In Singapore, dose-response bioassays of Aedes aegypti (L.) adults have been conducted, but the mechanisms underlying resistance to insecticides remain unclear. In this study, we evaluated insecticide resistance and its underlying mechanism in field populations of Ae. aegypti adults.
Routine surveillance on resistant status of field mosquito populations is important to implement suitable strategies in order to prevent pest outbreaks. WHO test kit bioassay is the most frequent bioassay used to investigate the susceptibility status of field-collected mosquitoes, as it is relatively convenient to be carried out in the field. In contrast, the topical application of active ingredient is less popular in investigating the susceptibility status of mosquitoes. In this study, we accessed the susceptibility status of Aedes albopictus Skuse collected from two dengue hotspots on Penang Island: Sungai Dua and Persiaran Mayang Pasir. Two active ingredients: permethrin and deltamethrin, were used. WHO test kit bioassay showed that both wild strains collected were susceptible to the two active ingredients; while topical application assay showed that they were resistant. This indicated that WHO test kit bioassay less sensitive to low level of resistance compared to topical application assay. Hence, topical application is expected to be more indicative when used in a resistance surveillance programme.
Budding and relocation of nests are important characteristics of the Pharaoh ant, Monomorium pharaonis (L.), an important pest of artificial structures. Pharaoh ant colony movements induced by several types of disturbances were evaluated in the laboratory. The percentages of workers and brood in the source and new nest sites were determined at Days 0, 1, 3, and 5 following physical disturbance (temporal removal of nestmates), chemical disturbance (application of pyrethroid insecticide), invasion by heterospecific ants, food depletion, and moisture depletion in the laboratory. All disturbances were performed in the source nest, which was connected to an empty new nest site. Almost all workers moved and carried the entire brood to the new nest site when subjected to physical disturbance, chemical disturbance, and ant invasion on Day 1, whereas only <5% of workers were present in the new nest site in the undisturbed control. After these disturbances, the brood was never relocated back to the original nest site in this 5-d study. When subjected to food depletion, ∼60% of the brood were found in the new nest site and ∼40% of the brood remained in the original nest on Day 5, resulting in a polydomous population. In contrast, moisture depletion did not show any significant effect on colony movement. These results provide useful information about the causes of Pharaoh ant colony budding and guidance about how to develop effective control and prevention strategies.
Insecticide resistance has become a serious issue in vector management programs. Information on insecticidal resistance and its associated mechanisms is important for successful insecticide resistance management. The selection of a colony of permethrin-resistant Aedes albopictus (Skuse) (Diptera: Culicidae), originating from Penang Island, Malaysia, yielded high larval-specific resistance to permethrin and cross-resistance to deltamethrin. Synergism assays showed that the major mechanism underlying this resistance involves cytochrome P450 monooxygenase. The resistance is autosomal, polygenically inherited and incompletely dominant (D = 0.26). Resistant larvae were reared under different conditions to assess the fitness costs. Under high larval density, larval development time of the resistant SGI strain was significantly longer than the susceptible VCRU strain. In both high- and low-density conditions SGI showed a lower rate of emergence and survival compared with the VCRU strain. Resistant larvae were more susceptible to predation by Toxorhynchites splendens (Wiedemann) (Diptera: Culicidae) larvae. The body size of SGI females reared under high-density conditions was larger compared with females of the susceptible strain. SGI females survived longer when starved than did VCRU females. The energy reserve upon eclosion was positively correlated with the size of the adults.
Biweekly ovitrap surveillance (OS) was conducted for a year (August 2007 - September 2008) at two different dengue endemic sites in Shah Alam, Selangor, Malaysia, 50 km from Kuala Lumpur. Aedes aegypti collected from these 2 locations were raised to the F3 stage and subjected to a WHO standard bioassay method to determine lethal time (LT) against pyrethroids (permethrin 0.75%, cyfluthrin 0.15%), organophosphates (malathion 5.0%, fenitrothion 1.0%), carbamates (propoxur 0.1%, bendiocarb 0.1%) and organochlorine (DDT 4.0%). Insecticide susceptibilities were analyzed for one year. Aedes aegypti were resistant to DDT with a mortality range of 0 - 13.3% throughout the year at both sites. Susceptibilities to pyrethroids and carbamates varied throughout the year. In contrast, susceptibilities to pyrethroids and carbamates varied throughout the year: resistant to propoxur, bendiocarb and permethrin with mortality of < 80% in most months; but, showed incipient resistant to cyfluthrin in most months. Mosquitoes were consistently susceptible to malathion and fenitrothion, with complete mortality during most months. They were especially susceptible to malathion with LT50 values of 21.32 - 36.37 minutes, suggesting effectiveness of malathion for control of dengue.
Wild caught female Culex quinquefasciatus (Say) from Kuala Lumpur were blood fed and reared in the insectarium. The late third stage of the F1 larvae which survived the high selection pressure of malathion and permethrin were reared and colonies were established from adults that emerged. Larvae from these colonies were then subjected in the subsequent 9 generations to higher selection pressure. The rate of resistance development were measured by LC50 value of larval bioassay, LT50 value of adult bioassay and the frequency of the elevated esterase levels. In another set of experiments using the same batch of Culex mosquitos, the larvae were not exposed to any insecticides and the decrease in resistance rate was monitored in each subsequent 9 generations by using similar methods. The heterozygous standard laboratory strain was selected for susceptibility using the single raft sib-selection method. The result showed that the field collected F1 generation was 96.0 and 6.3 fold more resistant to malathion and permethrin, respectively. After selection for about 9 generations the resistance ratio to malathion and permethrin was 6.2 and 767.3 fold more compared to the LC50 values of F1 generations, respectively. Esterase in F1 larvae was 6.0 fold more than the standard laboratory strain.
Two highly active synthetic pyrethroid insecticides, lambdacyhalothrin and cypermethrin, were evaluated as thermal fogs against houseflies (Musca domestica Linnaeus) and mosquitos (Aedes aegypti Linnaeus). Lambdacyhalothrin (OMS 3021) showed an average of 2.5 times more knockdown activity and over 5 times more adulticidal activity than cypermethrin against Musca domestica and Aedes aegypti. These results demonstrate that lambdacyhalothrin is highly effective at very low rates as a thermal fog against Ae. aegypti and M. domestica. Commercially available formulations of 2.5% and 5% lambdacyhalothrin can be diluted either with water for ULV cold aerosol space-spraying or with diesel/kerosene for thermal fogging at recommended application rates of 0.5-1 g ai/ha for mosquito control and 2 g ai/ha for housefly control. Due to the very low rates of application, formulated products of lambdacyhalothrin are unlikely to present any acute hazards in normal use. The low dosages required to bring about rapid control of houseflies and mosquitos make this new pyrethroid insecticide particularly cost-effective. Coupled with its good residual activity (Jutsum et al, 1984), lambdacyhalothrin can be adopted as a powerful tool in integrated pest management program for the control of medically important pests and vectors.
Many insect species display daily variation of sensitivity to insecticides when they are exposed to the same concentration at different times during the day. To date, this has not been investigated in bed bugs. To address this, we explored circadian rhythms in insecticide susceptibility, xenobiotic metabolizing (XM) gene expressions, and metabolic detoxification in the common bed bug, Cimex lectularius. An insecticide susceptible Monheim strain of C. lectularius was most tolerant of deltamethrin during the late photophase at ZT9 (i.e. nine hours after light is present in the light-dark cycle (LD) cycle) and similarly repeated at CT9 (i.e. nine hours into the subjective day in constant darkness (DD)) suggesting endogenous circadian involvement in susceptibility to deltamethrin. No diel rhythm was observed against imidacloprid insecticide despite significant daily susceptibility in both LD and DD conditions. Rhythmic expressions of metabolic detoxification genes, GSTs1 and CYP397A1 displayed similar expression patterns with total GST and P450 enzyme activities in LD and DD conditions, respectively. The oscillation of mRNA levels of GSTs1 and CYP397A1 was found consistent with peak phases of deltamethrin susceptibility in C. lectularius. This study demonstrates that circadian patterns of metabolic detoxification gene expression occur within C. lectularius. As a consequence, insecticide efficacy can vary dramatically throughout a 24 hour period.
Resistance status of Aedes albopictus (Diptera: Culicidae) collected from Sabah, East Malaysia, was evaluated against four major classes of adulticides, namely pyrethroid, carbamate, organochlorine, and organophosphate. Adult bioassays conforming to WHO standard protocols were conducted to assess knockdown and mortality rates of Ae. albopictus. Among tested pyrethroid adulticides, only cyfluthrin, lambda-cyaholthrin, and deltamethrin were able to inflict total knockdown. The other adulticide classes mostly failed to cause any knockdown; the highest knockdown rate was only 18.33% for propoxur. With regards to mortality rate, Ae. albopictus was unanimously susceptible toward all pyrethroids, dieldrin, and malathion, but exhibited resistance toward bendiocarb, propoxur, dichlorodiphenyltrichloroethane, and fenitrothion. Additionally, correlation analysis demonstrated cross-resistance between bendiocarb and propoxur, and malathion and propoxur. In conclusion, this study has disclosed that pyrethroids are still generally effective for Aedes control in Sabah, Malaysia. The susceptibility status of Ae. albopictus against pyrethroids in descending order was cyfluthrin > lambda-cyhalothrin > deltamethrin > etofenprox > permethrin.
Insecticide-based vector control approaches are facing challenges due to the development of resistance in vector mosquitoes. Therefore, a proper resistance surveillance program using baseline lethal concentrations is crucial for resistance management strategies. Currently, the World Health Organization's (WHO) diagnostic doses established for Aedes aegypti and Anopheles species are being used to study the resistance status of Aedes albopictus. In this study, we established the diagnostic doses for permethrin, deltamethrin, and malathion using a known susceptible reference strain. Five field-collected populations were screened against these doses, following the WHO protocol. This study established the diagnostic dose of malathion at 2.4%, permethrin at 0.95%, and deltamethrin at 0.28%, which differ from the WHO doses for Aedes aegypti and Anopheles spp. Among the insecticides tested on the 5 wild populations, only deltamethrin showed high effectiveness. Different susceptibility and resistance patterns were observed with permethrin, malathion, and dichloro-diphenyl-trichloroethane (DDT) at 4%. This study may assist the health authorities to improve future chemical-based vector control operations in dengue-endemic areas.
Bed bugs [both Cimex hemipterus (F.) and Cimex lectularius L.] are highly resistant to pyrethroids worldwide. An important resistance mechanism known as 'knockdown resistance' (kdr) is caused by genetic point mutations on the voltage-gated sodium channel (VGSC) gene. Previous studies have identified two point mutations (V419L and L925I) on the VGSC gene in C. lectularius that are responsible for kdr-type resistance. However, the kdr mutations in C. hemipterus have not been investigated.
In this study, 13 weeks (October to December 2012) of ovitrap surveillance was conducted in two suburban residential areas in Kampar town, Perak. A total of 17,310 Aedes mosquitoes were found in Taman Kampar Jaya, whereas Taman Juloong recorded a higher number at 19,042. Less than 1% of these were identified as Aedes aegypti, with the remaining confirmed as Aedes albopictus. The female Ae. albopictus were subsequently subjected to WHO standard diagnostic test kits against two pyrethroids (0.05% deltamethrin and 0.75% permethrin) and two organophosphates (1% fenitrothion and 5% malathion). The Ae. albopictus from both research sites were the most susceptible to deltamethrin, recording KT50 and KT95 response values of 15.84 minutes and 16.18 minutes; and 48.18 minutes and 49.44 minutes respectively. This was followed by permethrin (20.57 minutes and 17.52 minutes; 29.54 minutes and 54.54 minutes) and malathion (48.46 minutes and 62.69 minutes; 87.72 minutes and 141.04 minutes). Fenitrothion was found to be least effective towards Ae. albopictus; recording KT50 and KT95 response values of 150.29 minutes and 293.41 minutes for Taman Kampar Jaya, and 203.32 minutes and 408.07 minutes respectively for Taman Juloong. All tested Ae. albopictus showed 100% mortality after 24 hours post exposure. As both residential areas were fogged periodically by the municipal council; alternating between organophosphates and pyrethroids, thus, constant monitoring is crucial in light of the emergence of resistance noted in Ae. albopictus towards fenitrothion.
A 14-months survey was carried out to identify the species composition of Anopheles mosquitoes from Kampung Bongor, Grik, Perak. Adding to that, a preliminary one month mosquito population screening was done at Kampung Tepin, Serian, Sarawak. Consequently, the insecticide susceptibility status of a pyrethroid was tested against two selected species of Anopheles collected from these two locations in Malaysia. A total of 4,497 Anopheles from 11 species were identified from collections in Kampung Bongor, whereas 2,654 An. letifer were collected from Kampung Tepin. The An. maculatus of Kampung Bongor and An. letifer of Kampung Tepin were then selected and tested using WHO standard diagnostic test kits and impregnated papers with 0.75% permethrin. The response values of KT50 and KT95 for An. maculatus were recorded at 28.09 minutes and 62.98 minutes respectively. Anopheles letifer recorded much slower response values of KT50 and KT95, which was at 35.09 minutes and 73.03 minutes respectively. Both An. maculatus and An. letifer showed 100% mortality after 24 hours holding period. The results indicate that both species were still susceptible to the tested pyrethroid. For effective vector control and resistance management, accurate and periodic insecticide resistance monitoring should be undertaken especially in rural areas with agricultural usage of insecticides.
Toxicological studies of four insecticides (malathion, carbaryl, bioresmethrin, and GH 74) against Musca domestica vicinia (Ampang strain) were undertaken with particular reference to age, sex and posttreatment temperature. It was found that bioresmethrin and GH 74, both with a negative temperature coefficient, have great potential for use against houseflies. In vitro inhibitory studies of head and body esterases showed that unlike malathion and carbaryl, bioresmethrin had only negligible effect on these enzymes. The possibilities of using bioresmethrin and GH 74 for controlling the housefly problem in the Cameron Highlands, West Malaysia are discussed.
The emergence of pyrethroid resistance in Aedes aegypti (L.) has limited the success of vector control. Early detection of resistance could assist authorities in deciding well-suited control strategies to minimize operational failures of Ae. aegypti control. Herein, biochemical analysis was performed to investigate the mechanisms involved in pyrethroid resistance in nine populations of Indonesian Ae. aegypti. Enzymes of adult Ae. aegypti such as esterases (ESTs), glutathione-S-transferases (GSTs), and mixed-function oxidases (MFOs) were characterized. Elevated MFO activity was correlated with resistance phenotype, indicating the role of this enzyme in contributing to pyrethroid resistance. No significant correlations were shown between pyrethroid resistance phenotype and α-ESTs, suggesting that marginally exceeded enzyme levels relative to the reference strain in some pyrethroid-susceptible populations were causative factor for insecticide resistance in other groups of insecticides. However, significant correlation was demonstrated between β-ESTs and pyrethroid resistance phenotype. The lowest enzyme levels in GSTs indicated that this enzyme was not predominant in causing pyrethroid resistance, despite the presence of significant correlations. Because metabolic detoxification fails to comprehensively explain the pyrethroid resistance in some Indonesian Ae. aegypti, additional mechanisms such as altered target sites in voltage-gated sodium channel may also contribute to the high pyrethroid resistance in Ae. aegypti.
Cynoff 25ULV (cypermethrin 25 g/l) and Solfac UL015 (cyfluthrin 1.5% w/v) were evaluated against the sentinel sugar-fed adults and 4th-instar larvae of Aedes aegypti in a housing estate endemic of dengue in Malaysia. The impact of both pyrethroids on field populations of Aedes albopictus and Aedes aegypti larvae was monitored weekly using bottle containers. Both Cynoff 25ULV and Solfac UL015 showed adulticidal effects and larvicidal effects. This field trial using Cynoff 25ULV against dengue vectors showed its potential for use in dengue vector control programs.
BACKGROUND: Pediculiasis is treated aggressively in the United States, mainly with permethrin- and pyrethrin-containing pediculicides. Increasingly frequent anecdotal reports of treatment failure suggest the emergence of insecticidal resistance by these lice.
OBJECTIVE: To confirm or refute the susceptibility of head lice sampled in the United States to permethrin.
DESIGN: Survey. Head lice were removed from children residing where pediculicides are readily available and where such products are essentially unknown. Their survival was compared following exposure to residues of graded doses of permethrin in an in vitro bioassay.
SETTING: School children from Massachusetts, Idaho, and Sabah (Malaysian Borneo).
SUBJECTS: In the United States, 75 children aged 5 to 8 years. In Sabah, 59 boys aged 6 to 13 years. Virtually all sampled US children had previously been treated with pediculicides containing pyrethrins or permethrin; none of the Sabahan children were so exposed.
MAIN OUTCOME MEASURE: Survival of head lice exposed to permethrin.
RESULTS: Permethrin did not affect head lice sampled from chronically infested US children who had previously been treated for pediculiasis. The slope of the dose-response regression line for these lice did not differ significantly from zero (P = .66). This pediculicide immobilized lice sampled in Sabah. Mortality correlated closely with permethrin concentration (P = .008).
CONCLUSIONS: Head lice in the United States are less susceptible to permethrin than are those in Sabah. The pyrethroid susceptibility of the general population of head lice in the United States, however, remains poorly defined. Accordingly, these relatively safe over-the-counter preparations may remain the pediculicides of choice for newly recognized louse infestations.
The effect of permethrin impregnated bednets on Anopheles maculatus Theobald was studied in four villages in Pos Betau, Pahang, Malaysia from August 1990 to July 1992. Collections of mosquitos were carried out indoors and outdoors from 1900 to 0700 hours. All mosquitos were dissected for sporozoites and parity. In May 1991 two villages received bednets impregnated with permethrin at 0.5 g/m2 and two villages received placebo bednets. There was a significant difference in the sporozoite and parous rates between the treated and control villages after the distribution of bednets (p < 0.05). There was no significant difference in the bites/man/night of An. maculatus between the pre and post treatment periods in the control villages. However there was a significant difference in bites/man/night between pre and post treatment in the treated villages (p < 0.001).
There has been a worldwide surge in the number and severity of dengue in the past decades. In Singapore, relentless vector control efforts have been put in to control the disease since the 1960's. Space spraying, fogging, chemical treatment and source reduction are some commonly used methodologies for controlling its vectors, particularly Aedes aegypti. Here, as we explored the use of a commercially available delthamethrin-treated net as an alternative strategy and the efficacy of the treated net was found to be limited. Through bioassays and molecular studies, the failure of the treated net to render high mortality rate was found to be associated with the knockdown resistance (kdr) mutation. This is the first report of kdr- mutations in Singapore's Ae. aegypti. At least one point mutation, either homozygous or heterozygous, at amino acid residue V1016G of DIIS6 or F1269C of DIIIS6 was detected in 93% of field strains of Ae. aegypti. Various permutations of wild type and mutant amino acids of the four alleles were found to result in varying degree of survival rate among local field Ae. aegypti when exposed to the deltamethrin treated net. Together with the association of higher survival rate with the presence of both V1016G and F1269C, the data suggest the role of these mutations in the resistance to the deltamethrin. The high prevalence of these mutations were confirmed in a country wide survey where 70% and 72% of the 201 Ae. aegypti analysed possessed the mutations at residues 1016 and 1269 respectively. The highest mutated frequency combination was found to be heterozygous alleles (VG/FC) at both residues 1016 and 1269 (37.8%), followed by homozygous mutation at allele 1269 (24.4%) and homozygous mutation at allele 1016 (22.9%). The kdr- type of resistance among the vector is likely to undermine the effectiveness of pyrethroids treated materials against these mosquitoes.