Fertiliser application promotes the proliferation of mosquitoes, notably in rice fields and food crops. The concomitant use of insecticides increases the risk of resistance. New strategies to combat these disease-transmitting insects are required.
What if the use of fertilisers promoted mosquito infestation? This has just been demonstrated by a study conducted on two species, Anopheles gambiae and Aedes aegypti1. “In domestic environments as well as rice fields and vegetable crops which develop in rural and peri-urban areas, a peak in the density of mosquito larvae is often observed a few days after fertiliser has been applied, explains medical entomologist Frédéric Darriet. This is why we took to the laboratory to compare the growth of A. gambiae and A. aegypti in artificial breeding sites, with or without fertiliser”.
Nurturing breeding sites
The result? Females, when looking for a water body to lay their eggs, are attracted to the major components of fertilisers: nitrogen, phosphorus and potassium. These components are conducive to the development of algae, fungi and bacteria that mosquito larvae feed on. “The females therefore prefer to lay their eggs in breeding sites with fertiliser. Their survival rate in these areas is two to three times higher than in breeding sites without fertiliser”, continues the researcher.
The other major finding of the study is that the larvae growth rate is two to four times higher in breeding sites containing fertiliser. This second result is most worrisome as, if the larva grows faster, it matures – and can therefore reproduce - before the breeding site dries up. Consequently, the addition of fertiliser is one of the factors responsible for mosquito infestation. This would also explain the spread of malaria, which is transmitted by anopheles, in previously unaffected regions. “In Madagascar, the Hauts-Plateaux region for example was free from this disease, Frédéric Darriet continues. After the planting of the first rice fields in the 17th century, the number of irrigated areas increased, along with fertiliser application. Malaria appeared one century later”.
How can this proliferation caused by fertilisers and detrimental to health be controlled? By integrating insecticides into fertilisers? In fact, farmers already use this type of product to combat the pests that jeopardise their crops. But as they are confronted with insecticides from such an early stage, mosquito larvae become increasingly resistant to them, and reach adulthood nonetheless. “We must provide rice farmers with new-generation insecticides to which mosquitoes have not yet developed resistance, Frédéric Darriet opines. They would help combat disease-transmitting mosquitoes as well as crop pests”.
1. Darriet F. Synergistic Effect of Fertilizer and Plant Material Combinations on the Development of Aedes aegypti (Diptera: Culicidae) and Anopheles gambiae (Diptera: Culicidae) Mosquitoes, Journal of Medical Entomology, 22 décembre 2017 ; doi: 10.1093/jme/tjx231
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