The 2007 vole plague originated in early summer 2006 in the province of Palencia, located in the autonomous community of Castile and León, Spain. By the summer of 2007, rodent populations had severely devastated crops in the plateau fields. Following a summer marked by significant agricultural losses, the density of voles decreased by September 2007, leading to the institutional declaration that the plague was over. However, vole populations remained abundant in the months that followed. It was only the winter frosts and low temperatures in November and December that reduced their numbers to normal levels.
The common vole (Microtus arvalis) was primarily responsible for the crop devastation across the northern plateau. This Eurasian species had previously been confined to the Cantabrian Mountains, where it differentiated into the subspecies called Microtus arvalis asturianus. Over time, it expanded its habitat southward, escaping its natural predators, particularly birds of prey. Under normal conditions, the population of common voles did not exceed 100 million; however, estimates for the summer of 2007 suggested their numbers had surged to at least 700 million. The voles affected approximately 500,000 hectares of crops, resulting in estimated losses of 15 million euros. Their voracity led to their characterization as a significant agricultural pest in Castile.
The plague affected the entire community of Castile and León, with the provinces of Valladolid, Segovia, Palencia, and Zamora experiencing the most severe impacts. The areas of Tierra de Campos and the region bordering Tierra de Medina were particularly affected, extending into the municipalities of Aliste and nearing the border with Portugal.
Causes
The 2007 vole plague was driven by several factors that led to the devastation of crops, particularly beets, potatoes, onions, and carrots. Despite the typically harsh winters in the Meseta region of the Iberian Peninsula, the winter of 2007 experienced unusually high temperatures, resulting in a significant reduction in frost occurrences. This, combined with a warmer-than-average spring, facilitated a population explosion of voles. These rodents mature rapidly, reproducing frequently with multiple offspring per litter and several litters each year, enabling their populations to grow swiftly.
Additionally, environmental groups filed a complaint with the Junta of Castile and León in March 2007 regarding the use of poison to combat the emerging vole population. The Young Farmers Association (Asociación de Jóvenes Agricultores, ASAJA) attributed the severity of the crisis to the administration and these environmental groups, claiming that the halting of preventive measures could have mitigated the outbreak's impact.
Voles are known to transmit diseases to humans, such as tularemia, through direct contact or via dust particles contaminated with their feces. The plague has been linked to 42 officially recognized cases in the region, according to the Junta of Castile and León, while the PSOE of Castile and León has suggested that the actual number may be as high as 270. Consequently, the regional government has advised against the consumption of game during the hunting season due to concerns over potential infection from this disease.
Extent of the vole by province Under typical conditions, the population density of common voles (Microtus arvalis) ranges from 5 to 10 individuals per hectare. However, during the vole plague in Castile and León, densities reached alarming levels, with reports of up to 1,500 rodents per hectare in the most affected areas.
The province of Palencia, where the outbreak originated, was the hardest hit, followed closely by Valladolid and Zamora. In contrast, the province of Soria experienced the least impact from the plague, indicating significant regional variability in the extent of the infestation.
Consequences of common vole infestations. From an ecological perspective, population explosions of common voles (Microtus arvalis) yield both beneficial and detrimental effects. Vole burrows can enhance soil fertility by increasing organic matter, such as buried vegetation, feces, and decomposing bodies. This activity improves soil aeration and creates a more spongy texture, which facilitates water filtration and promotes edaphological processes. Some researchers have noted that the abundance of voles in Castile and León over recent decades has contributed positively to the faunal diversity of the Douro Valley.
However, voles can also pose significant threats. They are known carriers of various diseases that affect both humans and other animals. Specific diseases associated with Microtus arvalis include viral diseases such as rabies and hantavirus, as well as bacterial diseases like leptospirosis (or Weil's disease), listeriosis, borreliosis (or Lyme disease), and tularemia. Additionally, voles host numerous parasites, including protozoa (babesiosis) to helminths (hydatidosis).The presence of abundant prey can support predator populations, offering ample food sources and alleviating predation pressure on other species. However, since vole population outbreaks tend to be short-lived compared to the life cycles of many predators, a rebound effect can occur when vole numbers decline.See the case of the Short-eared Owl (Asio flammeus). Some specialists assert that the impact of mammalian predators is more significant than that of migratory birds, with species like the weasel exhibiting population dynamics closely mirroring those of voles. In short, if the number of animals that need to feed on meat increases, eventually they will target other animals, sometimes attacking endangered species that are vulnerable or of particular interest to humans.
For humans, these population explosions are often viewed as harmful plagues, leading to various repercussions in the economy, public health, leisure activities, and societal dynamics. The situation can incite social alarm and political debate, with different ideological groups using the issue as a focal point for conflict.
Consequences for harvests The common vole (Microtus arvalis) is recognized as one of the most damaging vertebrates for agriculture across Europe, including Spain. This species is prevalent throughout the Northern Hemisphere, contributing to significant agricultural challenges. The primary diet of common voles consists of tender green stems, though they also consume leaves and remnants of crops such as corn. The 2007 vole plague is believed to have originated in Tierra de Campos, Palencia, and subsequently spread throughout the autonomous community of Castile and León . It has been estimated that more than 200,000 hectares were affected during this outbreak, with potential agricultural losses exceeding one million euros.During the sowing season, common voles create burrows in ditches and field borders to evade plowing. After sowing, they tend to move into the interior of arable land, particularly in cereal and pasture fields, leading to the formation of empty stands. Data from the United States indicate that a density of approximately 200 rodents per hectare can result in a 5% loss of alfalfa. This figure is likely exceeded during vole outbreaks in Castile, illustrating the potential scale of damage. In addition to cereals, common voles also feed on beets, typically consuming the tuber and causing rapid decay of the plant. They gnaw the stems of sunflowers, leading to their collapse, and have been observed feeding in vineyards. While they generally target tender shoots, damage to the base of vine shoots can adversely affect future harvests. Often, the full extent of the damage may not become evident until it is too late for corrective measures. Common voles also pose a threat to fruit trees by gnawing at the bark at the base of their trunks, which can weaken or destroy the trees.
Health consequences
Rodents comprise nearly half of all known mammalian species and are significant contributors to human health challenges, serving as a major natural reservoir for zoonotic pathogens—diseases that can be transmitted from animals to humans. Common voles (Microtus arvalis) are particularly active zoonotic agents, especially during population surges when contact with humans and domestic animals is more likely.One of the diseases that cause most social alarm is Tularemia.
Many risks associated with pathogen transmission from rodents are linked to recreational activities, which have often received less attention than economic impacts. Rodent plagues frequently coincide with holiday periods, exposing children and teenagers in rural areas to large populations of voles in parks, gardens, and urban green spaces. Despite their less aggressive nature compared to house mice or rats, common voles can create a misleading sense of safety, leading to increased interactions such as chasing or capturing them for amusement. This behavior poses significant risks, including potential exposure to serious diseases, as children may encounter decomposing vole carcasses in play areas, where parasites like fleas and ticks can thrive. Additionally, voles may drown in swimming pools or irrigation reservoirs, raising further health concerns.
Another associated risk involves small game hunting. The Junta of Castile and León has issued guidelines to mitigate issues related to rodent pests. Historically, these concerns were often dismissed as minor nuisances primarily affecting farmers.Recommendations of the Junta of Castile and Leon before the opening of the hunting season (In Spanish) However, incidents involving poisoned dogs or hunters contracting diseases like tularemia have raised alarm.Similar cases to those described in The lack of foresight from hunting organizations and authorities has led to increased tension and blame among stakeholders, highlighting the need for better management strategies regarding rodent populations and associated health risks.
Administrative consequences The inaction of the Junta of Castile and León in addressing the rapid expansion of the common vole plague prompted widespread protests among farmers. On August 2, thousands gathered in front of the Junta's headquarters in Valladolid to demand urgent solutions.
In the absence of timely administrative intervention, farmers took matters into their own hands. They began digging ditches filled with water to drown the voles, with Villalar de los Comuneros in Valladolid spearheading these efforts. The town’s mayor even devised a plow designed to destroy the rodents' burrows. However, these grassroots solutions proved insufficient in mitigating the plague's impact.
Institutional measures were implemented belatedly on August 9, by which time voles had already infiltrated urban areas in several localities. The first action taken was the burning of stover, initiated in Fresno el Viejo, Valladolid.
The plague's effects extended into the wine industry, causing an estimated 40% loss in the grape harvest, which compounded the anticipated losses in the livestock sector. As a result, many local mayors requested the declaration of a catastrophe zone to facilitate a more coordinated response to the crisis.
Cyclical plagues of common voles in Castile and León The phenomenon of common vole (Microtus arvalis) "plagues" was largely unknown in Spain until recent decades. Naturalist Juan Delibes de Castro noted in 1989 that, historically, vole populations in Spain were quite small, primarily located in specific mountainous areas. He remarked: