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      Efficacy of the mermithid nematode, Romanomermis iyengari, for the biocontrol of Anopheles gambiae, the major malaria vector in sub-Saharan Africa

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          The intensive use of chemical insecticides against mosquitoes has led to the development of widespread insecticide resistance. Control of Anopheles mosquitoes in malaria endemic areas of sub-Saharan Africa has become increasingly difficult. There is an urgent need for malaria control programmes to adopt more integrated mosquito management approaches that include sustainable, nonchemical solutions. The mermithid nematode Romanomermis iyengari is one of several natural control alternatives to synthetic pesticides for mosquito suppression. This study evaluated the effectiveness of the nematode R. iyengari for control of Anopheles gambiae.


          The nematode R. iyengari was mass-produced, and pre-parasitic stage (J2) were used for laboratory and field experiments. In laboratory experiments, two concentrations of pre-parasitics (5 and 10 J2 per larva) were tested against first- (L1), second- (L2) and third-instar (L3) larvae of An. gambiae. Infected larvae were observed daily to determine their mortality rate and the number of post-parasitic nematodes emerging from dead larvae. In field experiments, 3500, 4000 and 5000 J2/m 2 were sprayed in separate natural Anopheles breeding sites. After treatment, the larval mosquito density in the breeding sites was assessed every 5–7 days.


          Laboratory results showed that larval An. gambiae is susceptible to nematode infection: 100% L1 larvae died within 24 hours post-treatment, and 100% of both L2 and L3 larvae died within 7 days, regardless of nematode concentrations. The average number of post-parasitic nematodes emerging per larva increased with increasing nematode concentration. In field experiments, the monthly applications of 3500 to 5000 pre-parasitic nematodes per m 2 eliminated larval mosquito development in Anopheles- and mixed breeding sites. Larval mosquito density dramatically decreased five days after the first treatment in all treated sites and was maintained at a very low level during the whole experimental period. Basically, only early instar larva were detected in treated sites throughout the test period. The average number of post-parasitic nematodes emerging per larva collected in treated sites was 1.45, 2, and 5.7 respectively for sites treated with 3500, 4000, and 5000 J2/m 2.


          Malaria mosquito larvae is susceptible to R. iyengari infection in West Africa. Parasitism intensity depends on tested nematode concentrations. Monthly application of 3500 J2/ m 2 was enough to control effectively larval An. gambiae in wetlands and floodable locations in West Africa.

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          Insecticide resistance in the Anopheles gambiae complex in Benin: a nationwide survey.

          Benin has embraced World Health Organization-recommended preventive strategies to control malaria. Its National Malaria Control Programme is implementing and/or coordinating various actions and conducting evaluation trials of mosquito control strategies. Mosquito control is based on the use of insecticide-treated nets and indoor residual spraying, but the efficacy of these strategies to control malaria vectors is endangered by insecticide resistance. Here, we present the results of a nationwide survey on the status of insecticide susceptibility and resistance in Anopheles gambiae s.l. (Diptera: Culicidae) carried out in Benin in 2006-2007 (i.e. before extensive vector control was undertaken). Overall, our study showed that the S molecular form of An. gambiae s.s. predominates and is widely distributed across the country, whereas the frequency of the M form shows a strong decline with increasing latitude. Susceptibility to DDT, permethrin, carbosulfan and chlorpyrifos-methyl was assessed; individual mosquitoes were identified for species and molecular forms, and genotyped for the kdr and ace-1 loci. Full susceptibility to chlorpyrifos-methyl was recorded and very few samples displayed resistance to carbosulfan. High resistance levels to permethrin were detected in most samples and almost all samples displayed resistance to DDT. The kdr-Leu-Phe mutation was present in all localities and in both molecular forms of An. gambiae s.s. Furthermore, the ace-1(R) mutation was predominant in the S form, but absent from the M form. By contrast, no target modification was observed in Anopheles arabiensis. Resistance in the An. gambiae S molecular form in this study seemed to be associated with agricultural practices. Our study showed important geographic variations which must be taken into account in the vector control strategies that will be applied in different regions of Benin. It also emphasizes the need to regularly monitor insecticide resistance across the country and to adapt measures to manage resistance. © 2010 The Authors. Medical and Veterinary Entomology © 2010 The Royal Entomological Society.
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            The future of microbial insecticides as vector control agents.

            Insect vectors of human diseases are subject to diseases of their own caused by viruses, bacteria, fungi, protozoans, and nematodes. Over the past 30 years, many members of these groups have been evaluated as vector control agents, particularly for mosquito control. Most pathogens and nematodes occur primarily in larvae, and are only effective against this stage. The principal candidate control agents studied include iridescent and nuclear polyhedrosis viruses, the bacteria Bacillus thuringiensis and Bacillus sphaericus, the fungi Lagenidium giganteum, Culicinomyces clavosporus, and species of the genus Coelomomyces, the protozoan Nosema algerae, and the mermithid nematode Romanomermis culicivorax. Of these, the only one considered an operational success is the bacterium, Bacillus thuringiensis subsp. israelensis (B.t.i.), which has proven useful for control of both mosquito and blackfly larvae in programs where larviciding has been traditionally employed as a vector control tactic. The reasons for the success of B.t.i. are its cost-effectiveness and relative ease of use, which are due, respectively, to the ability of B.t.i. to be grown on artificial media and the development of formulations that can be applied using conventional insecticide application technology. Because few microbial insecticides are cost-effective, and those that are are only effective against larvae, these agents will likely play only a minor, but in some cases important, role in most future vector control programs.
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              Nematodes as Biological Control Agents: Part II


                Author and article information

                Parasit Vectors
                Parasit Vectors
                Parasites & Vectors
                BioMed Central (London )
                22 May 2019
                22 May 2019
                : 12
                [1 ]ISNI 0000 0001 0382 0205, GRID grid.412037.3, Laboratoire d’Entomology Appliquée/Centre Edward Platzer, , Université d’Abomey-Calavi, ; BP:215, Godomey, Benin
                [2 ]ISNI 0000 0001 2165 8782, GRID grid.418275.d, CIIDIR Oaxaca, Instituto Politécnico Nacional, ; Xoxocotlan, Oaxaca, 71230 Mexico
                [3 ]ISNI 0000 0001 2222 1582, GRID grid.266097.c, Department of Nematology, , University of California, ; Riverside, CA 92521-0415 USA
                © The Author(s) 2019

                Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

                Funded by: FundRef http://dx.doi.org/10.13039/501100004828, Grand Challenges Canada;
                Award ID: 0122-01
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