Despite intense national and international efforts, malaria remains one of the major tropical challenges in the world today . In Benin, the primary tools for malaria vector control are long-lasting insecticidal nets (LLIN) and indoor residual spraying (IRS). However, insecticide resistance development in vector populations could impede the success of malaria control programmes in endemic areas. In West Africa, the resistance of Anopheles gambiae s.l. to the four major classes of insecticides available for public health has been reported [2–4].
Pyrethroids are the only option for net treatment due to their relative safety for humans at low dosage, excito-repellent properties, rapid rate of knock-down and killing effects .
Resistance to this insecticide class is now widespread in the main malaria vectors An. gambiae s.l, Anopheles arabiensis and Anopheles funestus [6–9]. Both enhanced detoxification [10, 11] and mutations in the gene encoding the voltage-gated sodium channel  have been shown to be important resistance mechanisms. The Leucine to Phenylalanine substitution at position 1014 (L1014F) was found predominant in West and central Africa [7, 13] whereas the Leucine to Serine substitution (L1014S), originated from Kenya , has now spread in the central region including Cameroon [15, 16], Equatorial Guinea , Gabon , Angola , Uganda  and Ethiopia .
The impact of this resistance on pyrethroid-based control is largely unquantified. Until recently, pyrethroid resistance based on kdr mutation or metabolic mechanisms in An. gambiae in Côte d'Ivoire [20, 21] and Kenya [22, 23], did not adversely affect the efficacy of pyrethroid-treated nets. However, a longitudinal survey recently conducted in southern Benin showed neither reduction of asymptomatic infection nor malaria attack by the use of LLINs in an area of pyrethroid resistance .
In Benin recent (2006-2007) entomological surveys reported cross-resistance to DDT and pyrethroids in An. gambiae s.l. with strong geographic variations in a south-north transect [13, 25]. Molecular studies showed the presence of the kdr L1014F mutation and an overexpression of two P450 genes (CYP6M2 & CYP6P3) potentially involved in pyrethroid resistance . The presence of a single point mutation (glycine to serine at position 119) in the oxyanion hole of the acetylcholinesterase enzyme [3, 27, 28] conferring resistance to carbamates and to a lesser extend to organophosphates was also detected in Burkina-Faso, Côte d'Ivoire and Benin [13, 29, 30].
The intense use of DDT in agricultural settings and during the WHO malaria eradication programme in the 1950s and 1960s were suspected to be the main factors selecting for pyrethroids and DDT resistance in An. gambiae populations . Various insecticidal products (organophosphates, pyrethroids, etc.) are also used for crop protection but the amount applied is far higher than that consumed in public health against malaria vectors . Benin is still an important producer of cotton in West Africa and 90% of pesticide products are directed against cotton pests [32, 33]. Small-scale vegetable farming is an important source of livelihood in urban and peri-urban environments  and provides income and food for tens of thousands of families . Akogbéto et al [31, 36] reported that mosquito species, An. gambiae in particular, lay their eggs in breeding sites located around agricultural settings. These eggs undergo a selection pressure from agricultural pesticides, which leads to the emergence of resistant populations of An. gambiae, thereby impeding malaria vector control operations.
The Beninese National Malaria Control Programme received financial and technical support from World Bank, Global fund and WHO to implement large-scale and free distribution of LLIN since 2007 . Several authors have studied the effect of insecticide treated nets (ITNs) with pyrethroids on An. gambiae populations and the possible selection of kdr alleles either in laboratory experiments  or experimental huts trials  or in the field [40, 41]. Increasing resistance of malaria vectors may have important implications for vector control programmes, especially considering the scaling up of LLINs and IRS in Africa. Hence, knowledge on spatio-temporal changes in insecticide resistance level is a basic requirement to guide the use of insecticides in malaria control programmes.
In this study, the dynamic of insecticide resistance was evaluated in An. gambiae populations collected in four sentinel sites selected on the basis of different agricultural practices, use of insecticides and environment (urban/rural areas). From 2008 to 2010, temporal changes in insecticide resistance level, sibling species among An. gambiae s.l, enzymatic activity and frequency of resistant alleles were measured twice per year through a combination of insecticide bioassay, biochemical and molecular techniques.