Semi-field evaluation of novel chemical lures for Aedes aegypti, Culex quinquefasciatus, and Anopheles minimus (Diptera: Culicidae) in Thailand.

BACKGROUND: Entomological surveillance is an important means of assessing the efficacy of insect vector management programs and estimating disease transmission thresholds. Among baited traps, Biogents' BG-Sentinel (BGS) trap baited with BG-Lure is considered to have the most similar outcome to, and be a possible replacement for, human-landing catches for the epidemiologically relevant monitoring of adult Aedes aegypti and Culex quinquefasciatus. In contrast to the BGS trap, the Black Hole ultraviolet (UV) light trap, which is widely used to catch nocturnal flying insects, is not baited with synthetic human odor-mimicking lures. METHODS: We evaluated the L-lactic acid-based Kasetsart University (KU)-lures nos. 1-6 as novel candidate chemical lures for the diurnal species Ae. aegypti and the nocturnal species Cx. quinquefasciatus using two commercial traps (the BGS trap and the Black Hole UV light trap) in a semi-field screen (SFS) house. Firstly, we optimized the dose of each KU-lure in an SFS house (140 m3). Secondly, six different candidate KU-lures were screened by comparing their percent attraction using a single discriminating dose (0.5 g). Finally, we evaluated the synergism of the KU-lures selected in this way with commercially available traps. RESULTS: BGS traps baited with KU-lure no. 1 exhibited the greatest percent attraction for Ae. aegypti (29.5% ± 14.3%), whereas those baited with KU-lure no. 6 most strongly attracted Cx. quinquefasciatus (33.3% ± 10.7%). Interestingly, BGS traps treated with 10 g BG-Lure did not significantly attract more Ae. aegypti or Cx. quinquefasciatus than the untreated BGS traps. CO2 at a flow rate of 250 ml/min most strongly attracted both Ae. aegypti and Cx. quinquefasciatus (42.2% ± 14.2% and 75.1% ± 16.9%, respectively). BGS and Black Hole UV light traps with KU-lure no. 6 exhibited a stronger attraction for Cx. quinquefasciatus than untreated traps, and the percent attraction did not differ between the treated traps. CONCLUSIONS: Synergistic effects of KU-lures nos. 1 and 6 with the mosquito traps were demonstrated for both the diurnal and nocturnal species in the SFS house assays. However, further studies are urgently needed for the development of species-specific lures to increase trap efficacy in the field for local vector mosquitoes in Thailand.

Evaluation of an ultraviolet LED trap for catching Anopheles and Culex mosquitoes in south-eastern Tanzania.

BACKGROUND: Improved surveillance techniques are required to accelerate efforts against major arthropod-borne diseases such as malaria, dengue, filariasis, Zika and yellow-fever. Light-emitting diodes (LEDs) are increasingly used in mosquito traps because they improve energy efficiency and battery longevity relative to incandescent bulbs. This study evaluated the efficacy of a new ultraviolet LED trap (Mosclean) against standard mosquito collection methods. METHODS: The study was conducted in controlled semi-field settings and in field conditions in rural south-eastern Tanzania. The Mosclean trap was compared to commonly used techniques, namely CDC-light traps, human landing catches (HLCs), BG-Sentinel traps and Suna traps. RESULTS: When simultaneously placed inside the same semi-field chamber, the Mosclean trap caught twice as many Anopheles arabiensis as the CDC-light trap, and equal numbers to HLCs. Similar results were obtained when traps were tested individually in the chambers. Under field settings, Mosclean traps caught equal numbers of An. arabiensis and twice as many Culex mosquitoes as CDC-light traps. It was also better at trapping malaria vectors compared to both Suna and BG-Sentinel traps, and was more efficient in collecting mosquitoes indoors than outdoors. The majority of An. arabiensis females caught by Mosclean traps were parous (63.6%) and inseminated (89.8%). In comparison, the females caught by CDC-light traps were 43.9% parous and 92.8% inseminated. CONCLUSIONS: The UV LED trap (Mosclean trap) was efficacious for sampling Anopheles and Culex mosquitoes. Its efficacy was comparable to and in some instances better than traps commonly used for vector surveillance. The Mosclean trap was more productive in sampling mosquitoes indoors compared to outdoors. The trap can be used indoors near human-occupied nets, or outdoors, in which case additional CO2 improves catches. We conclude that this trap may have potential for mosquito surveillance. However, we recommend additional field tests to validate these findings in multiple settings and to assess the potential of LEDs to attract non-target organisms, especially outdoors.

Impact of gamma irradiation on the development and reproduction of Culex pipiens (Diptera; Culicidae).

PURPOSE: The aim was to evaluate the effects of irradiation of Culex pipiens irradiated as pupae on the larval and pupal mortality percentages, the larval and pupal duration, adult emergence percentages and ovarian development in comparison with the un-irradiated group. MATERIAL AND METHODS: The mosquito, C. pipiens Linnaeus (L) was irradiated as pupae with three gamma doses 40, 60 and 80 Gy. Different biological aspects include the ovarian development of irradiated females tested. RESULTS: Gamma doses used significantly increased the larval and pupal mortality percentages, the larval and pupal duration and significantly decreased the adult emergence percentages. Moreover, fecundity and egg hatchability percentages were severely decreased with increasing gamma doses. While the sterility% was positively correlated with the doses used. Gamma doses caused a dose-dependent decrease in the number of developed eggs, reduction in the number of developed oocytes, signs of degenerative processes in oocytes, yolk mass and egg size decrease. CONCLUSIONS: Results obtained indicated that gamma radiation of C. pipiens induced remarkable effects on the biological, developmental and reproductive activity.

Vector competence of northern European Culex pipiens biotypes and hybrids for West Nile virus is differentially affected by temperature.

BACKGROUND: Outbreaks of West Nile virus (WNV) have not occurred in northern Europe despite nearby circulation of WNV in the southern part of the continent. The main vector for WNV, the mosquito Culex (Cx.) pipiens, consists of two behaviorally distinct biotypes, pipiens and molestus, which can form hybrids. Although temperature has been shown to influence vector competence of Cx. pipiens for WNV and biotypes are differentially susceptible towards infection, the interaction between the two has not been elucidated. METHODS: We determined vector competence of the Cx. pipiens biotypes and hybrids, after 14 days of incubation at 18, 23 and 28 °C. Mosquitoes were orally infected by providing an infectious blood meal or by injecting WNV directly in the thorax. Infection and transmission rates were determined by testing the bodies and saliva for WNV presence. In addition, titers of mosquitoes with WNV-positive bodies and saliva samples were determined. RESULTS: Orally infected biotype pipiens and hybrids showed significantly increased transmission rates with higher temperatures, up to 32 and 14 %, respectively. In contrast, the molestus biotype had an overall transmission rate of 10 %, which did not increase with temperature. All mosquitoes that were infected via WNV injections had (close to) 100 % infection and transmission rates, suggesting an important role of the mosquito midgut barrier. We found no effect of increasing temperature on viral titers. CONCLUSIONS: Temperature differentially affected vector competence of the Cx. pipiens biotypes. This shows the importance of accounting for biotype-by-temperature interactions, which influence the outcomes of vector competence studies. Vector competence studies with Cx. pipiens mosquitoes differentiated to the biotype level are essential for proper WNV risk assessments.

Geographic variation in the response of Culex pipiens life history traits to temperature.

BACKGROUND: Climate change is predicted to alter the transmission of many vector-borne pathogens. The quantitative impact of climate change is usually estimated by measuring the temperature-performance relationships for a single population of vectors, and then mapping this relationship across a range of temperatures or locations. However, life history traits of different populations often differ significantly. Specifically, performance across a range of temperatures is likely to vary due to local adaptation to temperature and other factors. This variation can cause spatial variation in pathogen transmission and will influence the impact of climate change on the transmission of vector-borne pathogens. METHODS: We quantified variation in life history traits for four populations of Culex pipiens (Linnaeus) mosquitoes. The populations were distributed along altitudinal and latitudinal gradients in the eastern United States that spanned ~3 °C in mean summer temperature, which is similar to the magnitude of global warming expected in the next 3-5 decades. We measured larval and adult survival, development rate, and biting rate at six temperatures between 16 and 35 °C, in a common garden experiment. RESULTS: Temperature had strong and consistent non-linear effects on all four life history traits for all four populations. Adult female development time decreased monotonically with increasing temperature, with the largest decrease at cold temperatures. Daily juvenile and adult female survival also decreased with increasing temperature, but the largest decrease occurred at higher temperatures. There was significant among-population variation in the thermal response curves for the four life history traits across the four populations, with larval survival, adult survival, and development rate varying up to 45, 79, and 84 % among populations, respectively. However, variation was not correlated with local temperatures and thus did not support the local thermal adaptation hypothesis. CONCLUSION: These results suggest that the impact of climate change on vector-borne disease will be more variable than previous predictions, and our data provide an estimate of this uncertainty. In addition, the variation among populations that we observed will shape the response of vectors to changing climates.

Efficacy of Two Larvasonic™ Units Against Culex Larvae and Effects on Common Aquatic Nontarget Organisms in Harris County, Texas.

The Larvasonic™ Field Arm Mobile Wetlands Unit and SD-Mini were tested for efficacy against Culex larvae, and effects on aquatic nontarget organisms (NTO). The Field Arm provided 84.61% to 100% control of caged Culex larvae out to 0.91-m distance in shallow ditches and 60.45% control of Culex larvae at 0.61-m without any effects to caged NTO. Slow ditch treatment achieved 77.35% control compared to fast treatment (20.42%), whereas 77.65% control was obtained along edges of a neglected swimming pool, compared to near the middle (23.97%). In bucket tests, the SD-Mini provided >97% control of Culex and 85.35% reduction of immature giant water bugs, which decreased slightly (83.45%) over the monitoring period, which was not significantly different from cannibalistic damselflies (62.80%), with reduction of both being significantly higher than other NTO tested. There was a small (0.37%) reduction of dragonflies (naiads), due to cannibalism. Both Larvasonic units could effectively augment conventional larvicide operations in smaller areas without causing resistance within mosquito populations or harming NTO when used properly.

Effects of larval rearing temperature on immature development and West Nile virus vector competence of Culex tarsalis.

BACKGROUND: Temperature is known to induce changes in mosquito physiology, development, ecology, and in some species, vector competence for arboviruses. Since colonized mosquitoes are reared under laboratory conditions that can be significantly different from their field counterparts, laboratory vector competence experiments may not accurately reflect natural vector-virus interactions. METHODS: We evaluated the effects of larval rearing temperature on immature development parameters and vector competence of two Culex tarsalis strains for West Nile virus (WNV). RESULTS: Rearing temperature had a significant effect on mosquito developmental parameters, including shorter time to pupation and emergence and smaller female body size as temperature increased. However, infection, dissemination, and transmission rates for WNV at 5, 7, and 14 days post infectious feeding were not consistently affected. CONCLUSIONS: These results suggest that varying constant larval rearing temperature does not significantly affect laboratory estimates of vector competence for WNV in Culex tarsalis mosquitoes.

Characterization of biochemical based insecticide resistance mechanism by thermal bioassay and the variation of esterase activity in Culex quinquefasciatus.

Biochemical mechanisms of insecticide resistance of thermal exposed and unexposed Culex quinquefasciatus strains are evaluated, which were not studied earlier. The activity of alpha- and beta-carboxylesterases and acetylcholinesterase of malathion susceptible and resistant strains were compared after thermal treatment. Three-day-old adult females were used for the malathion susceptibility test and biochemical assays, and males were used only for the susceptibility test. Thermal exposure brought about increase in resistance levels from 85% to 90% in males and 91% to 96.6% in females of resistant strain. The resistance status of the susceptibility strain was unchanged after thermal exposure. The activities of alpha- and beta-carboxylesterase of susceptible mosquitoes were within 800 and 700 U/mg protein, respectively. The alpha-carboxylesterase activity of the thermal exposed malathion-resistant population was significantly (t test, P < 0.05) higher than the unexposed resistant population, and the reverse was recorded in beta-carboxylesterase. The alpha-carboxylesterase activity of susceptible population was lower than the resistant population. The activity of alpha-carboxylesterase was higher than the beta-carboxylesterase in both the strains. Among the malathion resistant C. quinquefasciatus population, 2.3% population exhibited 30-40% inhibition which increased to 5.8% after the thermal exposure. Thermal exposure of mosquitoes increased the activity of both alpha-carboxylesterases and acetylcholinesterase but decreased the activity of beta-carboxylesterase.

Effect of different thermal conditions on the pre-imaginal biology of Culex apicinus (Philippi, 1865) (Diptera: Culicidae).

The time of development and survival of the pre-imaginal period and the adult body size of Culex apicinus were analyzed in individuals reared from the 1st instar larva to adult emergence under laboratory and field conditions. In the laboratory, insects were exposed to three constant temperatures (15, 20, and 25 degrees C) and a photoperiod of 14:10 (L:D). In the field, temperature and photoperiod were not manipulated; during the study period water temperature ranged between 15.5 and 24.2 degrees C, and photoperiod changed from 13:11 to 14:10 (L:D). Survival to the imaginal stage at 15 degrees C (85.4%) and in the field (88.8%) was higher than that at 20 degrees C (45.8%) and 25 degrees C (8.3%). Based on femur and wing length measurements, the mosquitoes reared under field conditions (mean water temperature = 20.5 degrees C) were smaller than those laboratory-reared, even when the time of development in the field (29.5 - 36 days) was longer than that observed at 15 degrees C (24 days), 20 degrees C (15 days), and 25 degrees C (11 days). Male emergence was earlier than that of females (protandry) only in the field. Although laboratory conditions included a photoperiod and temperature range comparable with the observed values in the field, the disagreements between field and laboratory results suggest that the characteristics examined in this work could be affected by the joint variation of several factors not controlled in field conditions, such as photoperiod, temperature regime, and/or food quality of larval habitat.

Preliminary study on the effectiveness of mosquito repelling lamp, E Da.

Photon (light) technology has already been widely used in make-up, medical treatment etc, but repelling mosquitoes by photon technology is an innovation. The objective of this study was to determine the efficacy of a mosquito repelling lamp, E Da under indoor conditions. E Da lamp is a lamp coated with yellow luminous pigment on the inner part of the glass bulb of the lamp which is used to screen out the UV radiation, and when it is turned on, the yellow illuminating wavelength will drive the mosquitoes away. The tests were conducted inside 2 cabins measuring 8' X 8' X 20'. The mosquito population was estimated by using the Bare Leg Catch (BLC) techniques. For treated test, E Da lamp was placed indoor 2 - 3 meters away from a human bait. Another cabin without the lamp was used as untreated control. BLC was conducted in both sites simultaneously. The mosquitoes collected in this study were solely those of Culex quinquefasciatus and Aedes albopictus. There was an 91.34% reduction of Cx. quinquefasciatus population in the treated test compared with the untreated cabin during the 4 hours catches (p < 0.05). E Da mosquito repelling lamp used in this study exerted repellency effect against the mosquitoes especially Cx. quinquefasciatus.

Some naturally occurring phytophototoxins for mosquito control.

Alpha-terthiophene (alpha-T) and erythrosin-B, the naturally occurring plant secondary metabolites, were tried for their phototoxic properties against Anopheles and Culex larvae under dark, ordinary tube light (1.9-2.4 w/m2) and sun light (680-840 w/m2). LC50 values of alpha-T for Anopheles larvae (4th instar) were found to be 154, 92 and 11 ppb under dark, tube light and sunlight, respectively. For Culex larvae corresponding LC50 values under different light conditions were 129, 97 and 22 ppb. Erythrosin-B under all photoregimens was found to be less toxic to larvae of both Anopheles and Culex sps. Also, the susceptibility of the mosquito species decreased with age, towards alpha-T and erythrosin-B. Cumulative effects in terms of delay in metamorphosis were also observed among survivors of such exposures. The effects of these compounds were also seen on the adults and developing unhatched embryos of a common aquatic snail (Lymnaea sps). The LC50 values of alpha-T for adults were found to be 39, 23 ppm and 77 ppb under dark, tube light and sunlight and for developing unhatched embryos the corresponding values were 620, 41 and 13 ppb. Erythrosin-B was found to be much less toxic under sunlight and dark, to both adults and embryos as compared to the toxicity of alpha-T. Potential use of such biodegradable and eco-friendly compounds of natural origin in mosquito control is discussed.