Pyrethroid insecticides susceptibility of Stomoxys calcitrans (Linnaeus) and Stomoxys indicus Picard (Diptera: Muscidae) from cattle farms in southern Thailand

The susceptibility to six pyrethroid insecticides (permethrin, deltamethrin, alpha-cypermethrin, cypermethrin, lambda-cyhalothrin and bifenthrin), each at the recommended concentration, was evaluated for the two stable fly species Stomoxys calcitrans (Linnaeus) and Stomoxys indicus Picard, through tarsal contact using a World Health Organization (WHO) cone bioassay procedure. The field populations of stable flies were collected from three study sites (Songkhla, Phattalung and Satun provinces) in Thailand. The stable flies were exposed to insecticide-treated filter paper for 30 min and their knockdown counts at 30 min and 60 min and mortality counts at 12 hr and 24 hr were recorded. The S. calcitrans and S. indicus in Songkhla and Phattalung populations were moderately susceptible to pyrethroids for 24-hr mortality. Nonetheless, the Satun population of S. indicus was completely susceptible to permethrin with 100% mortality and the lowest susceptible to deltamethrin and bifenthrin. The results indicate a generally low susceptibility of stable flies to pyrethroids in the southern provinces of Thailand.

Chemical composition and the insecticidal activity of Aeollanthus pubescens leaf essential oil against Anopheles gambiae sensu stricto

Background The excessive use of synthetic insecticides is responsible for many cases of resistance in insects. Therefore, the use of natural molecules of ecological interest with insecticidal properties is an alternative approach to the use of synthetic insecticides. The aim of this study is to investigating the larvicidal and adulticidal activity and the chemical composition of the essential oil of Aeollanthus pubescens on the major malaria vector, Anopheles gambiae. Methods Three reference strains of Anopheles gambiae sensu stricto (Kisumu, Kiskdr and Acerkis) were used in this study. The leaves of A. pubescens were collected in southern Benin. The standard World Health Organisation (WHO) guidelines for larvicide evaluation were used, and the chemical composition of the essential oil was analysed by gas chromatography coupled to mass spectrometry. Adult mosquitoes of each strain were exposed to pieces of net coated with the essential oil for 3 min using the WHO cone bioassay method. Probit regression analysis was used to determine the concentrations that would kill 50 and 95% of each test population (LC50, LC95) and the knockdown time for 50 and 95% of each test population (KDT50, and KDT95). The difference between the mortality–dose regressions for the different strains was analysed using the likelihood ratio test (LRT). The log-rank test was performed to evaluate the difference in survival between the strains. Results A total of 14 components were identified, accounting for 98.3% of total oil content. The major components were carvacrol (51.1%), thymyle acetate (14.0%) and ɣ-terpinene (10.6%). The essential oil showed larvicidal properties on the Kisumu, Acerkis and Kiskdr strains, with LC50 of 29.6, 22.9 and 28.4 ppm, respectively. With pieces of netting treated at 165 µg/cm2, the KDT50 of both Acerkis (1.71 s; Z = 3.34, P < 0.001) and Kiskdr (2.67 s; Z = 3.49, P < 0.001) individuals were significantly lower than that of Kisumu (3.8 s). The lifespan of the three mosquito strains decreased to 1 day for Kisumu (χ2 = 99, df = 1, P < 0.001), 2 days for Acerkis (χ2 = 117, df = 1, P < 0.001) and 3 days for Kiskdr (χ2 = 96.9, df = 1, P < 0.001). Conclusion Our findings show that A. pubescens essential oil has larvicide and adulticide properties against the malaria vector An. gambiae sensu stricto, suggesting that this essential oil may be a potential candidate for the control of the resistant malaria-transmitting vectors. Graphical Abstract

Evaluation of Perma Net® 2.0 in the Control of Culex quinquefasciatus and Aedes aegypti from Awka, Anambra State, Nigeria

Long Lasting Insecticide Nets (LLINs) reduce human-mosquito contact through the provision of physical barrier, repellent effect or mortality effect. This study evaluated the physical integrity and residual effect of insecticides in Perma Net® 2.0 of 6 months, 1 year, 2 years, 3 years and 4 years duration of use. Perma Net®2.0 used were obtained from systematically picked households within Ifite Awka and new Perma Net® 2.0 serving as control were collected from Society for Family Health. Adult mosquitoes were collected from picked households using Pyrethrum knockdown Collection method before the Perma Net® 2.0 were removed. The top and four sides of the nets were examined for presence of holes. Also, net pieces measuring 25cm × 25cm were obtained from Perma Net® 2.0 of different duration and subjected to WHO cone bioassay using Randomized Block Experimental Design. The Culex quinquefasciatus and Aedes aegypti used for the bioassay were collected from breeding sites within Ifite Awka, reared and maintained in an insectary. There was at least one hole in 10%, 30% and 40% of LLINs with 2 years, 3years and 4 years duration of use respectively. Significant differences were found in the physical integrity of the nets (P < 0.05). Culexquinquefasciatus and Aedes aegypti constituted 40.9% and 27.0% of all mosquitoes collected indoors respectively. There was no significant difference in the indoor abundance of the two mosquito species (P >0.05). However, their abundance depended on the duration of LLINs usage (P < 0.05). The mortality effects of16%, 8%, 7%, 5%, 5% and 2% on Culex quinquefasciatus were recorded for Perma Net® 2.0 of 0 month, 6 months, 1 year, 2 years, 3 years and 4 years respectively. The mortality effects of 23%, 12%, 9%, 6%, 5% and 3% on Aedes aegypti were recorded for Perma Net® 2.0 of 0 month, 6 months, 1 year, 2 years, 3 years and 4years respectively. The new LLINs produced significantly highest mortality effect (P < 0.05) on both mosquito species than nets of other duration of usage. The mortality effect of Perma Net® 2.0 on Culex quinquefasciatus, and that on Aedes aegypti were similar (P > 0.05). It shows that Perma Net® 2.0 offer physical barrier more than repellency and mortality effect against Culex quinquefasciatus, and Aedes aegypti.