Insecticides used for vector control
Research into insecticides that can potentially be used for vector control
To be effective and sustainable, insecticide-based vector control must be based on the alternate use of active substances with various modes of action, in order to limit the development in these insect vectors of resistance to insecticides. To identify active substances potentially interesting for use in vector control, ANSES undertook extensive, coordinated research into the insecticides used for vector control, which led to 32 substances being identified. ANSES then selected a reduced number of adulticide or larvicide compounds that could potentially be used in the medium term and whose use would comply with the European regulations, and carried out risk assessments on four of them.
Primarily because of climate change and the globalisation of trade, vector-borne diseases have been reappearing over the last few years: malaria in French Guiana and Mayotte, dengue in the French départements in the Americas, Reunion Island and the Pacific region, and chikungunya in the Indian Ocean. The Zika virus also emerged in 2015 in South America, and then spread to Guadeloupe, French Guiana and Martinique. Since 2004, the Asian tiger mosquito, Aedes albopictus, vector of several of these diseases, has settled in metropolitan France, and imported or indigenous cases of dengue fever, chikungunya and Zika are regularly detected in metropolitan départements. In this context, and in the absence of a vaccine or treatment against these diseases, curbing their insect vectors, especially through the use of insecticides (biocides), remains the primary means of control.
With the entry into force of the European Biocides regulation, various formerly-used active substances are gradually being withdrawn from the market. The reasons for this include a toxicological or ecotoxicological profile that does not meet requirements, or sometimes an unwillingness by the manufacturer to support its products.
Currently, very few larvicidal and adulticidal active substances are used for vector control (VC) in France. Vector control programmes are primarily organised around an active substance targeting mosquito larvae (Bti) and an active substance targeting adult mosquitoes (deltamethrin). The widespread use of deltamethrin, without alternating between other active substances, has led to the emergence of proven resistance among mosquitoes in the overseas départements. Agricultural and veterinary use of this compound has aggravated this problem. A broader range of active substances is therefore vitally needed, not only in the short term but also in the medium term, because the fight against vectors of arboviruses and other pathogens seems likely to spread throughout the whole country. Although authorised by the European regulations under exceptional circumstances, the use of prohibited substances or products under an exemption no longer seems an appropriate, sustainable strategy.
ANSES’s work
In this context, ANSES undertook extensive, coordinated research into the insecticides used for vector control. This is a completely novel approach.
To do this, an inventory was produced of active insecticidal substances with reported or assumed potency on the Diptera order, biting insects (flies, mosquitoes, horseflies), and even mosquitoes, for some substances. 129 substances were thus identified. In order to identify substances with the greatest potential, these compounds were then compared according to criteria of toxicity, ecotoxicity and environmental contamination using the SIRIS method.
Through this work 32 substances were identified as potentially useful in the fight against mosquito larvae or adults.
The different insecticides identified from this analysis are potential candidates, whose selection mainly took into account the acute toxicity/ecotoxicity effects, which are currently the best documented. If the use of such substances were considered, it would be necessary before any use to conduct a comprehensive assessment of the risks and efficacy of these products.
In this context, ANSES undertook extensive, coordinated research into the insecticides used for vector control. To do this, an inventory was produced of active insecticidal substances with a reported or assumed effect on the Diptera order of biting insects (flies, mosquitoes, horseflies), as well as on mosquitoes, for some substances. It led to 129 substances being identified. In order to identify substances with the greatest potential, these compounds were then compared according to criteria of toxicity, ecotoxicity and environmental contamination.
In 2012, ANSES proposed a selection of 32 active substances on the basis of a multi-criteria analysis combining toxicity, ecotoxicity, exposure and environmental fate. These substances were grouped into three classes (use for the short, medium or long term). Then in 2016, the Agency published an updated list of active substances that can potentially be used in vector control, with regard to known uses and the substances’ regulatory status in Europe. This was not a risk assessment, but a list of active substances that may be of interest for use in vector control, on which efforts should be concentrated to encourage manufacturers and stimulate research. In its opinion, ANSES recommended developing efficacy trials and exposure models designed to estimate exposure for operators, the population and the environment, devoted to the highly specific uses involved in vector control, so as to carry out a detailed risk assessment and compare the different products.
ANSES then undertook work to assess the risks associated with the use of four of the active substances identified as potential alternatives to deltamethrin: bendiocarb, chlorpyriphos-methyl, dinotefuran and imidacloprid. In an opinion published in January 2017, the Agency concluded that three of the four substances assessed could be used as alternatives to deltamethrin, but their use would entail major constraints in terms of protective equipment. In addition, an assessment of the environmental risks showed that the risks to non-target organisms in the environment cannot be ruled out for these four substances. If any of these substances were to be used in vector control, measurements of the environmental impact should be carried out at the time of treatment.
ANSES reiterates that vector control should be rational and sustainable, based on surveillance of vectors and pathogens, and should seek to minimise adverse effects. Adulticide treatments are one means of combating the spread of mosquitoes that are vectors of disease, but cannot by themselves be regarded as the only solution for vector control. Sustainable effectiveness of active substances is achieved through a strategy that prevents the development of resistance in vectors, by alternating the active substances used. It should also be remembered that vector control is not only achieved with biocides and should be integrated, i.e. it should be combined with all the other methods and in a complementary manner.
Information campaigns on the importance of mechanical removal of breeding sites and use of personal protective measures are the preferred solutions. In the framework of biocidal control, larvicide treatments should also be prioritised to avoid the proliferation of mosquitoes. Adulticide treatments should be limited to ad hoc use intended to prevent the spread of the disease around contamination outbreaks in a well-defined framework, particularly with regard to the epidemiological context and local conditions.