Completion of the sequence of the nuclear ribosomal DNA subunit of Simulium sanctipauli,with descriptions of the 18S, 28S genes and the IGS

We describe the IGS-ETS, 18S and 28S ribosomal gene sequences of Simulium sanctipauli Vajime & Dunbar, a member of the S. damnosum Theobald (Diptera: Simuliidae) complex of blackflies (Diptera: Simuliidae). These regions, together with the ITS-1, ITS-2 and 5.8S rDNA presented elsewhere (accession number U36206), constitute the composite sequence of the entire rDNA unit, making S. sanctipauli the second dipteran species of medical importance for which the entire rDNA has been sequenced. Despite the lack of sequence identity, the IGS of S. sanctipauli showed some structural similarities to other Diptera, i.e. the mosquito Aedes albopictus Skuse (Culicidae), the fruitfly Drosophila melanogaster Meigen (Drosophilidae) and the tsetse Glossina (Glossinidae). Two blocks of tandemly repeated subunits were present in the IGS of S. sanctipauli and, unlike other species of Diptera, they contained no duplications of promoter-like sequences. However, two promoter-like sequences were identified in the unique DNA stretches of the IGS by their sequence similarity to the promoter of Aedes aegypti L. (Diptera: Culicidae). The observed sequence variation can be explained, as in the case of Drosophila spp., by the occurrence of slippage-like and point mutation processes, with unequal crossing-over homogenizing (to a certain extent) the region throughout the gene family and blackfly population. The 18S and 28S rDNA genes show more intraspecific variability within the expansion segments than in the core regions. This is also the case in the interspecific comparison of these genes from S. sanctipauli with those of Simulium vittatum, Ae. albopictus and D. melanogaster. This pattern is typical of many eukaryotes and likely to be the result of a more relaxed functional selection in the expansion segments than on the core regions. The A + T content of the S. sanctipauli genes is high and similar to those of other Diptera. This could be the result of a change in the mutation pressure towards AT in the Diptera lineage.     

Elimination of the Djodji form of the blackfly Simulium sanctipauli sensu stricto as a result of larviciding by the WHO Onchocerciasis Control Programme in West Africa

Cytotaxonomic identifications of larvae of members of the Simulium damnosum Theobald (Diptera: Simuliidae) complex collected in forest zones of southeast Ghana and southwest Togo between 1977 and 1996 showed that the Djodji form of Simulium sanctipauli Vajime & Dunbar, a vector of onchocerciasis, was eliminated in 1988 by larvicide operations conducted by the World Health Organization (WHO) Onchocerciasis Control Programme (OCP) in West Africa. No members of the form were identified amongst 997 larvae collected up to 8 years after systematic control operations began in February 1988. The results are discussed in relation to estimates of the numbers of samples required to certify elimination and the possibility that other members of the S. damnosum complex were also eliminated by the OCP.     

Efficiency of Simulium sanctipauli as a vector of Onchocerca volvulus in the forest zone of Ghana

The role of Simulium sanctipauli Vajime & Dunbar (Diptera: Simuliidae) as a vector of Onchocerca volvulus (Leuckart) (Spirurida: Onchocercidae) in the forest zone of central Ghana was studied in the Upper Denkyira district, where onchocerciasis is hyperendemic. Simulium sanctipauli was found to be a highly efficient vector, with a mean of 377 infective (L3) larvae in the heads of 1000 parous and 122 in the heads of 1000 biting flies. The overall infection rate of 44% of the parous flies with L1, L2 and L3 stages of O. volvulus (identity confirmed by polymerase chain reaction) demonstrates marked anthropophily. Female flies dispersed over a wide area and can transmit onchocerciasis up to at least 10 km away from their breeding sites. Annual community-directed treatments with ivermectin did not have a noticeable effect on the infection rates and parasitic loads of fly populations, which were as high 2 months after as 3 months before the distribution of ivermectin. This failure can be attributed to poor coverage, with treatment taken by only 24.4% of the population of the six study villages.     

Field and laboratory studies on water conditions affecting the potency of VectoBac (Bacillus thuringiensis serotype H-14) against larvae of the blackfly, Simulium damnosum

River water conditions that might influence the efficacy of VectoBac, a formulation of the microbial insecticide Bacillus thuringiensis H-14 Berliner against Simulium damnosum sensu lato Theobald (Diptera: Simuliidae) larvae were investigated. A standard formulation was assayed 130 times over 15 months using a mini-gutter system at a field station beside the River Pra in Ghana. The lethal concentration (LC) values, river temperature, conductivity, turbidity and pH were analysed using univariate and multivariate statistics to identify which of these parameters influenced its performance. River temperature, conductivity and turbidity (in that order) were identified as having direct effects on the potency of VectoBac. Water temperature and conductivity were negatively correlated, whereas turbidity and pH were positively correlated with LC values. Analyses of river water samples revealed that despite observed differences in total solids, sodium and potassium cations and chloride concentrations, all the parameters measured did not differ significantly between wet and dry seasons. A simple method for rearing S. damnosum s.l. in the laboratory was then adopted to study the effect of conductivity on potency of VectoBac under controlled conditions. Increasing the conductivity of the water medium up to 3,000 microS enhanced potency by about 42%, whereas increasing that of the insecticide alone raised it by 37%. The results obtained suggest that for effective use of VectoBac for blackfly control in West Africa, river temperature, conductivity and turbidity should be taken into consideration, perhaps by only selecting rivers with optimal conditions for treatment. The laboratory-based system developed for assaying the product overcomes the vagaries associated with field conditions and also the demand for huge logistic requirements of the mini-gutter system, which has to be sited near rivers.     

Sibling species distributions of the Simulium damnosum complex in the West African Onchocerciasis Control Programme area during the decade 1984–93, following intensive larviciding since 1974

Abstract .During the decade from 1984 to 1993, nine species of the Simulium damnosum complex of blackflies (Diptera: Simuliidae) were identified from the area covered by the Onchocerciasis Control Programme. These were S. damnosum s.s., S. dieguerense, S. konkourense, S. leonense, S. sanctipauli, S. sirbanum, S. soubrense, S. squamosum, and S. yahense. Some of these species were found to consist of two chromosomal variant populations. These were S. konkourense‘Konkouré' and ‘Menankaya' forms, S. sanctipauli sensu stricto and‘Djodji' form, S. soubrense‘Chute Milo' and ‘Beffa' forms. The distribution of these twelve cytological taxa was assessed in relation to the two main vegetation zones of West Africa (forest and savanna), topography, river size and other factors. The range of each species was influenced by seasonal climatic changes in wind movement and river water level. The most widely distributed species were S. sirbanum and S. damnosum s.s., associated with savanna areas, recorded from all river basins. Simulium dieguerense was restricted mainly to Western Mali on the Rivers Bafing and Bakoye in the Senegal River basin. Simulium squamosum was identified from rivers draining mountainous areas in both the forest and savanna zones. Simulium yahense was found in small permanent rivers along a wide forested band parallel to the coast and was absent from the plains of Togo and Benin. Members of the S. sanctipauli subcomplex had restricted distributions except for S. sanctipauli s.s., which was widespread in large rivers of the forest zone from Sierra Leone to the Volta Lake in Ghana. Simulium soubrense‘Beffa' form occurred in Togo and Benin, S. soubrense‘Chutes Milo' form in Guinea, both ‘Konkouré' and ‘Menankaya' forms of S. konkourense occurred predominantly in Guinea and S. leonense in Sierra Leone. The relevance of the distribution maps and the importance of the data bank to vector control larvicidal operations are discussed.     

Evaluation of the impact of Bacillus thuringiensis serovar israelensis and Temephos,used for the control of Simulium (Chirostilbia) pertinax Kollar, 1832 (Diptera,Simuliidae) on the associated entomofauna,Paraty, state of Rio de Janeiro,Brazil

The study was set up to evaluate the impact of two commercial larvicide formulations, Bacillus thuringiensis serovar israelensis base (Bti) at 15 ppm/1 min and temephos at 0.03 ppm of active ingredient, used to control Simulium pertinax populations, on associated non-target entomofauna occupying the same breeding sites. The experiments were carried out on the Pedra Branca and Muricana rivers, on the slopes of Serra do Mar massif, municipality of Paraty, state of Rio de Janeiro, Brazil. Bti was applied to the river Pedra Branca and temephosto theriver Muricana. On both rivers, treatment and control sections were labeled as such, each one with two observation posts: slow moving water and fast water regions respectively. Artificial substrata was used to evaluate the abundance of associated entomofauna. Attached immature stages of arthropods were removed from both of its surfaces fortnightly. Were collected, from the two rivers, 28 477 specimens of the entomofauna associated with S. pertinax. The families Hydropsychidae, Chironomidae, Bactidae, Simuliidae, Blephariceridae and Megapodagrionidae were represented. These was an impact of temephos on the entomofauna associated with S. pertinax only in Simuliidae and Chironomidae, and to Bti only in Simuliidae. However, the reduction in their numbers was not statistically significant.     

Management of insecticide resistance in control of the Simulium damnosum complex by the Onchocerciasis Control Programme, West Africa: potential use of negative correlation between organophosphate resistance and pyrethroid susceptibility

1. Resistance of some populations of the Simulium damnosum complex to temephos (100-fold at the LC50 level), with degrees of cross-resistance to chlorphoxim (14-fold) and other organophosphate insecticides, follows intensive larvicidal control of S. damnosum s.l. in West African river systems since 1975 by the WHO Onchocerciasis Control Programme. 2. Larvae of at least three sibling species of the S. damnosum complex have become organophosphate-resistant: these are the forest species S. sanctipauli Vajime & Dunbar and the savanna species S. sirbanum V. & D. and S. damnosum Theobald sensu stricto. 3. Organophosphate-resistant S. damnosum s.l. larvae show increased susceptibility to some organochlorine and pyrethroid insecticides, especially to permethrin (up to 11-fold) and OMS 3002 (up to 17-fold), as compared with organophosphate-susceptible populations. 4. This differential susceptibility is reflected by increased pyrethroid efficacy in operational use for river treatments against organophosphate-resistant field populations of S. damnosum s.l. larvae. Treatment of 100 km of the lower Bandama River in 1985 showed that permethrin at the highly selective dosage of 10 min exposure to 0.01 mg/l caused reversion towards organophosphate-susceptibility of the target population of S. sanctipauli. This effect was less pronounced when the Comoe River was treated at the lower dosage of 0.005 mg/l for 10 min. 5. To overcome temephos-resistance, it is proposed that the most rational usage of currently available larvicides would involve the following annual sequence of treatments: Bacillus thuringiensis serotype H-14 when river discharge is below 75 m3/s; chlorphoxim for about eight weekly treatment cycles after river discharge rises; permethrin (or alternative pyrethroid) for up to six treatment cycles--this should eliminate any incipient selection for chlorphoxim-resistance; resume chlorphoxim (or perhaps carbosulfan) treatments until river discharge falls below 75 m3/s permitting resumed use of B.t. H-14.     

DNA in situ hybridization on polytene chromosomes of Simulium sanctipauli at loci relevant to insecticide resistance

A DNA technique for in situ hybridization developed by Kumar & Collins (1994) for use on polytene chromosomes of adult Anopheles mosquitoes (Diptera: Culicidae) was modified for use with Simulium larval salivary gland chromosomes (Diptera: Simuliidae). Cloned fragments of several Simulium genes (coding for aspartate amino transferase, cytochrome P450 and DNA polymerase) were successfully mapped physically by assigning specific band locations in Simulim sanctipauli V. & D. This represents the first attempt at locating genes beyond the resolution of linkage to inversions in any blackfly species.     

Assessment of rDNA IGS as a molecular marker in the Simulium damnosum complex

For five cytospecies of the Simulium damnosum Theobald complex of blackflies (Diptera: Simuliidae) from West Africa, both ends of the intergenic spacer region (IGS) of the rDNA have been sequenced with the aim of developing specific molecular markers. No specific differences in these two regions were detected between Simulium sanctipauli V. & D., Simulium sirbanum V. & D., Simulium soubrense V. & D., Simulium squamosum Enderlein and Simulium yahense V. & D., except in the number of A subrepeats at the 5' end of the IGS (two in S. squamosum and four or five in the others) and in position 310 of the 3' end (a C in S. squamosum and a G in the others). However, genetic distances within and between species overlapped. These DNA sequences had no strong phylogenetic signal, and the trees obtained were mostly unresolved. Although most sequences from S. squamosum clustered together, a few of them were more similar to those in other cytospecies. These results could be explained either by hybridization with genetic introgression or by ancestral polymorphism and recent speciation.     

An epizootic of patent iridescent virus disease in multiple species of blackflies in Chiapas, Mexico

Simulium blackfly larvae (Diptera: Simuliidae) were collected from rivers and streams at 500-1500 m a.s.l. in Chiapas State of southern Mexico. Among 45 sites surveyed over an area of 2300 km2 (around 15 degrees 15'N 92 degrees 20'W), some Simulium larvae from three sites were opalescent violet-blue, interpreted as patent infection with invertebrate iridescent virus (IIV). Dissection confirmed the presence of putative Iridovirus particles, 130nm diameter, but no IIV isolates were obtained from homogenates injected into Galleria mellonella (L) larvae (Lepidoptera: Pyralidae). All Simulium with patent IIV infection died before metamorphosis, whereas approximately 60% of asymptomatic Simulium survived to adulthood in the laboratory. During 1997, standard monthly samples from two parallel rivers 42-50 km north-west of Tapachula comprised the following species proportions (and rates of patent IIV infection): 41.8% (47%) Simulium mexicanum Bellardi complex, 31.3% (31.4%) S. rubicundum Knab, 10.1% (13.1%) S. paynei, 6.5% (2.9%) S. callidum (Dyar & Shannon), 6.3% (5.1%) S. ochraceum Walker complex, 3.1% (0.7%) S. downsi Vargas et al., 0.7% S. samboni Jennings and 0.2% S. metallicum Bellardi complex, showing a strong correlation between blackfly abundance and the prevalence of patent infection. An epizootic of IIV in January and February (infection rates 41-100%) was followed by absence of larvae (March-August) until the end of the rainy season, when numbers collected on nylon strings rose to approximately 1/cm with patent IIV infection rates of 0-12.5% during September-December. Further investigations are underway to isolate this IIV and assess its potential usefulness for biological control of Simulium pests and vectors of onchocerciasis.     

Sex chromosome variation and cytotaxonomy of the onchocerciasis vector Simulium squamosum in Cameroon and Nigeria

On the basis of sex chromosome variation, three cytotypes of Simulium squamosum (Enderlein) (Diptera: Simuliidae) are described from Cameroon and Nigeria. Simulium squamosum A is the typical form as originally described by Vajime & Dunbar (1975) with chromosome I as the sex chromosome. It occurs throughout most of Cameroon and south-east Nigeria. A second cytotype, S. squamosum B, is described from the river Sanaga (Cameroon). It also has chromosome I as the sex chromosome, but the nature of the sex differential region is different. Simulium squamosum C has no sex-linked chromosomal rearrangements. It is widespread in Nigeria and occurs near Mount Cameroon, where it seems to hybridize with S. squamosum A.     

Revision of the Ketaketa subcomplex of blackflies of the Simulium damnosum complex

A revision of the taxonomy of the Ketaketa subcomplex of the Simulium damnosum Theobald complex (Diptera: Simuliidae) is presented including new material from Tanzania, Malawi and South Africa. The cytotaxonomy, morphology and molecular identity of known and new taxa are described. The Ketaketa subcomplex is cytotaxonomically defined by the paracentric inversion 1L-7. We recognize three sibling species, namely Simulium latipollex (Enderlein), Simulium plumbeum Krueger, sp.n. and Simulium kipengere Krueger, sp.n., the latter comprising three cytoforms: 'Typical', 'Linthipe' and 'Mombo'. The cytoforms 'Mwamphanzi', 'Ketaketa' and 'Hammerkopi' are synonymized with S. plumbeum. Identification keys are provided on the basis of chromosomal and morphological characters. In view of their potential role as vectors of human onchocerciasis (river blindness) we also discuss the possible medical importance of the different cytoforms and their geographical distribution.     

A new species of the Simulium damnosum complex from Uganda, and comparative morphology of the tarsal claws in females of this complex

The female, male, pupa and larva of Simulium (Edwardsellum) pandanophilum sp. nov. (Diptera: Simuliidae) from western Uganda are described. Diagnostic features are the shape of the tarsal claws of females and, cytogenetically, new intraspecific inversions on chromosomes I and II. Simulium pandanophilum is assigned to the Simulium damnosum complex. Different tarsal claw shapes of West African members of the S. damnosum complex are also described.     

The Beffa form of Simulium soubrense of the S.damnosum complex in Togo and Benin

The Beffa form of Simulium soubrense Vajime & Dunbar, a member of the S. sanctipauli sub-complex of the S. damnosum complex, was found breeding throughout rivers in the Togo-Benin Gap, as far north as 9 degrees 30'N. Its distribution changed with the season. In southern Togo there were seasonal fluctuations in the relative abundancies of the Beffa form and of S. damnosum/S.sirbanum. There was considerable temporal and regional variation in the frequencies of different colour morphs of adult flies. The flies in Benin tended to be darker. Infections with Onchocerca volvulus (Leuckart) appeared to be independent of the host's colour morph category. Larger flies harboured significantly more first stage Onchocerca larvae but no significant relations with fly size were found for second and third stage larvae.     

Anthropophilic blackflies (Diptera: Simuliidae) and onchocerciasis transmission in southwest Ethiopia

Abstract. From May 1984 to May 1985, the transmission of onchocerciasis by blackflies (Dipteria: Simuliidae) was studied around Gilgel Ghibe and Gojeb rivers in southwest Ethiopia. The predominant anthropophilic species of Simulium at both localities were S.(Edwardsellum) damnosum sensu law and S.(Lewisellum) ethiopiense. Some S.(Anasolen) dentulosum also landed on human bait at Gilgel Ghibe river.
Dissections of S. damnosum from Gilgel Ghibe and Gojeb revealed mean parous rates of 92% (n = 18,291) and 84% (n = 9530), respectively. S. ethiopiense from Gilgel Ghibe and Gojeb showed mean parous rates of 53% (n = 322) and 93% (n = 14), respectively. Of the parous S. damnosum , 1.3% at Gilgel Ghibe and 0.5% at Gojeb harboured infective third-stage larvae (L3) of Onchocerca volvulus (or morphologically indistinguishable from it). Unknown filariae of animal origin, indicative of zoophily, were found in 0.3% and 0.7% of Simulium damnosum complex females from Gilgel Ghibe and Gojeb, respectively. S. ethiopiense harboured developing (L,) larvae only, with 7.7% infection rate in both localities. In contrast, S. dentulosum did not harbour any filaria larvae. The annual infective biting rate (AIBR) and transmission potential (ATP) of the S. damnosum complex at Gilgel Ghibe river were 858 and 5478, respectively. The AIBR and ATP of S. damnosum s.I. at Gojeb river were 519.5 and 1963, respectively. These results emphasize the predominant role of the S. damnosum complex in the transmission of O. volvulus in southwest Ethiopia     

Blackfly vectors of zoonotic onchocerciasis in Japan

Studies of blackfly vectors of Onchocerca dewittei japonica Uni, Bain & Takaoka (Spirurida: Onchocercidae), a parasite of wild boar implicated in the aetiology of zoonotic onchocerciasis in Japan, and six other zoonotic Onchocerca species of this country are reviewed. Molecular identification of infective larvae found in wild‐caught female blackflies showed that Simulium bidentatum (Shiraki) (Diptera: Simuliidae) is a natural vector of O. dewittei japonica, and also Onchocerca sp. sensu Fukuda et al., another parasite of wild boar. Inoculation experiments demonstrated that Simulium arakawae Matsumura and four other Simulium species are putative vectors. Similarly, S. arakawae, S. bidentatum and Simulium oitanum (Shiraki) are putative vectors of Onchocerca eberhardi Uni & Bain and Onchocerca skrjabini Rukhlyadev, parasites of sika deer. Morphometric studies of infective larvae indicated that Onchocerca lienalis Stiles, a bovine species, is transmitted by S. arakawae, Simulium daisense (Takahasi) and Simulium kyushuense Takaoka, and that Onchocerca sp. sensu Takaoka & Bain, another bovine species, is transmitted by S. arakawae, S. bidentatum, S. daisense and S. oitanum. Prosimulium sp. (Diptera: Simuliidae) and Simulium japonicum Matsumura are suspected vectors of Onchocerca suzukii Yagi, Bain & Shoho and O. skrjabini [Twinnia japonensis Rubtsov (Diptera: Simuliidae) may also transmit the latter], parasites of Japanese serow, following detection of the parasites' DNA genes in wild‐caught blackflies.     

Mechanisms of resistance to DDT and pyrethroids in Patagonian populations of Simulium blackflies

Mixed populations of the pest blackflies Simulium bonaerense Coscarón & Wygodzinsky, S. wolffhuegeli (Enderlein) and S. nigristrigatum Wygodzinsky & Coscarón (Diptera: Simuliidae) are highly resistant to DDT and pyrethroids in the Neuquén Valley, a fruit-growing area of northern Patagonia, Argentina. As these insecticides have not been used for blackfly control, resistance is attributed to exposure to agricultural insecticides. Pre-treatment with the synergist piperonyl butoxide (PBO) reduced both DDT and fenvalerate resistance, indicating that resistance was partly due to monooxygenase inhibition. Pre-treatment with the synergist tribufos to inhibit esterases slightly increased fenvalerate toxicity in the resistant population. Even so, biochemical studies indicated almost three-fold higher esterase activity in the resistant population, compared to the susceptible. Starch gel electrophoresis confirmed higher frequency and staining intensity of esterase electromorphs in the resistant population. Incomplete synergism against metabolic resistance indicates additional involvement of a non-metabolic resistance mechanism, such as target site insensitivity, assumed to be kdr-like in this case. Glutathione S-transferase activities were low and inconsistent, indicating no role in Simulium resistance. Knowing these spectra of insecticide activity and resistance mechanisms facilitates the choice of more effective products for Simulium control and permits better coordination with agrochemical operations.     

Oviposition pheromone in the Simulium damnosum complex: biological activity of chemical fractions from gravid ovaries

Abstract. Communal oviposition in the Afrotropical blackfly species complex Simulium damnosum Theobald (Diptera: Simuliidae) is mediated by a pheromone emitted by freshly laid eggs. Previously, two compounds (designated peaks A and B) emanating from fresh eggs were shown to be associated with attractiveness to gravid blackflies in bioassay. The present study investigated the role of these compounds by testing the responses of wild-caught Simulium yahense in Ghana to fractionated hexane extracts of gravid ovaries prepared by gas chromatography (GC). Although the fractions were prepared from Sierra Leonean Simulium leonense , GC analysis of the emissions from fresh S.yahense eggs showed that the volatile blends of both species were similar. When tested in a two-choice bioassay, 66% of ovipositing blackflies chose the substrate baited with a mixture of the four fractions recombined. In a series of bioassays testing responses to the four individual fractions presented with a control in a multiple-choice arrangement, only fraction 3 (containing peaks A and B) attracted significantly more ovipositions than the other fractions and control. However, fraction 3 failed to elicit a significant response when presented as the sole attractant with a control in a two-choice bioassay. It was concluded that fraction 3 , though mainly responsible for mediating aggregated oviposition by S.yahense , was acting in tandem with additional cues, probably further chemicals, which remain to be isolated and characterized.     

Molecular phytogeny and typing of blackflies (Diptera: Simuliidae) that serve as vectors of human or bovine onchocerciasis

Abstract. A subregion of the mitochondrial large subunit (16s) rRNA gene was amplified by polymerase chain reaction (PCR) from nine species of blackflies (Diptera: Simuliidae) which serve as natural or experimental vectors of human or bovine Onchocerca parasites. PCR products from each species of blackfly were tested by directed heteroduplex analysis (DHDA), and their genotypes established according to diagnostic banding patterns of the heteroduplex products. Three alleles of mitochondrial 16s rRNA were found to exist in members of the Simulium (Ewardsellum) damnosum sensu lato complex from West Africa, and two alleles were found in the Neotropical Simulium (Psilopelmia) ochraceum Walker complex and the Simulium (Simulium) metallicum Bellardi complex. Different single alleles were detected in Austrosimulium bancrofti , in English S.(S.)noelleri and in two North American laboratory vectors: Simulium (Psilozia) vittatum Zetterstedt and S.(S.)decorum Walker. Phylogenetic analysis of 16s sequences indicated that blackflies from West Africa and the Americas formed distinct clades. Neotropical onchocerciasis vectors were found to be more closely related to Nearctic and Palaearctic non-vector Simulium species than to the African vectors of onchocerciasis.     

Phosphoglucomutase and trehalase isoenzymes of Venezuelan simulium vectors of Onchocerca volvulus

Phosphoglucomutase (PGM) and trehalase (Tre) isoenzymes of five species of Simulium blackflies (Diptera: Simuliidae), vectors of onchocerciasis in Venezuela, were investigated by means of a portable electrophoresis field kit. Tre differed between S. incrustatum and S. oyapockense s.l. Electrophoretic variation of Tre in other members of the S. amazonicum and S. incrustatum groups merit further investigation. PGM appears to be more useful for separating populations within species complexes. Multiple populations and/or seasonal changes in population structure of S. guianense s.l., S. exiguum s.l. and S. metallicum s.l. were inferred from elecrophoretic variation of PGM.