Ecological alternatives to mosquito control in Singapore

Mosquito control has intensified in Singapore over the recent months to contain the current Dengue outbreak. Although they still rely heavily on chemical insecticides, strategies are slowly evolving towards greener alternatives.

There are 80 different types of mosquitoes in Singapore, distributed among the genera Aedes, Culex and Anopheles. These insects are potential vectors of pathogens that cause infectious diseases such as Dengue, Zika, Chikungunya, Yellow fever, Japanese encephalitis, Malaria, Lymphatic filariasis and Elephantiasis. Dengue fever, transmitted by Aedes aegypti and Aedes albopictus, is considered endemic to Singapore. Because of its stronger vectorial capacity, efforts are generally focused on controlling the life cycle of A.aegypti. Mosquito development takes place in 4 phases: egg, larva, nymph, adult. The first 3 stages are aquatic. The complete metamorphosis takes 7 to 14 days. The female is the only adult biting. In Singapore, mosquito control is mainly based on the spread of chemical or biological insecticides (Bacillus thurigiensis), associated with many problems: appearance of resistant mosquitoes, toxicity to other non-harmful insects, limited access to A.aegypti who tends to rest sheltered in houses. The development of more efficient and more ecological alternative techniques therefore represents a major challenge for the National Environment Agency (NEA) and the Environmental Health Institute (EHI) today. It is in response to this need that ecological traps (Gravitraps) are dispersed in Singapore, as well as sterile male mosquitoes (A.aegypti-Wolbachia). At the individual level, ecological mosquito control can also involve the use of repellents based on essential oils. In addition, these plant extracts, as well as nanoparticles, constitute new alternatives to synthetic insecticides.

Fogging

The most popular mosquito control methods in Singapore

Adulticide treatments consist of treating adult mosquitoes’ resting or activity areas. Thermal fogging and misting are the most common methods used in Singapore. Thermal fogging consists of spraying a diesel-based insecticide mixture. This operation, which generates a smelly white smoke, can treat a large area in a very short time. Diesel can be replaced with water and a misting agent, leaving no oily residue on plants or surfaces. Water being heavier than smoke, the treated surface is narrower. Thermal fogging takes place preferably at dawn or dusk, when the Aedes are most active. Its effectiveness is limited at the time of application, mosquitoes tending to return once the cloud has dissipated. The second technique, misting, involves spreading an insecticide using an Ultra Low Volume (ULV) Misting. It aims to deposit an insecticidal residue in places where mosquitoes are used to resting. The device diffuses very fine droplets which very quickly eliminate mosquitoes coming into contact with. This odourless, stain-free treatment can also be performed indoors. Its persistence lasts from several days to 1 month. However, in addition to the harmful impact on the ecosystem, as well as on humans (e.g. respiratory damage), the repeated application of chemical insecticides tends to favour the emergence of resistant mosquitoes.

A. aegypti aquatic larvae

Bti, a larvicide of biological origin

Larvicidal treatments generally involve substances such as organophosphates (e.g. Temephos), growth regulators (stop metamorphosis) or microbial control agents. In recent years, the use of Bacillus thuringiensis as an ecological alternative has turned popular. The subspecies in use in Singapore is Bacillus thuringiensis Israelensis (Bti). Its toxicity for many insects is linked to the ability of this bacterium to sporulate while producing toxic protein crystals. Spores and toxins ingested by the larvae are activated by the insects’ digestive enzymes, allowing rapid infection of their entire organism, eventually leading to their death. Bti is available in liquid or solid form (tablets, briquettes, pellets). Liquid treatment can be spread by ULV misting. The use of Bti is recommended for places including fish, aquatic organisms, plants and pets. This treatment nevertheless has 2 major drawbacks, due to its toxicity for many non-target insects (e.g. beetles, lepidoptera) and the appearance of resistant mosquitoes.

The use of female mosquito traps

Some traps attract insects by emitting molecules released naturally by humans such as carbon dioxide (CO2), lactic acid or octenol. Others reproduce human warmth, UV rays recognized from mosquitoes or female mosquito synthetic pheromones. Indeed, females tend to lay eggs where other congeners have already left theirs (and their pheromones). Other devices involve attracting pregnant females into a trap containing a larvicidal mixture. Once the eggs are laid in the receptacle, the females leave covered with active substances. These block the virus replication, weaken the females by reducing their ability to bite, and kill them in 7 to 14 days. Over this time gap, they contaminate the other places visited, thus eliminating the larvae present on the breeding sites. As the insecticide mixture is toxic to aquatic organisms and other insects, the use of this type of trap is supervised by professionals. The Gravitraps developed by the EHI represent an ecological alternative. These devices contain a hay infusion, covered with a fine mesh net and a sticky substance. The females are attracted by the device’s dark colour and the smell of hay. Once the layings have been done, they are trapped in the Gravitrap. The net helps prevent the release of mosquitoes from the laid eggs. To date, the NEA regularly monitors and renews more than 64,000 Gravitraps through the City-State.

Gravitrap (source: NEA)

Wolbachia, a bacterium at the service of mosquito control

Since 2016, Singapore has implemented an in natura introduction program of A.aegypti males carrying a Wolbachia genus bacteria. By acting in competition with the Dengue virus, Wolbachia blocks its replication in the mosquito. Furthermore, by infecting the cells of the mosquito's sexual organs, it renders males sterile. Mating between an A. aegypti - Wolbachia male and a "urban" A. aegyti female produces eggs that do not hatch. But the opposite is not true. An A. aegypti - Wolbachia female will produce viable offspring infected with Wolbachia, whether they mate with an A. aegypti - Wolbachia or "urban" male. It is therefore preferable to release only A.aegypti - Wolbachia males, so as to favour mixed matings and the absence of offspring. In 2012, NEA and EHI demonstrated the ability of Wolbachia males to compete with "urban" males, allowing a 50% reduction of A.aegypti populations in the release areas. The opening of a new production unit at Techplace II, Ang Mo Kio, in 2019, has made it possible to increase the production of A. aegypti Wolbachia males. This facility now includes a low-dose X-ray irradiation step, to sterilize females inadvertently released alongside males. Irradiated insects are not radioactive. Since Wolbachia is transmitted to eggs by injection, mosquitos do not undergo genetic manipulation. The bacterial strain is harmless to humans and the environment. And finally, male mosquitoes are harmless to humans because they are strictly vegetarian.

Essential oils with repellent properties

Personal protection remains essential to prevent the transmission of Dengue and other vector-borne diseases. It can be carried out using synthetic products or natural essential oils. The most commonly used insect repellent is N, N-diethyl-3-methylbenzamide (DEET). Although very effective, this substance can cause mucosal irritations and other toxicities. Plant extracts, especially essential oils, offer healthy alternatives to chemical insect repellents. Combining several extracts together causes synergies increasing the insect repellent effect. Adding 5% vanillin (fixative) to the mixture increases its shelf life. Since essential oils are composed of volatile substances, their application must be renewed frequently during the day. Essential oils can potentially cause skin irritation, varying in intensity from one plant species to another. They should be applied at a distance from mucous membranes, especially the eyes. If using a photosensitizing oil (e.g. Lemon (citrus x limon)), any sun exposure of the treated parts should be avoided. For a skin application, it is preferable to dilute the essential oil to 5% in vegetable oil beforehand. In the event of a skin reaction, protection can be carried out via treated clothing by spraying essential oils diluted in a hydroalcoholic solution. Wearing clear, long-sleeved clothing and installing mosquito nets is also recommended.

Examples of A. aegypti repelling essential oils

Neem vegetable oil (Azadirachta indica) is also a good insect repellent. It is to be diluted to 10% in another vegetable oil. It can act in synergy with essential oils.

Ceylon cinnamon (Cinnamomum verum); Neem (Neem) (Azadirachta indica); Lemongrass (Cymbopogon flexuosus)

Other alternatives to synthetic insecticides

The research subjects dedicated to ecological mosquito control are extremely numerous. Due to the ubiquitous nature of mosquitoes vectors of infectious diseases, this work is now deployed on all continents. Today, plant extracts such as essential oils or nanoparticles produced from plants (e.g. Silver AgNp), stand on top position in the development of insecticides alternatives. These substances have already shown very promising ovicidal and larvicidal effects, at very low dosages. Nanoparticles also exert a triggering effect on mosquito larvae predators, such as fish, amphibians and copepods. However, the environmental impact of these new control agents remains to be specified. Indeed, the undesirable effects of nanoparticles on non-target organisms (e.g. larvae of other species), as well as their residues in the environment, still raise many questions. Otherwise, a whole other area of ​​research is currently focusing on developing genetically modified A.aegypti mosquitoes.

For the NEA recommendations on mosquito prevention, click here.

French version published on lepetitjournal.com/singapour, on July 13th, 2020.

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