First Detection of Acinetobacter baumannii in Pediculus humanus capitis from Latin America.

Several studies have documented the presence of Acinetobacter baumannii, a known multi-drug-resistant pathogen, in the human head louse, Pediculus humanus capitis. Since no reports from countries in Latin America have been published, the aim of the present study was to determine whether A. baumannii was present in head lice specimens collected in this geographic region. Head lice specimens from Argentina, Colombia, and Honduras were analyzed. PCR assays were performed to confirm the specimens' species and to investigate whether the DNA of A. baumannii was present. The products of the latter were sequenced to confirm bacterial identity. Altogether, 122 pools of head lice were analyzed, of which two (1.64%) were positive for A. baumannii's DNA. The positive head lice had been collected at the poorest study site in Honduras. The remaining specimens were negative. This study is the first to report the presence of A. baumannii in human head lice from Latin America. Further investigations are required to elucidate whether these ectoparasites can serve as natural reservoirs or even effectively transmit A. baumannii to humans.

Global Pattern of kdr-Type Alleles in Musca domestica (L.).

Purpose of Review: Houseflies, Musca domestica L., are an important sanitary pest that affects human and domesticated animals. They are mechanical carriers of more than 100 human and animal diseases including protozoan, bacterial, helminthic, and viral infections. Recently, it was demonstrated that houseflies acquired, harbored, and transmitted SARS-CoV-2 (COVID-19) for up to 1 day post-exposure. The most widely used control strategy relays on the application of pyrethroid insecticides due to their effectiveness, low mammalian toxicity, low cost, and environmental safety. The main mechanism of action of pyrethroids is to exert their toxic effects through affecting the voltage-sensitive sodium channel (VSSC) modifying the transmission of the nerve impulse and leading to the death of the insects. Target site insensitivity of the VSSC is due to the presence of single nuclear polymorphisms (SNPs) named knockdown mutations (kdr). In this review, we synthetize recent data on the type and distribution of these mutations globally. Recent Findings: Housefly resistance is reported in several countries. Increased applications of pyrethroids to control housefly populations led to the emergence of multiple evolutionary origins of resistance determined by five amino acid substitutions or specific mutations in the VSSC: kdr (L1014F), kdr-his (L1014H), super-kdr (M918T + L1014F), type N (D600N + M918T + L1014F), and 1B (T929I + L1014F). According to the global map obtained, high levels of resistance to pyrethroids are associated with the L1014F mutation found mostly in North America, Europe, and Asia, while the super-kdr mutation was mostly found in the American continent. The level of protection conferred by these alleles against pyrethroids was generally kdr-his < kdr < Type N ≤ super-kdr ≤ 1B. The relative fitness of the alleles under laboratory conditions was susceptible ≅ kdr-his > kdr > super-kdr suggesting that the fitness cost of an allele was relative to the presence of other alleles in a population and that the reversion of resistance in a free insecticide environment might be quite variable from one region to another. Summary: An adequate integrated pest management program should consider monitoring susceptibility to pyrethroids to detect early levels of resistance and predict the spread and evolution of resistant phenotypes and genotypes. From this review, the pyrethroid resistance status of housefly population was determined in very few countries and has evolved independently in different areas of the world affecting chemical control programs.

Head lice were also affected by COVID-19: a decrease on Pediculosis infestation during lockdown in Buenos Aires.

Pediculosis is a worldwide disease affecting school-aged children produced by the presence of the head louse, Pediculus humanus capitis De Geer, an obligate ectoparasite on the human scalp feeding exclusively on blood. Transmission occurs primarily through direct physical head-to-head contact. In March 2020, the World Health Organization (WHO) declared the COVID-19 outbreak as a pandemic. COVID-19 is caused by SARS-CoV-2, a variant of the coronavirus. Therefore, on March 18, 2020, the Argentinean government established mandatory isolation for an indefinite period. This obligatory isolation interrupted regular classes avoiding direct contact between children, thus affecting the dispersal route of individuals and the evolution of head louse populations. In this study, we evaluated through an online survey how confinement affected the prevalence of lice during lockdown compared to the situation prior to confinement. The survey allowed to discriminate the different control strategies, the number of treatments, and the amount of insects recorded by parents. Data of 1118 children obtained from 627 surveys were analyzed. As the main result, it was observed that prevalence of lice decreased significantly from before (69.6%) to during (43.9%) COVID-19 lockdown. Moreover, head lice infestation was more effectively controlled in households with up to 2 children in comparison to households with 3 or more children. This is the first study that analyzed the prevalence of head lice during COVID-19 pandemic. In addition, this work demonstrated the impact of social distance in the population dynamics of head lice and how it could affect the control strategies in the future.

First Determination of Pyrethroid Knockdown Resistance Alleles in Human Head Lice (Phthiraptera: Pediculidae) From Chile.

The infestation with the human ectoparasite, Pediculus humanus capitis (De Geer), is a common public health problem affecting schoolchildren worldwide. In Chile, the main active ingredients present in the over-the-counter pediculicides contain pyrethroids. Despite the extended use of these products, there is no evidence of the insecticide resistance status of the head lice geographically located in Chile. The most extended resistant mechanism of pyrethroids consists of the target site insensitivity (Kdr) determined by the presence of mutations linked to insecticide-binding sites in the voltage-sensitive sodium channel. T917I is recognized as the main mutation in head lice, and detection is considered to be a biomarker of resistance. The goal of the present study was to detect the presence and distribution of T917I mutation in five geographic locations of Chile. All five geographically selected louse populations had a frequency of pyrethroid resistance genes that ranged from 36 to 77%, and 94.9% of the collected head lice had one or two T917I mutant alleles. Moreover, the frequency of the aggregate resistant alleles was 50.5%. This is the first evidence that head lice in Chile had the mutations commonly associated with the resistance to pyrethroids. Moreover, the overrepresentation of heterozygotes in the studied populations suggests that head lice in Chile are currently under active selective pressure.

The Sensory Machinery of the Head Louse Pediculus humanus capitis: From the Antennae to the Brain.

Insect antennae are sophisticated sensory organs, usually covered with sensory structures responsible for the detection of relevant signals of different modalities coming from the environment. Despite the relevance of the head louse Pediculus humanus capitis as a human parasite, the role of its antennal sensory system in the highly dependent relation established with their hosts has been barely studied. In this work, we present a functional description of the antennae of these hematophagous insects by applying different approaches, including scanning electron microscopy (SEM), anterograde antennal fluorescent backfills, and behavioral experiments with intact or differentially antennectomized lice. Results constitute a first approach to identify and describe the head louse antennal sensilla and to determine the role of the antenna in host recognition. SEM images allowed us to identify a total of 35-40 sensilla belonging to seven different morphological types that according to their external architecture are candidates to bear mechano-, thermo-, hygro-, or chemo-receptor functions. The anterograde backfills revealed a direct neural pathway to the ipsilateral antennal lobe, which includes 8-10 glomerular-like diffuse structures. In the two-choice behavioral experiments, intact lice chose scalp chemicals and warm surfaces (i.e., 32°C) and avoided wet substrates. Behavioral preferences disappeared after ablation of the different flagellomeres of their antenna, allowing us to discuss about the location and function of the different identified sensilla. This is the first study that integrates morphological and behavioral aspects of the sensory machinery of head lice involved in host perception.

Insights About Head Lice Transmission From Field Data and Mathematical Modeling.

Head lice infest millions of school-age children every year, both in developed and developing countries. However, little is known about the number of lice transferred among children during school activities, because direct methods to study this are almost impossible to implement. This issue has been addressed following an indirect method, which consist in collecting data of real infestation from several children groups and using a mathematical model of lice colonies to infer how the infestation observed might have evolved. By determining the events that would most likely lead to infestations as those observed, we find that severe infestations are most likely initiated by a relatively large number of lice transferred at the same moment or within relatively short time spans. In turn, analysis of the data obtained from screenings of the same groups of children a few days apart shows evidence of such transmission events. Interestingly, only children with severe infestations could harbor the lice necessary for this type of transmission. Thus, they play the same role as 'superspreaders' in epidemiology. As part of our experimental study it is also shown that a simple procedure of combing can be very effective to remove all mobile lice, and thus could be used as an effective preventive measure against those severe infestations that are responsible for the spread of pediculosis.

Ivermectin lipid-based nanocarriers as novel formulations against head lice.

The use of pyrethroids to control the human head louse, Pediculus humanus capitis De Geer (Anoplura: Pediculidae), has suffered considerable loss of efficacy due to the evolution of resistance. Thus, the development of efficiently insecticide delivery systems is imperative for the control of head lice. We studied the insecticidal activity of ivermectin-loaded lipid nanocapsules (IVM-LNC) against permethrin-resistant head lice from Argentina. The LNC, prepared by a phase inversion procedure, were characterized in terms of size, surface potential, and physical stability. These nanoparticles were nearly spherical with mean diameters of 55 nm and narrow size distribution (PI ≤ 0.2). The KT50 mortality values of head lice after exposure to two IVM-LNC formulations (0.11 and 0.28%) were significantly smaller (5 and 3 h, respectively) compared to those exposed only to LNC control group (8 h). This investigation showed the effectiveness in the encapsulation of ivermectin (IVM) into stable LNC dispersion with a potential clinical activity against head lice.

Novel polymeric micelles for insect pest control: encapsulation of essential oil monoterpenes inside a triblock copolymer shell for head lice control.

BACKGROUND: Essential oil components (EOCs) are molecules with interesting application in pest control, these have been evaluated against different insect pest from more than 100 years, but their practical use is rather limited. Thus, the enhancement of their bioavailability and manageability due to their dispersion in water can open new perspective for the preparation of formulations for the control of insect pest. In this work, we studied the encapsulation of different monoterpenes in a poloxamer shell in order to prepare aqueous formulations that can be used for the development of platforms used in pest control. METHODS: Micellar systems containing a 5 wt% of poloxamer 407 and 1.25 wt% of the different monoterpenes were prepared. Dynamic Light Scattering (DLS) experiments were carried out to characterize the dispersion of the EOCs in water. The pediculicidal activity of these micellar systems was tested on head lice using an ex vivo immersion test. RESULTS: The poloxamers allowed the dispersion of EOCs in water due to their encapsulation inside the hydrophobic core of the copolymer micelles. From this study, we concluded that it is possible to make stable micellar systems containing water (>90 wt%), 1.25 wt% of different monoterpenes and a highly safe polymer (5wt% Poloxamer 407). These formulations were effective against head lice with mortality ranging from 30 to 60%, being the most effective emulsions those containing linalool, 1,8-cineole, α-terpineol, thymol, eugenol, geraniol and nonyl alcohol which lead to mortalities above 50%. DISCUSSION: Since these systems showed good pediculicidal activity and high physicochemical stability, they could be a new route for the green fabrication of biocompatible and biosustainable insecticide formulations.

Cytogenetic Features of Human Head and Body Lice (Phthiraptera: Pediculidae).

The genus Pediculus L. that parasitize humans comprise two subspecies: the head lice Pediculus humanus capitis De Geer and the body lice Pediculus humanus humanus De Geer. Despite the 200 yr of the first description of these two species, there is still a long debate about their taxonomic status. Some authors proposed that these organisms are separate species, conspecifics, or grouped in clades. The sequencing of both forms indicated that the difference between them is one gene absent in the head louse. However, their chromosomal number remains to be determined. In this study, we described the male and female karyotypes, and male meiosis of head and body lice, and examined the chromatin structure by means of C-banding. In P. h. humanus and P. h. capitis, the diploid chromosome complement was 2 n = 12 in both sexes. In oogonial prometaphase and metaphase and spermatogonial metaphase, it is evident that chromosomes lack of a primary constriction. No identifiable sex chromosomes or B chromosomes were observed in head and body lice. Neither chiasmata nor chromatin connections between homologous chromosomes were detected in male meiosis. The meiotic behaviour of the chromosomes showed that they are holokinetic. C-banding revealed the absence of constitutive heterochromatin. Our results provide relevant information to be used in mapping studies of genes associated with sex determination and environmental sensing and response.

Is imidacloprid an effective alternative for controlling pyrethroid-resistant populations of Triatoma infestans (Hemiptera: Reduviidae) in the Gran Chaco ecoregion?

The prevention of Chagas disease is based primarily on the chemical control of Triatoma infestans (Klug) using pyrethroid insecticides. However, high resistance levels, correlated with control failures, have been detected in Argentina and Bolivia. A previous study at our laboratory found that imidacloprid could serve as an alternative to pyrethroid insecticides. We studied the delayed toxicity of imidacloprid and the influence of the blood feeding condition of the insect on the toxicity of this insecticide; we also studied the effectiveness of various commercial imidacloprid formulations against a pyrethroid-resistant T. infestans population from the Gran Chaco ecoregion. Variations in the toxic effects of imidacloprid were not observed up to 72 h after exposure and were not found to depend on the blood feeding condition of susceptible and resistant individuals. Of the three different studied formulations of imidacloprid on glass and filter paper, only the spot-on formulation was effective. This formulation was applied to pigeons at doses of 1, 5, 20 and 40 mg/bird. The nymphs that fed on pigeons treated with 20 mg or 40 mg of the formulation showed a higher mortality rate than the control group one day and seven days post-treatment (p < 0.01). A spot-on formulation of imidacloprid was effective against pyrethroid-resistant T. infestans populations at the laboratory level.

Is imidacloprid an effective alternative for controlling pyrethroid-resistant populations of Triatoma infestans (Hemiptera: Reduviidae) in the Gran Chaco ecoregion?

The prevention of Chagas disease is based primarily on the chemical control of Triatoma infestans (Klug) using pyrethroid insecticides. However, high resistance levels, correlated with control failures, have been detected in Argentina and Bolivia. A previous study at our laboratory found that imidacloprid could serve as an alternative to pyrethroid insecticides. We studied the delayed toxicity of imidacloprid and the influence of the blood feeding condition of the insect on the toxicity of this insecticide; we also studied the effectiveness of various commercial imidacloprid formulations against a pyrethroid-resistant T. infestans population from the Gran Chaco ecoregion. Variations in the toxic effects of imidacloprid were not observed up to 72 h after exposure and were not found to depend on the blood feeding condition of susceptible and resistant individuals. Of the three different studied formulations of imidacloprid on glass and filter paper, only the spot-on formulation was effective. This formulation was applied to pigeons at doses of 1, 5, 20 and 40 mg/bird. The nymphs that fed on pigeons treated with 20 mg or 40 mg of the formulation showed a higher mortality rate than the control group one day and seven days post-treatment (p < 0.01). A spot-on formulation of imidacloprid was effective against pyrethroid-resistant T. infestans populations at the laboratory level.

Influence of the formulations in removing eggs of Pediculus humanus capitis (Phthiraptera: Pediculidae).

Head lice lay eggs in human head hairs in order to reproduce. There is a difficulty associated to the process of detaching these eggs: they are tightly gripped to the hair by a secretion produced by female head lice. The physical removal of eggs has become an important part of treatment of louse infestations. The finding of new products to loosen the eggs is necessary to avoid mistaken diagnosis or reinfestations. This work aimed to compare different kinds of pediculicide formulations in order to find if their presentations represented differences in the egg remover effect. We also wanted to present a new device to test the efficacy of the egg remover formulations. Products with creamy presentations (Bio infant lice and egg remover and hair conditioner) and one containing dimethicone (Nyda) showed the lower mean forces compared with the control (lower mean forces represented best removal activity). Whereas, the Biferdil egg remover (gel) and Nopucid Tribit (hydroalcoholic lotion) had no egg removal effect, presenting the highest mean forces (177.82 and 189.99 mN, respectively) compared with the control. Additionally, we proposed a removal index (RI) to compare the efficacy of different products on the egg removal activity (RI > 0, good performance). The higher index values were for Bio infant lice and egg remover (0.72) and Biferdil hair conditioner (0.58). The lowest index values were for Biferdil egg remover (-0.26) and Nopucid Tribit (-0.35).The formulation of over the counter pediculicides in the egg remover effect was discussed.

Geographical distribution of pyrethroid resistance allele frequency in head lice (Phthiraptera: Pediculidae) from Argentina.

The human head louse, Pediculus humanus capitis De Geer (Phthiraptera: Pediculidae), is an obligate ectoparasite that causes pediculosis capitis and has parasitized humans since the beginning of humankind. Head louse infestations are widespread throughout the world and have been increasing since the early 1990s partially because of ineffective pediculicides. In Argentina, the overuse of products containing pyrethroids has led to the development of resistant louse populations. Pyrethroid insecticides act on the nervous system affecting voltage-sensitive sodium channels. Three point mutations at the corresponding amino acid sequence positions M815I, T917I, and L920F in the voltage-gated sodium channel gene are responsible for contributing to knockdown resistance (kdr). The management of pyrethroid resistance requires either early detection or the characterization of the mechanisms involved in head louse populations. In the current study, we estimated the distribution of kdr alleles in 154 head lice from six geographical regions of Argentina. Pyrethroid resistance kdr alleles were found in high frequencies ranging from 67 to 100%. Of these, 131 (85.1%) were homozygous resistant, 13 (8.4%) were homozygous susceptible, and 10 (6.5%) were heterozygous. Exact tests for the Hardy-Weinberg equilibrium for each location showed that genotype frequencies differed significantly from expectation in four of the six sites studied. These results show that pyrethroid resistance is well established reaching an overall frequency of 88%, thus close to fixation. With 30 yr of pyrethroid-based pediculicides use in Argentina, kdr resistance has evolved rapidly among these head louse populations.

Toxicity of non-pyrethroid insecticides against Triatoma infestans (Hemiptera: Reduviidae).

Triatoma infestans (Klug) is the main vector of Chagas disease, which is a public health concern in most Latin American countries. The prevention of Chagas disease is based on the chemical control of the vector using pyrethroid insecticides. In the last decade, different levels of deltamethrin resistance have been detected in certain areas of Argentina and Bolivia. Because of this, alternative non-pyrethroid insecticides from different chemical groups were evaluated against two T. infestans populations, NFS and El Malá, with the objective of finding new insecticides to control resistant insect populations. Toxicity to different insecticides was evaluated in a deltamethrin-susceptible and a deltamethrin-resistant population. Topical application of the insecticides fenitrothion and imidacloprid to first nymphs had lethal effects on both populations, producing 50% lethal dose (LD50) values that ranged from 5.2-28 ng/insect. However, amitraz, flubendiamide, ivermectin, indoxacarb and spinosad showed no insecticidal activity in first instars at the applied doses (LD50 > 200 ng/insect). Fenitrothion and imidacloprid were effective against both deltamethrin-susceptible and deltamethrin-resistant populations of T. infestans. Therefore, they may be considered alternative non-pyrethroid insecticides for the control of Chagas disease.

Toxicity of non-pyrethroid insecticides against Triatoma infestans (Hemiptera: Reduviidae)

Triatoma infestans (Klug) is the main vector of Chagas disease, which is a public health concern in most Latin American countries. The prevention of Chagas disease is based on the chemical control of the vector using pyrethroid insecticides. In the last decade, different levels of deltamethrin resistance have been detected in certain areas of Argentina and Bolivia. Because of this, alternative non-pyrethroid insecticides from different chemical groups were evaluated against two T. infestans populations, NFS and El Malá, with the objective of finding new insecticides to control resistant insect populations. Toxicity to different insecticides was evaluated in a deltamethrin-susceptible and a deltamethrin-resistant population. Topical application of the insecticides fenitrothion and imidacloprid to first nymphs had lethal effects on both populations, producing 50% lethal dose (LD50) values that ranged from 5.2-28 ng/insect. However, amitraz, flubendiamide, ivermectin, indoxacarb and spinosad showed no insecticidal activity in first instars at the applied doses (LD50 > 200 ng/insect). Fenitrothion and imidacloprid were effective against both deltamethrin-susceptible and deltamethrin-resistant populations of T. infestans. Therefore, they may be considered alternative non-pyrethroid insecticides for the control of Chagas disease.

Comparative efficacy of new commercial pediculicides against adults and eggs of Pediculus humanus capitis (head lice).

The use of pyrethroids to control head louse infestations have suffered considerable loss of efficacy due to the development of resistance. In the last past years, several new alternative products to synthetic pyrethroids have been developed and are sold in the Argentinean market against head lice. The present study investigated the efficacy of two new Argentinean products Nopucid Qubit® and Nopucid Bio Citrus® and its comparison with two reference products Nyda® and Hedrin®. Nopucid Qubit® is a two-phase lotion containing geraniol and citronellol (phase 1) and ciclopentaxiloxane (phase 2); while Nopucid Bio Citrus® contains dimethicone, ciclopentaxiloxane, and bergamot essential oil. These products are physically acting compounds. The sensitivity of two laboratory assays for testing insecticide activity of new formulations was also compared. Mortality (100%) of motile forms occurred after they were exposed to any product for 1 and 2 min, either by in vitro or ex vivo test. Concerning ovicidal activity, the most effective pediculicides were Nopucid Bio Citrus® and Nyda®, followed by Hedrin® and Nopucid Qubit®. The present study revealed, for the first time, the efficacy of over-the-counter commercial pediculicides available in Argentine (Nopucid Bio Citrus® and Nopucid Qubit®) on either motile stages or eggs against head lice.

Insecticide resistance of house fly, Musca domestica (L.) from Argentina.

The status of resistance to cyromazine, 2,2-dichlorovinyl dimethyl phosphate (DDVP), and permethrin relative to field populations of the house fly, Musca domestica L. from Argentinean poultry farms was studied. All the three studied populations (SV, Q, and C) showed resistant ratios (RRs) to cyromazine of 3.9, 10.98, and 62.5, respectively. We observed high levels of resistance toward the organophosphate DDVP and permethrin. The RRs to DDVP ranged from 45.4 to 62.5. No significant differences were found among the studied populations. All the house fly populations were permethrin-resistant, in comparison with the susceptible strain. Two of the analyzed populations (SV and Q) differed significantly in toxicity to the population C. This is the first evidence that house flies from Argentina showed a multi-resistance pattern. The implementation of an insecticide monitoring program on poultry farms of Argentina is needed to prevent field control failures. Furthermore, integrated control strategies are needed to delay detrimental development of insecticide resistance.

Differential patterns of insecticide resistance in eggs and first instars of Triatoma infestans (Hemiptera: Reduviidae) from Argentina and Bolivia.

Previous work at our laboratory has indicated high resistance levels to deltamethrin correlated with failures of chemical control in field populations of Triatoma infestans (Klug, 1834) (Hemiptera: Reduviidae) in northern Argentina and southern Bolivia. The aim of the present work was to study the resistance patterns in eggs and first instars of T. infestans in populations from Argentina and Bolivia. At the egg stage, a population from Salvador Mazza, Argentina, showed the highest resistance ratio to deltamethrin and lambda-cyhalothrin, and it was susceptible to fipronil and fenitrothion. A population from Mataral, Bolivia, showed very low resistance ratios to deltamethrin and lambda-cyhalothrin, and it was susceptible to fipronil and fenitrothion. A Sucre population was susceptible to deltamethrin and fenitrothion, and it showed very low resistant ratios to lambdacyhalothrin and fipronil. A Yacuiba population was susceptible to deltamethrin. At the first instar, the Salvador Mazza population was susceptible to fipronil, whereas the Mataral and Sucre populations were susceptible to fenitrothion, and they showed very low resistance ratios to lambda-cyhalothrin but the high resistance to fipronil. The Salvador Mazza population was resistant to deltamethrin at the larval stage. Remarkable differences were found in the resistance profile to fipronil in first instars and eggs from Sucre and Mataral. These indicated that the expression of insecticide resistance in eggs varies between populations and that the pyrethroid resistance diagnosed in T. infestans first instars is not indicative of resistance in the egg stage.

Interspecific hybridization of Eucalyptus as a potential tool to improve the bioactivity of essential oils against permethrin-resistant head lice from Argentina.

The essential oils extracted from Eucalyptus grandis, Eucalyptus camaldulensis, Eucalyptus tereticornis, and the hybrids E. grandisxE. camaldulensis, and E. grandisxE. tereticornis were analyzed by GC-MS, and evaluated for their fumigant and repellent effects on permethrin-resistant head lice. Fumigant activity of both hybrids was higher than that for pure species. E. grandisxE. tereticornis and E. grandisxE. camaldulensis showed KT50 values of 12.99 and 13.63min, respectively. E. grandis, E. camaldulensis, and E. tereticornis showed KT50 values of 25.57, 35.01, and 31.31, respectively. A simple regression analysis revealed a significant correlation between KT50 data and % of 1,8-cineole in these essential oils. Repellency varied from 47.80+/-16% to 80.69+/-6% for the five Eucalyptus essential oils tested. Interspecific hybridization improves the pediculicidal activity of Eucalyptus essential oils.

Fumigant and repellent properties of essential oils and component compounds against permethrin-resistant Pediculus humanus capitis (Anoplura: Pediculidae) from Argentina.

The repeated use of permethrin and other insecticides for the control of head lice, Pediculus humanus capitis De Geer (Anoplura: Pediculidae), during past decades has resulted in the development of marked levels of resistance. Thus, new alternative insecticides are needed for the control of head lice. We studied the fumigant and repellent properties of essential oils from 16 native and exotic plants in Argentina, and 21 chemical components against permethrin-resistant head lice from Argentina. With a direct vapor-exposure bioassay, the most effective oil was from the native Myrcianthes cisplatensis Cambess (Myrtaceae) with a time to 50% knockdown (KT50) of 1.3 min, followed by exotic species, Eucalyptus cinerea F.V. Muell., Eucalyptus viminalis Labill., and Eucalyptus saligna Smith. with KT50 values of 12.0, 14.9, and 17.4 min, respectively. The most effective components were 1,8-cineole and anisole, with KT50 values of 11.1 and 12.7 min, respectively. Regression analysis of KT50 values and vapor pressures and water-partition coefficients for the essential oil components revealed that the most effective fumigants were among the more volatile components. Repellency assays indicated that the essential oil from Mentha pulegium L. and its benzyl alcohol component were the most effective repellents, having repellency indices of 75.5 and 57.8%, respectively. Thus, some Argentinean plants contain essential oils and components that function as fumigants or as repellents and thereby show potential for development of new control products for head lice.