Abstract
BackgroundMosquitoes and other vectors are often exposed to sublethal doses of insecticides. Larvae can be exposed to the run-off of agricultural use, and adults can be irritated by insecticides used against them and move away before they have picked up a lethal dose. This sublethal exposure may affect the success of control of insect-borne diseases, for it may affect the competence of insects to transmit parasites, in particular if the insects are undernourished.MethodsWe assessed how exposure of larvae and adults to a sublethal dose of permethrin (a pyrethroid) and how larval competition for food affect several aspects of the vector competence of the mosquito Anopheles gambiae for the malaria parasite Plasmodium berghei. We infected mosquitoes with P. berghei and measured the longevity and the prevalence and intensity of infection to test for an effect of our treatments.ResultsOur general result was that the exposure to the insecticide helped mosquitoes deal with infection by malaria. Exposure of either larvae or adults decreased the likelihood that mosquitoes were infected by about 20%, but did not effect the parasite load. Exposure also increased the lifespan of infected mosquitoes, but only if they had been reared in competition. Larval competition had no effect on the prevalence of infection, but increased parasite load. These effects may be a consequence of the machinery governing oxidative stress, which underlies the responses of mosquitoes to insecticides, to food stress and to parasites.ConclusionsWe conclude that insecticide residues are likely to affect the ability of mosquitoes to carry and transmit pathogens such as malaria, irrespective of the stage at which they are exposed to the insecticide. Our results stress the need for further studies to consider sublethal doses in the context of vector ecology and vector-borne disease epidemiology.
Highlights
Mosquitoes and other vectors are often exposed to sublethal doses of insecticides
E.g. food limitation for larvae and exposure of insecticides to larvae and adults, may often co-occur in nature, we investigated how their combination would affect the development of malaria in adult mosquitoes, using the mosquito Anopheles gambiae (s.s.), a pyrethroid insecticide, and the rodent malaria parasite Plasmodium berghei
Determination of sublethal dose of permethrin To find a sublethal concentration of insecticide which is relevant in nature, we evaluated the effects of three concentrations (0.1, 0.15 and 0.2 μg/l), which are close to the median concentration of permethrin found in surface waters of agricultural areas (0.04 μg/l (25th percentile: 0.01 μg/l, 75th percentile: 0.31 μg/l)) [35]
Summary
Larvae can be exposed to the run-off of agricultural use, and adults can be irritated by insecticides used against them and move away before they have picked up a lethal dose This sublethal exposure may affect the success of control of insectborne diseases, for it may affect the competence of insects to transmit parasites, in particular if the insects are undernourished. The insecticides that are used in agriculture can runoff and end up at sublethal concentrations in surface or ground water, where they can affect many animals, including aquatic arthropods such as mosquito larvae [2] Such sublethal concentrations can increase oxidative stress [3, 4], modulate the immune responses [5], and increase the activity of detoxifying enzymes [6,7,8] and the metabolic cost associated with detoxification. Exposing adult mosquitoes to a low concentration of pyrethroid impedes the development of malaria parasites [13,14,15], and the effects of exposing larvae can carry over to adults to influence their vector competence (that is, their ability to acquire, maintain and transmit parasites [16]) for arboviruses [17, 18] and malaria [19, 20]
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