[Animals] Mosquitoes increase their resistance to the most common insecticides in Venezuela

[WilkerCSBD]

The fight against mosquitoes and any other insects treated as pests is complicated. Several studies have detected that these animals have increased their resistance to the usual chemical insecticides to extend the immune po[CENSORED]tion to entire areas, as reflected in the latest research published in PLOS Biology, and that this survival is generalized in many of the more than 3,500 species of mosquitoes of the world. In the common (Culex pipiens) resistance has been registered to the four types of insecticides currently authorized and even to DDT, banned in Spain and in a hundred countries due to its toxicity, but which the World Health Organization (WHO) allows in case of epidemic outbreaks.

This resistance is developed by the widespread use of chemical compounds to combat insect pests, which are transmitters of pathogens that cause global diseases such as Zika, dengue (six cases were detected in Spain in 2018), chikungunya, West Nile virus fever or malaria. “It is a genetic mechanism. If a certain biocide is used, the most resistant specimens survive, which are the ones that reproduce and transfer that capacity to new generations ”, explains Santiago Ruiz, biologist in charge of the Mosquito Control Service of the Huelva Provincial Council and one of the leading experts in this field in Spain.

The most common insecticides are pyrethroids (permethrin, deltamethrin, and cyhalothrin). They are synthetic molecules based on pyrethrin, a natural compound that is modified to infer particular characteristics, such as greater resistance to light. It is present in most domestic and industrial insecticides to kill adult specimens, and among its advantages, according to Ruiz, is its safety when used in areas with vertebrates. But carbamates (bendiocarb and propoxur), organophosphates (pirimiphos-methyl) and organochlorines (DDT) are also used.

Most insects treated as pests develop resistance
SANTIAGO RUIZ, BIOLOGIST RESPONSIBLE FOR THE MOSQUITO CONTROL SERVICE OF THE COUNCIL OF HUELVA
"Most insects treated as pests develop resistance," confirms Ruiz. An investigation carried out during five years in Spain and published in the journal PLOS ONE, showed that pyrethroids, although they are effective against the tiger mosquito, are no longer effective against the common mosquito (Culex pipiens), capable of resisting the most common insecticides and even the banned DDT.

The ability of insects, as reflected in the PLOS Biology study, occurs in several ways. The mutations in the species to survive the compounds include from the thickening of the cuticle (the outer layer of organic tissue that covers the body) so that the pesticides do not penetrate to the increase in the generation of detoxifying enzymes that biodegrade the insecticides.

At present, new generations of pyrethroids are marketed that are combined with regulators of the growth of insects or inhibitors of the metabolism that makes the mosquito resistant. "But the selection mechanism that favors the resistant ones continues to act, as it also happens with antibiotics", warns Jordi Figuerola, deputy director of Research at the Doñana Biological Station and leader of a CIBER group (Consorcio de Investigación Biomédica en Red) from Epidemiology and Public Health).

Mutations to survive the compounds range from thickening of the cuticle to increased generation of detoxifying enzymes that biodegrade insecticides
"The product portfolio is shrinking and you have to develop new effective methods or extend the useful life of existing ones," he adds. In this sense, Figuerola indicates investigations that are directed to the sterilization of males by radiation or by means of the genetic modification of the specimens so that the progeny are not viable.

"But the ideal strategy is to reduce the breeding areas", highlights the researcher after noting that "the remains that remain on a plate under a pot after watering it, an abandoned pool or the bucket that collects the air conditioning liquid can be breeding sites for mosquitoes ”.

Martina Ferraguti, a biologist trained in Rome who works at the University of Extremadura after receiving awards for her research at the Pablo de Olavide University in Seville and the Doñana Biological Station, advocates understanding how the environment and human activities influence distribution and abundance of mosquitoes to mitigate the diseases they cause.
 

The biologist points out that, given the resistance of mosquitoes to insecticides, other forms of control have been developed, such as Bacillus thuringiensis israelensis, a bacterium of natural origin known by the acronym BTI that is found in soils and whose spores produce toxins that attack mosquito larvae, black fly and fungus gnat.

It is the system applied by the Ruiz team in Huelva, but with preventive measures to identify if the mosquito also develops resistance to this bacterium. “Every year we do tests and we still haven't detected it. However, we alternate biocides so that the pressure on the pest is not done in one direction ”, she warns.

The method is effective, although not in 100% of the areas or in the entire po[CENSORED]tion, so the control of adults is carried out with pyrethroids and the resistance mechanism, complemented by its generalized domestic use, continues to work.

The mosquito is part of the natural system as a food source or part of the decomposition process of organic matter
JORDI FIGUEROLA, VICE DIRECTOR OF RESEARCH AT THE DOÑANA BIOLOGICAL STATION
Ferraguti also recalls the enormous impact on the environment of chemical products. The following warning can be read on most anti-mosquito or household insecticides: “Very toxic to aquatic organisms, with long-lasting harmful effects. To avoid it's releasing into the environment". "They also have serious effects on birds," adds the biologist.

In her opinion and in Figuerola's, the best insecticides are natural ones, such as bats, birds (such as swallows and airplanes) or spiders; those using BTI, which are safe for people, and human behavior. "If we do not favor their habitat by leaving traces of water or we reverse global warming, for example, the po[CENSORED]tions of these insects will decrease," she concludes.

In addition, according to the Doñana researcher, we must not forget that the mosquito is part of the natural system as a source of food or part of the process of decomposition of organic matter through the larvae. Therefore, extinction is not an alternative and a balance must be found.