Guillaume Ketoh

Insecticide resistance in field populations of Bemisia tabaci (Hemiptera: Aleyrodidae) in West Africa

BACKGROUND The tobacco whitefly, Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae), has developed a high degree of resistance to several chemical classes of insecticides throughout the world. To evaluate the resistance status in West Africa, eight insecticides from different chemical families were tested using the leaf-dip method on four field populations collected from cotton in Benin, Togo and Burkina Faso. RESULTS Some field populations showed a significant loss of susceptibility to pyrethroids such as deltamethrin [resistance ratio (RR) 3-5] and bifenthrin (RR 4-36), to organophosphates (OPs) such as dimethoate (RR 8-15) and chlorpyrifos (RR 5-7) and to neonicotinoids such as acetamiprid (RR 7-8) and thiamethoxam (RR 3-7). Bemisia tabaci was also resistant to pymetrozine (RR 3-18) and to endosulfan (RR 14-30). CONCLUSION The resistance of B. tabaci to pyrethroids and OPs is certainly due to their systematic use in cotton treatments for more than 30 years. Acetamiprid has been recently introduced for the control of whiteflies. Unfortunately, B. tabaci populations from Burkina Faso seem to be already resistant. Because cross-resistance between these compounds has never been observed elsewhere, resistance to neonicotinoids could be due to the presence of an invasive B. tabaci biotype recently detected in the region.

Susceptibility of the bruchid Callosobruchus maculatus (Coleoptera: Bruchidae) and its parasitoid Dinarmus basalis (Hymenoptera: Pteromalidae) to three essential oils.

Abstract The bruchid Callosobruchus maculatus (F.) causes major losses during the storage of seeds of Vigna unguiculata (Walp.) in West Africa. An endemic parasitoid, the pteromalid Dinarmus basalis (Rond.) reduces the increase in bruchid populations in stores and could be used for biological control. African farmers often introduce essential oils into granaries at harvest time. In Togo, essential oils were extracted from two Gramineae, Cymbopogon nardus (L.) and Cymbopogon schoenanthus (L.) and from a Lamiaceae, Ocimum basilicum (L.). The major components of these essential oils were citronellal in C. nardus, carene-2 and piperitone in C. schoenanthus and estragol in O. basilicum. Cymbopogon schoenanthus was the most toxic oil for C. maculatus adults. D. basalis adults were more susceptible to the three essential oils than the adults of their hosts C. maculatus. In the presence of cowpea seeds, the LC50s of the three essential oils were lower than in their absence, suggesting that the seeds may absorb a part of the volatiles. High doses of three essential oils slightly affected the survival of the fourth instar or the pupae of C. maculatus. This high survival was due to protection of larvae from volatiles by the surrounding seeds. The D. basalis were more affected by the oil volatiles than their hosts. Sub-lethal doses of essential oils reduced the duration of the adult life of both insect species and fecundity of the females. The differences in sensitivity of the host and its parasitoid could influence their population dynamics. The introduction of the essential oils into storage systems potentially could reduce density of parasitoid populations and increase seed losses.

Detection of genetically isolated entities within the Mediterranean species of Bemisia tabaci: new insights into the systematics of this worldwide pest

BACKGROUND The taxonomy of the species complex Bemisia tabaci, a serious agricultural pest worldwide, is not well resolved yet, even though species delimitation is critical for designing effective control strategies. Based on a threshold of 3.5% mitochondrial (mtCOI) sequence divergence, recent studies have identified 28 putative species. Among them, mitochondrial variability associated with particular symbiotic compositions (=cytotypes) can be observed, as in MED, which raises the question of whether it is a single or a complex of biological species. RESULTS Using microsatellites, an investigation was made of the genetic relatedness of Q1 and ASL cytotypes that belong to MED. Samples of the two cytotypes were collected in West Africa where they live in sympatry on the same hosts. Genotyping revealed a high level of differentiation, without evidence of gene flow. Moreover, they differed highly in frequencies of resistance alleles to insecticides, which were much higher in Q1 than in ASL. CONCLUSION Q1 and ASL are sufficiently reproductively isolated for the introgression of neutral alleles to be prevented, suggesting that they are actually different species. This indicates that nuclear genetic differentiation must be investigated within groups with less than 3.5% mtCOI divergence in order to elucidate the taxonomy of B. tabaci at a finer level. Overall, these data provide important information for pest management.

Dynamics of the invasive Bemisia tabaci (Homoptera: Aleyrodidae) Mediterranean (MED) species in two West African countries

Bemisia tabaci Gennadius is a major pest on cotton and vegetable plants in Africa. It is considered as a cryptic species complex. Identification of the most damaging species such as the Middle East–Asia Minor 1 (B biotype) and Mediterranean (MED) (which contains the Q and Africa silverleaf (ASL) biotypes) species represents an important step towards the management of B. tabaci. Some data on the geographical distribution of the B. tabaci species complex exist in Burkina Faso, Benin and Togo, but data on the pest’s invasion and displacement dynamics, in relation to time, are lacking. Here, molecular markers (mitochondrial cytochrome oxidase, mtCO1) were used to determine the identity of B. tabaci. Our results illustrate population dynamics on cotton and vegetable plants between 2007, 2009 and 2010. On cotton in southern Togo, ASL was predominant and found in sympatry with Q1. Its frequency decreased slightly over time, i.e. from 92% in 2009 to 90% in 2010. In Burkina Faso, Q1, Q3 and ASL biotypes showed different temporal and spatial distribution patterns. There, Q1 dominated on cotton plants throughout the study. This work provides relevant information about the population dynamics of B. tabaci MED species in two West African countries, Burkina Faso and Togo, in connection with pest management programmes.

Essential oils residual effects on Callosobruchus maculatus (Coleoptera: Bruchidae) survival and female reproduction and cowpea seed germination

Essential oils were extracted by steam distillation from Cymbopogon nardus, Cymbopogon schoenanthus and Ocimum basilicum. They were used in fumigation to investigate their absorption by cowpea, Vigna unguiculata seeds during treatment and their residual effects on adult Callosobruchus maculatus (Fabricius) survival and female reproduction, and seed germination. During the treatment, seeds absorbed the essential oil compounds differently. Treated seeds dried for 24 h and then maintained at 20, 30 and 50°C for degassing continued releasing terpenic compounds. After a 72-h exposure, adult C. maculatus mortality was less than 50% in the presence of treated and degassed seeds. The treated seeds did not affect reproduction in female C. maculatus nor seed germination.RésuméLes huiles essentielles ont été distillées de Cymbopogon nardus, Cymbopogon schoenanthus et de Ocimum basilicum par l’entraînement à la vapeur. Elles ont été évaluées dans un test de fumigation pour leur pouvoir d’absorption par des graines de niébé, Vigna unguiculata durant le traitement, et leurs effets résiduels sur la survie des adultes, la reproduction des femelles de Callosobruchus maculatus (Fabricius) et sur la germination des gaines. Durant le traitement, les graines absorbaient différemment les composants des huiles essentielles. Les graines traitées et séchées pendant 24 h et ensuite incubees à 20, 35 et 50°C pour leur dégazage continuaient de libérer des composés terpéniques, quelles que soient les conditions de dégazage. En presence des graines traitées et dégazées, le taux de mortalité des adultes de C. maculatus était inférieur à 50%, après 72 h d’exposition. Les substances fixées par les graines traitées n’ont aucun effet sur la reproduction des femelles de C. maculatus. Les résidus dans les graines traitées n’affectent pas non plus leur germination.

Pesticide resistance in Plutella xylostella (Lepidoptera: Plutellidae) populations from Togo and Benin

The diamondback moth, Plutella xylostella (L.) is the major insect pest of cabbage crops in Togo and Benin. For control, farmers very often resort to spraying chemical insecticides at high dosages with frequent applications. Bioassays were carried out on three populations of P. xylostella, two from Togo (Kara and Dapaong) and one from Benin (Cotonou), to assess their level of susceptibility to currently used insecticides. A reference strain of P. xylostella from Matuu in Kenya was used as a control. In the laboratory, three insecticide representatives of different chemical families (deltamethrin, chlorpyrifos ethyl and spinosad) were assayed against third instar larvae of P. xylostella. Results revealed that P. xylostella populations from Dapaong, Kara and Cotonou were more resistant to deltamethrin (13 to 59-fold at LC50 level, 149 to 1772-fold at LC90 level) and chlorpyrifos ethyl (5 to 15-fold at LC50 level, 9 to 885-fold at LC90 level) than the reference strain. Spinosad was more toxic to P. xylostella populations than the other insecticides with LC50 and LC90 values less than 1 ώg/ml and 15 ώg/ml, respectively. However, the population from Cotonou appeared significantly more resistant to spinosad compared to the reference strain. These results are discussed in the light of developing an integrated pest management strategy for reducing the selection pressure of spinosad.

First report of the presence of L1014S Knockdown-resistance mutation in Anopheles gambiae s.s and Anopheles coluzzii from Togo, West Africa

Background: To optimize the success of insecticide-based malaria control intervention, knowledge of the distribution of Anopheles gambiae species and insecticide resistance mechanisms is necessary. This paper reported an updated data on pyrethroids/DDT resistance in the An. gambiae s.l population from Togo. Methods: From December 2013 to April 2015, females of indoor-resting An. gambiae s.l were captured in three locations belonging to three different ecological zones. Resistance to DDT, permethrin and deltamethrin was screened in F1 progeny of collected mosquitoes using WHO susceptibility tests. The identification of species of An. gambiae complex and the detection of kdr and ace.1 R allele were carried out using DNA-based molecular techniques. Results: An. gambiae from Kovié and Nangbéto were highly resistant to DDT and permethrin with mortalities rate ranging from 0.83% to 1.58% for DDT and zero to 8.54% for permethrin. Mosquitoes collected in Nangbéto displayed 81.53% mortality with deltamethrin. An. coluzzii and An. gambiae s.s were found in sympatry in Nangbéto and Mango . The allelic frequency of L1014F was high, ranging from 66 to 100% in both An. coluzzii and An. gambiae s.s. For the first time we detected the L1014S allele in both An. coluzzii and An. gambiae s.s. from Togo at the frequency ranging from 5% to 13% in all the sites. The kdr N1575Y was present at various frequencies in both species ranging from 10% to 45%. Both An. gambiae s.s. and An. coluzzii shared the ace1 R mutation in all investigated sites with allelic frequency ranging from 4% to 16%. Conclusion: These results showed that multiple mutations are involved in insecticides resistance in An. gambiae populations from Togo including the kdr L1014F, L1014S, and N1575Y and ace.1 R G119S mutations.

Larvicidal Activity of a Natural Botanical Biostop Moustiques® andPhysiological Changes Induced in Susceptible and Resistant Strains ofAnopheles gambiae Giles (Diptera: Culicidae)

The larvicidal activity of Biostop Moustiques ® (BM), a botanical biocide, was studied on susceptible and resis- tant strains of Anopheles gambiae s.s. at the concentrations of 1, 5, 10 and 20 mL/L of water. In addition to mortality and total protein concentration, the effect of BM on the activity of the following metabolic enzymes was evaluated in fourth instar larvae: glutathione-S-transferase (GST), alkaline phosphatase (ALP), superoxide dismutase (SOD), lactate dehy- drogenase (LDH) and glucose 6-phosphate dehydrogenase (G6PDH). BM caused more than 90% mortality at different larval stages and had median lethal concentration (LC50) of 8.13 mL/L in susceptible Kisumu strain and 8.08 mL/L in re- sistant Acerkis strain. Protein concentration increased significantly in both strains when BM concentration was higher than 1 mL/L. GST and ALP activities increased significantly in both strains. LDH activity increased in Kisumu strain at 5 mL/L and decreased in Acerkis strain at all concentrations. G6PDH activity significantly increased with a maximum ef- fect at 1 mL/L for Acerkis and 5 mL/L for Kisumu. BM completely suppressed SOD activity at 10 mL/L for Kisumu strain and 1 mL/L for Acerkis strain. This study showed that BM had a high larvicidal activity against both strains of Anopheles gambiae and it elicited a wide range of physiological changes.

Molecular xenomonitoring for post-validation surveillance of lymphatic filariasis in Togo: no evidence for active transmission

BackgroundLymphatic filariasis (LF) is a mosquito-borne filarial disease targeted for elimination by the year 2020. The Republic of Togo undertook mass treatment of entire endemic communities from 2000 to 2009 to eliminate the transmission of the disease and is currently the first sub-Saharan African country to be validated by WHO for the elimination of LF as a public health problem. However, post-validation surveillance activities are required to ensure the gains achieved are sustained. This survey assessed the mosquito vectors of the disease and determined the presence of infection in these vectors, testing the hypothesis that transmission has already been interrupted in Togo.MethodMosquitoes were collected from 37 villages located in three districts in one of four evaluation units in the country. In each district, 30 villages were selected based on probability proportionate to size; eight villages (including one of the 30 villages already selected) where microfilaremia-positive cases had been identified during post-treatment surveillance activities were intentionally sampled. Mosquitoes were collected using pyrethrum spray collections (PSC) in households randomly selected in all villages for five months. In the purposefully selected communities, mosquitoes were also collected using human landing collections (HLC) and exit traps (ET). Collected mosquitoes were identified morphologically, and the identification of Wuchereria bancrofti DNA in the mosquitoes was based on the pool screening method, using the LAMP assay.ResultsA total of 15,539 mosquitoes were collected during the study. Anopheles gambiae (72.6%) was the predominant LF vector collected using PSC. Pool screen analysis of 9191 An. gambiae in 629 pools revealed no mosquitoes infected with W. bancrofti (0%; CI: 0–0.021).ConclusionsThese results confirm the findings of epidemiological transmission assessment surveys conducted in 2012 and 2015, which demonstrated the absence of LF transmission in Togo. The challenges of implementing molecular xenomonitoring are further discussed.

Efficacy of two PBO long lasting insecticidal nets against natural populations of Anopheles gambiae s.l. in experimental huts, Kolokopé, Togo

LLINs containing an insecticide plus the synergist, piperonyl butoxide (PBO) have been designed for increased efficacy against pyrethroid-resistant malaria vectors. In this study, two LLINs with PBO, PermaNet® 3.0 and Olyset® Plus, and a pyrethroid-only LLIN, Yorkool®, were evaluated in experimental huts against a free-flying, wild population of Anopheles gambiae s.l. in Kolokopé, a cotton cultivated area of Togo. WHO susceptibility tube tests and subsequent molecular assays determine the An. gambiae s.l. populations to be resistant to pyrethroids and DDT with both target site kdr and metabolic resistance mechanisms involved in the resistance observed. Anopheles gambiae s.s. and An. coluzzi were present in sympatry though the kdr (L1014F) mutation was observed at a higher frequency in An. gambiae s.s. The experimental hut results showed that both PermaNet® 3.0 and Olyset® Plus nets induced similar levels of deterrence, exophily, and reduced blood feeding rate against wild An. gambiae s.l. in contrast to the pyrethroid only LLIN, Yorkool®. The proportion of wild An. gambiae s.l. killed by unwashed PermaNet® 3.0 was significantly higher than unwashed Olyset® Plus (corrected mortality 80.5% compared to 66.6%). Similar blood feeding inhibition rates were observed for unwashed PermaNet® 3.0 and Olyset® Plus; however, PermaNet® 3.0 washed 20 times demonstrated significantly higher blood feeding inhibition rate than Olyset® Plus washed 20 times (91.1% compared with 85.6% respectively). Yorkool® performed the worst for all the parameters evaluated. In an area of pyrethroid resistance of An. gambiae s.l involving kdr target site and metabolic resistance mechanisms, LLINs with PBO can provide additional protection in terms of reduction in blood feeding and increase in mosquito mortality compared to a pyrethroid-only net, and should be considered in malaria vector control strategies.