Pests are becoming less and less seasonal: this is how rats, cockroaches, mosquitoes...

Being bitten by a mosquito in December or finding cockroaches in the street in January. These situations, more typical of the months of April and October, when temperatures are higher than in winter, could become increasingly common in the future.

Climate change and the increased comfort of indoor spaces already allow many pest species both to survive the winter season and to be more active than usual when the cold months arrive.

"Pests tend to be seasonal. They have their peak season in spring, summer and autumn because many of them are arthropods or insects and their body temperature depends on the temperature of the environment. Others are mammals, such as rats, and can be around all year round," says Jorge Galván, director of a sanitation company.

"Having said that, pests are becoming less and less seasonal. Although a few years ago there was a very important decrease in the winter months, we have realized that now many pests that were seasonal are more common at any time of the year," explains Galván, who blames it on "climate change and the air conditioning of the places": "This makes them able to move, feed and reproduce at any time of the year, although in winter there is a decrease in activity.

"More and more pests are on the move. Habitats that were once unsuitable are now more so."

But climate change not only causes some species to survive longer and extend their time of proliferation and maturation: it also allows them to extend their habitat to areas that were not before.

"There are pests that are more characteristic of one area than another, but this characterization is not fixed. More and more pests are moving with the change in temperatures. Habitats that were not suitable before are now more so. And places where they did not appear, appear," says the head of Anecpla.

Another element to understand the expansion of pests is globalization. The constant world movement of people and goods favours many species to move easily from one area to another.

The Asian wasp dominates the Cantabrian Coast

Many pests (rats, processionary caterpillars, cockroaches, mosquitoes, bedbugs...) are present throughout Spain, although they may be more important in some areas than in others (see graph). For example, the American cockroach (periplaneta americana) is present throughout Spain, although it is more prevalent in the south and east of the peninsula due to the heat and humidity. Something similar happens with the termites (isoptera), more abundant in the north and east of the peninsula.

Others, however, are specific to certain areas. This is the case of the Asian wasp (vespa velutina), established in the Cantabrian coast, especially in Galicia and León; or the tiger mosquito (aedes albopictus), which dominates the entire Levant peninsular; as well as the black fly (simulium erytrocephalum), which in addition to the Levantine coast is found in Madrid and parts of Andalusia. The common tick (ixodes ricinus), on the other hand, covers the entire territory except for the Levant and the Andalusian provinces of Seville and Cordoba.

Concern about mosquitoes

Asked which is the most relevant pest at the moment, Galván recognizes that it is "complicated" to speak of importance. He points out that "rats and cockroaches are always under constant control because they tend to transmit diseases" but that at present there is great concern about mosquitoes, present throughout the national territory, although with greater prominence in coastal areas.

"They have a relevant value in public health because they are important vectors (a living organism that transmits a virus from an infected animal to a human being or another animal) of diseases. It is now very easy for an insect to get anywhere and a virus can also get anywhere because of the way we live," says Galván.

An example of transmission of an infectious agent to a person is the so-called Nile virus. Infection occurs as a result of the bite of a mosquito, usually the common or trumpeter mosquito (Culex pipiens), which has previously fed on an infected bird during migration between Europe and Africa, where this virus is endemic.

When is a pest of concern?

At present, no pest in Spain is in a worrying process of expansion. Not even the black rat, which has proliferated in recent months in several Spanish cities such as Madrid. "They are outbreaks that are localized and with a controlled population," says Galván. 

"To determine that we have to act on a pest, we use what is called the tolerance threshold, which is set by man. This is the level above which action must be taken, but it is relative and depends on each area. For example, the tolerance threshold for cockroaches in an operating room is zero, but on a farm it can be four or ten," he explains.

This environmental health expert clarifies, however, that "pests are associated with a lack of hygiene". "The whole problem is ours. All pests are synanthropic species, which are those associated with the human environment. If I leave the garbage outside the container because it is full or I feed stray animals and then do not pick up the remains, I am favoring the appearance of pests. Pests are always linked to humans. Maintaining hygiene is essential to curb their appearance," he concludes.