1. INTRODUCTION

According to World Health Organization (WHO), vector control strategies led to major reductions in malaria cases in many areas around the world (WHO 2018). The scale-up of two principal tools, long-lasting insecticidal nets (LLINs) and to a lesser extent indoor residual spraying (IRS), have contributed up to 33 % reduction in malaria deaths in Africa, the most affected continent, during the last decade (Hemingway 2014).
For impregnating LLINs, the WHO currently recommends six pyrethroids and for IRS, twelve insecticides, belonging to four classes of insecticides (pyrethroids, organophosphates, carbamates and organochlorines) were selected. Unfortunately, malaria vectors are developing resistance to insecticides used in these tools, mainly to pyrethroids. Resistance to this insecticide class is now widespread in Anopheles gambiaes.l. populations across sub-Saharan Africa (Dabiré et al. 2009, Ranson et al. 2011). Mutations in para -type voltage-gated sodium channel gene (Hemingway and Ranson 2000) providing high levels of cross-resistance to pyrethroids and DDT that share the voltage-gated sodium channel as a target site in mosquitoes that is known to be the main mechanism resistance.
In West Africa, pyrethroid resistance in Anopheles mosquitoes is mainly attributed to mutation in the sodium channel target site, the L1014F kdr (Dabiré et al. 2009) while another L1014S mutation originated from Kenya known as kdr -East (Ranson et al. 2000), was recently detected in the same region, in An. arabiensispopulations from Benin (Djègbè et al. 2011) and in An. gambiae s.s and An. coluzzii from Burkina Faso (Namountougou et al. 2013) as well as in An. gambiae s.s from Togo, West Africa (Djègbè et al. 2018).
Insecticide resistance in malaria vector populations in Burkina Faso has been reported as early as in the 1960s, when An. funestus andAn. gambiae s.l. (thereafter, An. gambiae s.s ) populations showed reduced mortality to dieldrin and DDT (Coz et al. 1968, Hamon et al. 1968). After, other studies have confirmed that resistance to DDT is still prevailing at a high level in An. gambiae s.l populations from Burkina Faso, where cross-resistance to pyrethroids due to thekdr L1014F mutation was also increasingly reported (Diabaté et al. 2002, Diabate et al. 2004, Dabiré et al. 2009). Moreover, cross-resistance to organophosphates and carbamates due toace -1R G119S was also detected, although at a moderate frequency (Weill et al. 2003, Djogbénou et al. 2008, Dabiré et al. 2009). Finally, in the humid savannas regions of South-West Burkina Faso, a complex context of multiple resistance is established inAn. gambiae s.l. populations (Dabiré et al. 2008) with the concomitant presence of kdr L1014F andace -1R mutation genes. This particularly alarming situation constitutes a major impendence to the success of current vector control strategies.
The use of the same chemicals (organophosphates and pyrethroids) in both agriculture and public health could lead to a more rapid development of resistance to these compounds (Gnankiné et al. 2013). It is known that the trend towards insecticide resistance in malaria vectors is partly linked to the current use of these chemicals for crop protection (Diabaté et al. 2002, Gnankiné et al. 2013, Hien et al. 2017, Chabi et al. 2018). According to Akogbéto et al. (2006), residues from insecticide applied on crops can contaminate mosquito-breeding sites, resulting in promoting insecticide resistance selection among mosquito larvae .
Indeed, the population of Africa has more than doubled over the past 30 years, increasing the demand for food and intensifying agricultural production to achieve food security (NEPAD 2013). According to the New Partnership for Africa’s Development (NEPAD), 530 million of the total African population (around 48 %) depend on agriculture activities and this population expected to reach 580 million in 2020 (NEPAD 2013). Agrochemical use has increased dramatically in recent years and has resulted in an important reduction in crop losses (NEPAD 2013). At the same time, the African continent has experienced the massive deployment of long-lasting insecticidal nets (LLINs), the most widespread intervention tool against malaria vector, and to a lesser extent indoor residual spraying (IRS). In fine, the massive use of insecticides in both agriculture and public health could establish a double pressure of resistance selection in mosquito vectors. Cotton production remains a crop that requires an important use of insecticides belonging to Pyrethroids (PY) and Organophosphates (OP). Up to now, three cotton pests management strategies have been adopted in some African countries, particularly South Africa and Burkina Faso: i) conventional based on systematic use of insecticides belonging to Pyrethroids, Organophosphates and neonicotinoids classes, ii) organic when no chemicals are used and iii) transgenic Bt-cotton (Bacillus thuringiensis ) when OP and PY are not used in fields, except neonicotinoids (Gnankine et al. 2018). Transgenic cotton (Btcotton) has been adopted as new agricultural practice that aimed at reducing the insecticides amount and their frequency of applications but also at increasing yields (Gnankine et al. 2018). Since 2016,Bt-cotton cultivation has suspended by the government of Burkina Faso due to the poor quality of cotton fiber.
With respect to the insecticide resistance increase in diseases vectors, WHO launched the Global Plan for Insecticide Resistance Management in malaria vectors (GPIRM) in 2012 (WHO 2012). GPIRM comprises of five activities (described as five ‘pillars’) spanning in the short, medium and long term, aimed at controlling insecticide resistance to ensure the continued effectiveness of current and future vector control tools to prevent malaria transmission, morbidity and mortality (WHO 2012).
The current study aimed at assessing the impact of the three cotton pest’s management strategies in different ecological settings; conventional, organic and transgenic cotton growing areas on insecticide susceptibility in the main malaria vector in Burkina Faso, West Africa.