, and frequently requires the combination of different control methods.
- Look for signs of wild boar damage, and try to confirm the cause.
- Surveillance cameras can be useful, and are now relatively cheap.
- It is useful to know how many different individuals are involved when formulating a management plan.
Keeping wild boar out
- Fencing using traditional materials such as wood and rock walls are highly effective, but can be time consuming to erect and maintain. Biofences may also be effective if well-constructed.
- Electric fencing, using solar or hydro-electricity, is highly effective if properly constructed and maintained. For wild boar electric fencing needs to have four-strands and any depressions in the ground need to be blocked to prevent wild boar from getting under the bottom strand.
Scaring away wild boar
- Current methods aimed at scaring wild boar away are very time consuming, and the labour involved is one of greatest negative effects of wild boar in Bhutan. Traditional methods include scarecrows, banging cans, night guarding, shouting, stretching reels, hanging cans and using dummy tigers.
- In Bhutan, sound and light repellent was piloted in certain Dzongkhags. However, wildlife adaptation and the ineffectiveness of the light and sound to travel beyond 200 m hindered adoption in the field.
Suppressing wild boar populations
- Lethal methods are often the most practical way to suppress wild boar populations, and for removing problem individuals.
- Currently farmers can only kill a wild boar on their property if it is causing damage to crops or property. A statement from Chiwog Tshogpa shall be accepted in cases the animal was shot on private land while destroying crops and later died on government land within 200 m from the boundary of the agricultural land (Forest and Nature Conservation Rules of Bhutan 2006).
- Non-lethal methods such as sterilizing wild boar are possible, but have never been tested in Bhutan and are likely to not be feasible or economic.
Image acknowledgements: NPPC.