Larvicidal properties of terpenoid-based nanoemulsions against the dengue vector Aedes aegypti L. and their potential toxicity against non-target organism.

The development of insecticide resistance in mosquitoes of public health importance has encouraged extensive research into innovative vector control methods. Terpenes are the largest among Plants Secondary Metabolites and have been increasingly studied for their potential as insecticidal control agents. Although promising, terpenes are insoluble in water, and they show low residual life which limits their application for vector control. In this study, we developed and evaluated the performances of terpenoid-based nanoemulsions (TNEs) containing myrcene and p-cymene against the dengue vector Aedes aegypti and investigated their potential toxicity against non-target organisms. Our results showed that myrcene and p-cymene showed moderate larvicidal activity against mosquito larvae compared to temephos an organophosphate widely used for mosquito control. However, we showed similar efficacy of TNEs against both susceptible and highly insecticide-resistant mosquitoes from French Guyana, hence suggesting an absence of cross-resistance with conventional insecticides. We also showed that TNEs remained effective for up to 45 days in laboratory conditions. The exposure of zebrafish to TNEs triggered behavioral changes in the fish at high doses but they did not alter the normal functioning of zebrafish organs, suggesting a good tolerability of non-target organisms to these molecules. Overall, this study provides new insights into the insecticidal properties and toxicity of terpenes and terpenoid-based formulations and confirms that TNE may offer interesting prospects for mosquito control as part of integrated vector management.

A new WHO bottle bioassay method to assess the susceptibility of mosquito vectors to public health insecticides: results from a WHO-coordinated multi-centre study.

BACKGROUND: The continued spread of insecticide resistance in mosquito vectors of malaria and arboviral diseases may lead to operational failure of insecticide-based interventions if resistance is not monitored and managed efficiently. This study aimed to develop and validate a new WHO glass bottle bioassay method as an alternative to the WHO standard insecticide tube test to monitor mosquito susceptibility to new public health insecticides with particular modes of action, physical properties or both. METHODS: A multi-centre study involving 21 laboratories worldwide generated data on the susceptibility of seven mosquito species (Aedes aegypti, Aedes albopictus, Anopheles gambiae sensu stricto [An. gambiae s.s.], Anopheles funestus, Anopheles stephensi, Anopheles minimus and Anopheles albimanus) to seven public health insecticides in five classes, including pyrethroids (metofluthrin, prallethrin and transfluthrin), neonicotinoids (clothianidin), pyrroles (chlorfenapyr), juvenile hormone mimics (pyriproxyfen) and butenolides (flupyradifurone), in glass bottle assays. The data were analysed using a Bayesian binomial model to determine the concentration-response curves for each insecticide-species combination and to assess the within-bioassay variability in the susceptibility endpoints, namely the concentration that kills 50% and 99% of the test population (LC50 and LC99, respectively) and the concentration that inhibits oviposition of the test population by 50% and 99% (OI50 and OI99), to measure mortality and the sterilizing effect, respectively. RESULTS: Overall, about 200,000 mosquitoes were tested with the new bottle bioassay, and LC50/LC99 or OI50/OI99 values were determined for all insecticides. Variation was seen between laboratories in estimates for some mosquito species-insecticide combinations, while other test results were consistent. The variation was generally greater with transfluthrin and flupyradifurone than with the other compounds tested, especially against Anopheles species. Overall, the mean within-bioassay variability in mortality and oviposition inhibition were < 10% for most mosquito species-insecticide combinations. CONCLUSION: Our findings, based on the largest susceptibility dataset ever produced on mosquitoes, showed that the new WHO bottle bioassay is adequate for evaluating mosquito susceptibility to new and promising public health insecticides currently deployed for vector control. The datasets presented in this study have been used recently by the WHO to establish 17 new insecticide discriminating concentrations (DCs) for either Aedes spp. or Anopheles spp. The bottle bioassay and DCs can now be widely used to monitor baseline insecticide susceptibility of wild populations of vectors of malaria and Aedes-borne diseases worldwide.

Behavioral cost & overdominance in Anopheles gambiae.

In response to the widespread use of control strategies such as Insecticide Treated Nets (ITN), Anopheles mosquitoes have evolved various resistance mechanisms. Kdr is a mutation that provides physiological resistance to the pyrethroid insecticides family (PYR). In the present study, we investigated the effect of the Kdr mutation on the ability of female An. gambiae to locate and penetrate a 1cm-diameter hole in a piece of netting, either treated with insecticide or untreated, to reach a bait in a wind tunnel. Kdr homozygous, PYR-resistant mosquitoes were the least efficient at penetrating an untreated damaged net, with about 51% [39-63] success rate compared to 80% [70-90] and 78% [65-91] for homozygous susceptible and heterozygous respectively. This reduced efficiency, likely due to reduced host-seeking activity, as revealed by mosquito video-tracking, is evidence of a recessive behavioral cost of the mutation. Kdr heterozygous mosquitoes were the most efficient at penetrating nets treated with PYR insecticide, thus providing evidence for overdominance, the rarely-described case of heterozygote advantage conveyed by a single locus. The study also highlights the remarkable capacity of female mosquitoes, whether PYR-resistant or not, to locate holes in bed-nets.

Insecticidal activities of bark, leaf and seed extracts of Zanthoxylum heitzii against the African malaria vector Anopheles gambiae.

The olon tree, Zanthoxylum heitzii (syn. Fagara heitzii) is commonly found in the central-west African forests. In the Republic of Congo (Congo-Brazzaville) its bark is anecdotally reported to provide human protection against fleas. Here we assess the insecticidal activities of Z. heitzii stem bark, seed and leaf extracts against Anopheles gambiae s.s, the main malaria vector in Africa. Extracts were obtained by Accelerated Solvent Extraction (ASE) using solvents of different polarity and by classical Soxhlet extraction using hexane as solvent. The insecticidal effects of the crude extracts were evaluated using topical applications of insecticides on mosquitoes of a susceptible reference strain (Kisumu [Kis]), a strain homozygous for the L1014F kdr mutation (kdrKis), and a strain homozygous for the G119S Ace1R allele (AcerKis). The insecticidal activities were measured using LD50 and LD95 and active extracts were characterized by NMR spectroscopy and HPLC chromatography. Results show that the ASE hexane stem bark extract was the most effective compound against An. gambiae (LD50 = 102 ng/mg female), but was not as effective as common synthetic insecticides. Overall, there was no significant difference between the responses of the three mosquito strains to Z. heitzii extracts, indicating no cross resistance with conventional pesticides.

Pyrethroid resistance reduces the efficacy of space sprays for dengue control on the island of Martinique (Caribbean).

BACKGROUND: Dengue fever is reemerging on the island of Martinique and is a serious threat for the human population. During dengue epidemics, adult Aedes aegypti control with pyrethroid space sprays is implemented in order to rapidly reduce transmission. Unfortunately, vector control programs are facing operational challenges with the emergence of pyrethroid resistant Ae. aegypti populations. METHODOLOGY/PRINCIPAL FINDINGS: To assess the impact of pyrethroid resistance on the efficacy of treatments, applications of deltamethrin and natural pyrethrins were performed with vehicle-mounted thermal foggers in 9 localities of Martinique, where Ae. aegypti populations are strongly resistant to pyrethroids. Efficacy was assessed by monitoring mortality rates of naturally resistant and laboratory susceptible mosquitoes placed in sentinel cages. Before, during and after spraying, larval and adult densities were estimated. Results showed high mortality rates of susceptible sentinel mosquitoes treated with deltamethrin while resistant mosquitoes exhibited very low mortality. There was no reduction of either larval or adult Ae. aegypti population densities after treatments. CONCLUSIONS/SIGNIFICANCE: This is the first documented evidence that pyrethroid resistance impedes dengue vector control using pyrethroid-based treatments. These results emphasize the need for alternative tools and strategies for dengue control programs.

Efficacy of an insecticide paint against insecticide-susceptible and resistant mosquitoes - part 1: laboratory evaluation.

BACKGROUND: The main malaria vector Anopheles gambiae and the urban pest nuisance Culex quinquefasciatus are increasingly resistant to pyrethroids in many African countries. There is a need for new products and strategies. Insecticide paint Inesfly 5A IGR™, containing two organophosphates (OPs), chlorpyrifos and diazinon, and insect growth regulator (IGR), pyriproxyfen, was tested under laboratory conditions for 12 months following WHOPES Phase I procedures. METHODS: Mosquitoes used were laboratory strains of Cx. quinquefasciatus susceptible and resistant to OPs. The paint was applied at two different doses (1 kg/6 m2 and 1 kg/12 m2) on different commonly used surfaces: porous (cement and stucco) and non-porous (softwood and hard plastic). Insecticide efficacy was studied in terms of delayed mortality using 30-minute WHO bioassay cones. IGR efficacy on fecundity, fertility and larval development was studied on OP-resistant females exposed for 30 minutes to cement treated and control surfaces. RESULTS: After treatment, delayed mortality was high (87-100%) even against OP-resistant females on all surfaces except cement treated at 1 kg/12 m2. Remarkably, one year after treatment delayed mortality was 93-100% against OP-resistant females on non-porous surfaces at both doses. On cement, death rates were low 12 months after treatment regardless of the dose and the resistance status. Fecundity, fertility and adult emergence were reduced after treatment even at the lower dose (p < 10(-3)). A reduction in fecundity was still observed nine months after treatment at both doses (p < 10(-3)) and adult emergence was reduced at the higher dose (p < 10(-3)). CONCLUSIONS: High mortality rates were observed against laboratory strains of the pest mosquito Cx. quinquefasciatus susceptible and resistant to insecticides. Long-term killing remained equally important on non-porous surfaces regardless the resistance status for over 12 months. The paint's effect on fecundity, fertility and adult emergence may continue to provide an additional angle of attack in reducing overall population densities when the lethal effect of OPs diminishes over time. Some options on how to deal with porous materials are given. Implications in vector control are discussed.

Insecticide resistance of Triatoma infestans (Hemiptera, Reduviidae) vector of Chagas disease in Bolivia.

OBJECTIVE: To define the insecticide resistance status of Triatoma infestans to deltamethrin (pyrethroid), malathion (organophosphate) and bendiocarb (carbamate) in Bolivia. METHODS: Fifty populations of T. infestans were sampled in Bolivian human dwellings. Quantal response data were obtained by topical applications of 0.2 µl of insecticide-acetone solutions on nymphs N1 of the F1 generations. For most populations, dose-mortality relationships and resistance ratios (RR) were analysed. Discriminating concentrations were established for each insecticide with a susceptible reference strain and used on the other field populations. A tarsal-contact diagnostic test using insecticide impregnated papers was designed to rapidly identify deltamethrin-resistant populations in the field. RESULTS: Discriminating concentrations for topical applications were 5, 70 and 120 ng active ingredient per insect for deltamethrin, bendiocarb and malathion, respectively. The diagnostic concentration for deltamethrin was 0.30% for the 1-h exposure by tarsal contact. All populations sampled in human dwellings exhibited significant levels of resistance to deltamethrin, from 6 to 491 and varied among regions. Resistant populations did not recover complete susceptibility to deltamethrin when the synergist piperonyl butoxide (PBO) was used. None of the sampled populations exhibited significant resistance to bendiocarb (all RR(50) < 1.8) or malathion (all RR(50) < 2.2). CONCLUSION: In Bolivia, most 'domestic' T. infestans populations are resistant to deltamethrin. Because insecticide vector control is the only selection pressure, resistance likely originates from it. Switching from pyrethroids to organophosphates or carbamates could be a short-term solution to control this vector, but other alternative integrated control strategies should also be considered in the long term.

Pyrethrum: a mixture of natural pyrethrins has potential for malaria vector control.

Pyrethrum is a natural mixture of six insecticidal esters, recognized for low mammalian toxicity and limited persistence in the environment. In this study, World Health Organization standard bioassays were used to evaluate the performance of pyrethrum against both susceptible and pyrethroid-resistant Anopheles gambiae s.s. The results showed that the intrinsic activity of pyrethrum was similar to that of permethrin but lower than that of deltamethrin against susceptible mosquitoes. However, pyrethrum was less affected by the presence of the kdr mutation than synthetic pyrethroids (with lower resistance ratios) and showed good knock-down effect, repellency, and blood-feeding inhibition against the pyrethroid-resistant strain. In laboratory condition, mosquito nets treated with 500-1,000 mg/m2 (pyrethrum) remained effective, i.e., > 80% mortality and/or > 95% KD effect, for 9 mo. Conversely, the efficacy and residual activity of pyrethrum (Pynet 5% EC) on substrates was not conclusive, especially concerning mud, which is a porous substrate (mortality < 80% after 3 mo at 2 g/m2). These findings suggested that pyrethrum may be a potential alternative candidate for the impregnation of mosquito nets and textiles in areas where resistance to pyrethroids has become problematic.

Multi-function oxidases are responsible for the synergistic interactions occurring between repellents and insecticides in mosquitoes.

BACKGROUND: With the spread of pyrethroid resistance in mosquitoes, the combination of an insecticide (carbamate or organophosphate) with a repellent (DEET) is considered as a promising alternative strategy for the treatment of mosquito nets and other relevant materials. The efficacy of these mixtures comes from the fact that they reproduce pyrethroid features and that positive interactions occur between insecticides and repellent. To better understand the mechanisms involved and assess the impact of detoxifying enzymes (oxidases and esterases) in these interactions, bioassays were carried out in the laboratory against the main dengue vector Aedes aegypti. METHODS: Topical applications of DEET and propoxur (carbamate), used alone or as a mixture, were carried out on female mosquitoes, using inhibitors of the two main detoxification pathways in the insect. PBO, an inhibitor of multi-function oxidases, and DEF, an inhibitor of esterases, were applied one hour prior to the main treatment. RESULTS: Results showed that synergism between DEET and propoxur disappeared in the presence of PBO but not with DEF. This suggests that oxidases, contrary to esterases, play a key role in the interactions occurring between DEET and cholinesterase inhibitors in mosquitoes. CONCLUSION: These findings are of great interest for the implementation of "combination nets" in the field. They support the need to combine insecticide with repellent to overcome insecticide resistance in mosquitoes of public health importance.

Costs and benefits of multiple resistance to insecticides for Culex quinquefasciatus mosquitoes.

BACKGROUND: The evolutionary dynamics of xenobiotic resistance depends on how resistance mutations influence the fitness of their bearers, both in the presence and absence of xenobiotic selection pressure. In cases of multiple resistance, these dynamics will also depend on how individual resistance mutations interact with one another, and on the xenobiotics applied against them. We compared Culex quinquefasciatus mosquitoes harbouring two resistance alleles ace-1R and KdrR (conferring resistance to carbamate and pyrethroid insecticides, respectively) to mosquitoes bearing only one of the alleles, or neither allele. Comparisons were made in environments where both, only one, or neither type of insecticide was present. RESULTS: Each resistance allele was associated with fitness costs (survival to adulthood) in an insecticide-free environment, with the costs of ace-1R being greater than for KdrR. However, there was a notable interaction in that the costs of harbouring both alleles were significantly less than for harbouring ace-1R alone. The two insecticides combined in an additive, synergistic and antagonistic manner depending on a mosquito's resistance status, but were not predictable based on the presence/absence of either, or both mutations. CONCLUSION: Insecticide resistance mutations interacted to positively or negatively influence a mosquito's fitness, both in the presence or absence of insecticides. In particular, the presence of the KdrR mutation compensated for the costs of the ace-1R mutation in an insecticide-free environment, suggesting the strength of selection in untreated areas would be less against mosquitoes resistant to both insecticides than for those resistant to carbamates alone. Additional interactions suggest the dynamics of resistance will be difficult to predict in populations where multiple resistance mutations are present or that are subject to treatment by different xenobiotics.

Trend of temephos resistance in Aedes (Stegomyia) mosquitoes in Thailand during 2003-2005.

The diagnostic dose for temephos susceptibility test was established based on Aedes aegypti, the susceptible Bora (French Polynesia) strain, for practical and routine use. The diagnostic dose was subsequently used to evaluate the susceptibility/resistance status in F1 progenies of field-collected samples from Bangkok and various parts of Thailand. It appeared that Ae. aegypti mosquitoes of one collection site each in Bangkok, Nakhon Sawan (northcentral), and Nakhon Ratchasrima (northeast) were resistant to temephos, with mortality ranging from 50.5 to 71.4%. Moreover, there was a trend of resistance to temephos among Ae. aegypti populations of all studied districts of Nakorn Ratchasima and most areas of Nakhon Sawan, of which those in one area were susceptible. However, various levels of temephos susceptibility were found in Bangkok populations, including resistance and incipient resistance. In Chonburi Province (eastern), all mosquitoes were susceptible to temephos with an indication of tolerance in one sample. Additionally, mosquitoes from Songkhla (south), Chiang Rai (north), Kanchanaburi (west), and Chanthaburi (east) remained susceptible to temephos during the sample collecting period. Bioassay tests on Aedes albopictus populations collected in this study from Nakhon Sawan, Nakorn Ratchasima, Songkhla, and Kanchanaburi revealed high susceptibility to temephos. Although the use of temephos seems to be potentially effective in many areas of the country, a noticeable trend of resistance indicated that alternative vector control methods should be periodically applied.

Insecticide susceptible/resistance status in Aedes (Stegomyia) aegypti and Aedes (Stegomyia) albopictus (Diptera: Culicidae) in Thailand during 2003-2005.

Susceptibility baselines and diagnostic doses of the technical grade insecticides deltamethrin, permethrin, fenitrothion, and propoxur were established based on Aedes aegypti (L.), Bora (French Polynesia), a reference susceptible strain. Field-collected Aedes mosquitoes from each part of Thailand were subjected to bioassay for their susceptibility to the diagnostic doses of each insecticide. Almost all Ae. aegypti collected were incipient resistant or resistant to deltamethrin and permethrin, except those from some areas of Songkhla (southern) and Phan district of Chiang Rai (northern) province. Susceptibility to fenitrothion was found in mosquitoes from Bangkok (central), Chonburi (eastern), Chiang Rai, Kanchanaburi (western), and Songkhla, whereas they were resistant in almost all areas of Nakhon Sawan (north central) and Nakhon Ratchasima (northeastern) provinces. Most of Ae. aegypti were susceptible to propoxur except those from Mae Wong, Nakhon Sawan province. Various levels of insecticide resistance and susceptibility in adjacent areas revealed a focal susceptible/resistance profile in the country. It could be noted that almost all of Ae. albopictus were susceptible to the insecticides tested at the same diagnostic doses. In conclusion, resistance to pyrethroids (permethrin and deltamethrin) has developed in Ae. aegypti in most of the collected areas, suggesting that an alternative choice of insecticide or other control measures should be applied.

Assessment of laboratory and field assays of sunlight-induced killing of mosquito larvae by photosensitizers.

The effectiveness of light-induced killing of mosquito larvae in the presence of photosensitizers was studied with larvae of Aedes aegypti (L.), Anopheles stephensi (Liston), and Culex quinquefasciatus Say grown in the laboratory and of Cx. quinquefasciatus grown under field conditions. Tested photosensitizers included xanthene, chlorin, and porphyrin derivatives. All the larvae were treated at the fourth instar. Preliminary laboratory experiments showed a light-induced lethal effect of Rose Bengal (RB) on three species of mosquito larvae. Compared with other photosensitizers, RB seemed to be more efficient at even lower concentration than chlorin (e6) and chlorophyllin on Ae. aegypti larvae. Among the four porphyrin derivatives, i.e., chloroquinoline tetraphenyl propioamidoporphine, tetraphenyl porphine tetrasulfonate, hematoporphyrin (HP), and tetraphenylporphinepropionic acid porphine, HP was the only effective photosensitizer on Ae. aegypti larvae. The best conditions for field tests using RB were conducted on Cx. quinquefasciatus in Bobo-Dioulasso, Burkina Faso. The mortality induced by RB varied from 80 to 96% obtained with unfiltered cesspit water to 0.4 to 6.7% in cesspits with a heavy load of organic materials, thus providing the basis for further developments of this technique under field conditions.

Spinosad: a new larvicide against insecticide-resistant mosquito larvae.

Among the characteristics of spinosad that are worth noting is its environmentally favorable toxicity profile. It is a compound of biological origin, whose insecticide action is highly effective against a great number of insects. Laboratory larval bioassays of spinosad on Aedes aegypti, Culex quinquefasciatus, and Anopheles gambiae (specimens that were either susceptible or resistant to pyrethroids, carbamates, and organophosphates) showed that this product had a lethal action (mortality after 24 h of exposure) regardless of the original status, susceptible or resistant, of the mosquito larvae.

Dinotefuran: a potential neonicotinoid insecticide against resistant mosquitoes.

Because pyrethroid, organophosphate, and carbamate resistance is more and more developed in mosquitoes of medical importance, there is an urgent need for alternative insecticides for vector control. Dinotefuran, a new neonicotinoid insecticide commercialized by Mitsui Chemicals (Tokyo, Japan), could be a useful candidate in public health because it shows low mammalian toxicity and great insecticidal activity against a broad range of pests. In this study, the intrinsic toxicity of dinotefuran was evaluated by larval bioassay and topical application against different mosquito strains of Anopheles gambiae Giles, Culex quinquefasciatus Say, and Aedes aegypti L. having none, one, or several resistance mechanisms, respectively, to insecticides. The results showed that dinotefuran was less toxic than most of the commonly used insecticides (e.g., deltamethrin, carbosulfan, and temephos) against the susceptible mosquitoes tested (between 6- and 100-fold at the LD50 level). However, the toxicity of dinotefuran was not strongly affected by the presence of common resistance mechanism, i.e., kdr mutation and insensitive acetylcholinesterase (resistance ratio [RR] from 1.3 to 2.3). More interestingly, the carbamate-resistant strain of Cx. quinquefasciatus was significantly more affected by dinotefuran than the susceptible strain (RR = 0.70), probably because the insensitive acetylcholinesterase is less efficient to degrade nicotinic substrates than normal acetylcholinesterase. Despite the relatively low toxicity of dinotefuran against susceptible mosquitoes, the absence of cross-resistance with common insecticides (pyrethroids, carbamates, and organophosphates) makes neonicotinoids potential candidates for disease vector control, especially in area where mosquitoes are resistant to insecticides.

Evaluation of Olyset insecticide-treated nets distributed seven years previously in Tanzania.

BACKGROUND: Insecticide-treated nets represent currently a key malaria control strategy, but low insecticide re-treatment rates remain problematic. Olyset nets are currently one of two long-lasting insecticidal nets recommended by WHO. An assessment was carried out of the effect of Olyset nets after seven years of use in rural Tanzania. METHODS: A survey of Olyset nets was conducted in two Tanzanian villages to examine their insecticide dosage, bioassay efficacy and desirability compared with ordinary polyester nets. RESULTS: Of 103 randomly selected nets distributed in 1994 to 1995, 100 could be traced. Most nets were in a condition likely to offer protection against mosquito biting. Villagers appreciated mainly the durability of Olyset trade mark nets and insecticide persistence. People disliked the small size of these nets and the light blue colour and preferred a smaller mesh size, features that can easily be modified. At equal price, 51% said they would prefer to buy an Olyset net and 49% opted for an ordinary polyester net. The average permethrin content was 33%-41% of the initial insecticide dose of 20,000 mg/Kg. Bioassay results indicated high knock-down rates at 60 minutes, but the mosquito mortality after 24 hours was rather low (mean: 34%). No significant correlation was found between bioassay results and insecticide concentration in and on the net. CONCLUSIONS: Olyset nets are popular, durable and with a much longer insecticide persistence than ordinary polyester nets. Hence, Olyset nets are one of the best choices for ITN programmes in rural malaria-endemic areas.

Comparative performances, under laboratory conditions, of seven pyrethroid insecticides used for impregnation of mosquito nets.

OBJECTIVE: To compare the efficacy of seven pyrethroid insecticides for impregnation of mosquito nets, six currently recommended by WHO and one candidate (bifenthrin), under laboratory conditions. METHODS: Tests were conducted using pyrethroid-susceptible and pyrethroid-resistant strains of Anopheles gambiae and Culex quinquefasciatus. Knock-down effect, irritancy and mortality were measured using standard WHO cone tests. Mortality and blood-feeding inhibition were also measured using a baited tunnel device. FINDINGS: For susceptible A. gambiae, alpha-cypermethrin had the fastest knock-down effect. For resistant A. gambiae, the knock- down effect was slightly slower with alpha-cypermethrin and much reduced following exposure to the other insecticides, particularly bifenthrin and permethrin. For susceptible C. quinquefasciatus, the knock-down effect was significantly slower than in A. gambiae, particularly with bifenthrin, and no knock-down effect was observed with any of the pyrethroids against the resistant strain. Bifenthrin was significantly less irritant than the other pyrethroids to susceptible and resistant A. gambiae but there was no clear ranking of pyrethroid irritancy against C. quinquefasciatus. In tunnels, all insecticides were less toxic against C. quinquefasciatus than against A. gambiae for susceptible strains. For resistant strains, mortality was significant with all the pyrethroids with A. gambiae but not with C. quinquefasciatus. Inhibition of blood-feeding was also high for susceptible strains of both species and for resistant A. gambiae but lower for resistant C. quinquefasciatus; bifenthrin had the greatest impact. CONCLUSIONS: Efficacy for impregnation of mosquito nets against A. gambiae was greatest with alpha-cypermethrin. Bifenthrin is likely to have a significant comparative advantage over other pyrethroids in areas with pyrethroid resistance because of its much stronger impact on the nuisance mosquito, C. quinquefasciatus, despite its slower knock-down effect and irritancy. Selection of pyrethroids for mosquito vector control and personal protection should take into account the different effects of these insecticides, the status of pyrethroid resistance in the target area, and the importance of nuisance mosquitoes, such as C. quinquefasciatus.