The 2007 vole plague began to develop at the beginning of the summer of 2006 in the Spanish autonomous community of Castile and León, more specifically in the province of Palencia. The plague became relevant from the summer of 2007 onwards when crops in the plateau fields were devastated by these rodents, which put special emphasis on irrigated crops. In September 2007, after a summer of severe losses, the density of rodents decreased throughout the community and the plague was institutionally considered to be over. However, there was an abundance of voles during the following months, being only the winter frosts and the low temperatures of November and December which helped to put an end to the situation.
The common vole (Microtus arvalis) was responsible for devastating the crops of the northern plateau. This Eurasian species had only penetrated the Iberian Peninsula as far as the Cantabrian Mountains, where it differentiated and became a subspecies called Microtus arvalis asturianus, beginning to extend its habitat to the south, thus freeing itself from its natural predators, the birds of prey. In normal years, their population did not exceed 100 million, but in the summer of 2007, it is estimated that they reached at least 700 million. They devastated a total of 500,000 hectares of crops and caused losses of 15 million euros. Their voracity led them to be described as the scourge of Castile.
This plague was present throughout the community of Castile and León, being the most affected the provinces of Valladolid, Segovia, Palencia and Zamora, especially in the areas of Tierra de Campos, and in the area bordering with Tierra de Medina, where other provinces such as Salamanca and Avila converge. It also came to be located in the municipalities of Aliste, on the verge of crossing into Portugal.
Causes
A number of reasons caused this plague of rodents, which ruined entire crops, especially beets, potatoes, onions and carrots. Despite the fact that winters in the Meseta are usually the coldest in the Iberian Peninsula, in 2007 temperatures were higher than usual, reducing the number of frosts to almost zero, which, together with the arrival of spring (also with slightly above-average temperatures), led to a population explosion of voles. These animals are characterized by reaching sexual maturity very quickly, reproducing at high speeds with several offspring per litter and several births per year, causing the explosion to occur without too much difficulty.
It has also been pointed out, as a cause of the proliferation of voles, the complaint filed by environmental groups before the Junta of Castile and León, in March 2007, against the use of poison that was being used to stop the incipient plague. The Young Farmers Association (Asociación de Jóvenes Agricultores, ASAJA) has blamed the administration and the environmental groups for the paralyzation of preventive measures which, in their opinion, could have prevented the high magnitude of the crisis.
Voles can transmit diseases to humans, such as tularemia, by direct contact or with dust particles that were in contact with the animal or its feces. The plague could be the cause of 42 cases in the region, officially recognized by the Junta of Castile and León. According to the PSOE of Castile and León, the number of cases could reach 270. In addition, the opening of the hunting season has led the regional executive to request that game should not be consumed for fear of possible infection by this disease.
Extent of the vole by province
Under normal conditions, the typical density of common voles is 5–10 individuals per hectare. During the plague in Castile and León, up to 1,500 rodents per hectare have been recorded in the most affected areas.
The province of Palencia, where the plague originated, is the province most affected by the spread of voles, followed by Valladolid and Zamora. On the other hand, the province of Soria has been the one where the plague has had the least impact.
Consequences of common vole infestations. From an ecological point of view, vole population explosions both detrimental and beneficial effects; for example, their burrows generate diverse benefits for the soil, growing its fertility by increasing subway organic matter (such as buried vegetation, feces, or their own decomposing corpses), they increase soil aeration and make it more spongy, as well as favor edaphological processes by stirring up the soil in their excavations and facilitating water filtration. Some researchers have even stated that "The abundance of voles in Castilla y León during the last decades has contributed to increasing the faunal diversity of the Douro Valley".