A study carried out with two groups of mosquito larvae, one from the environment and the other from the laboratory, suggests that there is a slightly greater resistance for the larvae from the environment
Aedes aegypti mosquito larvae. By Marcos Santos, USP Journal/USP images.
A study recently carried out by researchers from the University of São Paulo (USP) has confirmed the effectiveness of bleach, which is a household cleaning product, in combating the larvae of Aedes aegypti, the dengue vector mosquito. The experiment, which was commissioned by Clorosur ( Latin American Chlor-Alkali and Derivatives Industry Association and Abiclor (the Brazilian Association of the Alkali, Chlorine and Derivatives Industry), used two groups of mosquito larvae, one of which originated from many generations reared in the laboratory at Fiocruz, and the other with larvae from mosquitoes born in the wild recently collected from the environment, where only two generations of larvae have been reared in the laboratory.
The study suggested a slightly higher resistance for the larvae of insects from the wild. The mortality rate of the group of larvae where a number of generations had been reared in the laboratory went up to as high as 95 per cent, while the mortality rate for the group of larvae from insects collected in the environment two generations previously did not go above 92.5 per cent. «The strain of larvae from insects in the field is more resistant because it is born into a hostile free environment,» explains Andre Machi, co-author of the study and a collaborator at the Radiobiology and Environment Laboratory (CENA/USP).
Even so, for both strains of insects, bleach – in proportions of 2 and 3 ml of product per liter of water – proved to be very effective in combating the pest during the larval stage, which takes place in containers of still water. This means that it achieved a mortality rate of more than 90 per cent.
«A mortality rate in excess of 90 per cent, which can go as high as 95 per cent, is regarded as excellent. This is in line with the results in the literature when working with insect control, as you can never eliminate 100 per cent of a pest in the field,» explains Valter Arthur, co-author of the study and senior lecturer at the Radiobiology and Environment Laboratory at CENA (USP).
How to use the product
Machi also stresses the benefits of using bleach to combat these larvae and other insects and organisms that develop in water: «Bleach should be used as an alternative method to control the dengue mosquito, not just because it is easily accessible for domestic use, but mainly because it is not a polluting resource and it doesn’t have an impact on the environment, unlike fumigation and the use of insecticides, which can cause insect resistance.»
It’s important to bear in mind that the action taken to combat the insect larvae depends on how long it was since the product was applied. The study showed that the bleach’s effectiveness decreased after five days, indicating that it is necessary to replenish the dosage of the product in the water containers.
For domestic purposes, the researchers recommend 1 dessert spoon of the product (equivalent to 5 ml) for each liter of water, with replacement every five days.
Quality of the bleach
According to Abiclor and Clorosur, in order to be effective in fighting mosquito larvae and for disinfecting and cleaning, the bleach should be purchased from reliable sources and pirated products should never be bought.
«Quality bleach contains 2.5 per cent active chlorine, which is the element that disinfects,» says Nelson Felipe, Clorosur/Abiclor’s Technical and Advocacy Officer. «It’s important to note that bleach should be sold in opaque packaging, as the impact of light can degrade the product and impair its effectiveness. Therefore, products sold in translucent PET bottles do not perform as well in terms of disinfecting and cleaning,» he adds.
Methodology
For the experiment, tap water was used, the pH of which was around 7.1. It was then placed in buckets with a five-liter capacity, together with the bleach, which was bought at a local store and was within its best-before date.
The product was added in the following concentrations: 1.0, 2.0 and 3.0 ml per liter of water, plus the control treatment which contained just tap water. For each dose used, four repetitions were made, each one containing 10 larvae, adding up to a total of 40 third and fourth instar larvae, which are the mosquito’s most resistant larval stages.
The experiment was carried out under ambient climate conditions, and all the treatments were given adequate quantities of fish feed. The number of dead larvae was assessed every 24 hours.
The study was carried out in order to update the results of a study that was carried out in 2016, and according to the mortality results, the control of the larvae was similar.