Relative efficacy of insecticide treated mosquito nets (Diptera: Culicidae) under field conditions

Insecticidal properties of etofenprox for the control of Ephestia kuehniella, Rhyzopertha dominica, Sitophilus oryzae, and Tribolium confusum on stored barley, maize, oats, rice, and wheat

Etofenprox is a novel pyrethroid insecticide that targets the nervous system of insects by affecting the function of the sodium channel. The current study examines the insecticidal activity of etofenprox against Ephestia kuehniella (Lepidoptera: Pyralidae) larvae, Rhyzopertha dominica (Coleoptera: Bostrychidae) adults, Sitophilus oryzae (Coleoptera: Curculionidae) adults, and Tribolium confusum (Coleoptera: Tenebrionidae) adults and larvae on different grain commodities. For this purpose, etofenprox was applied on barley, maize, oats, rice, and wheat at 0.1, 1, 5, and 10 ppm. Mortality levels were recorded after 7, 14, and 21 days of exposure. For E. kuehniella larvae, 10 ppm applied on whole rice killed 96.1% of the exposed individuals after 21 days of exposure. The application of etofenprox on oats at 5 ppm caused the death of 98.3% of the exposed R. dominica adults. Complete mortality was observed for R. dominica adults 21 days post-exposure to oats and whole rice treated with 10 ppm etofenprox. The highest concentration applied on barley caused 95.0% mortality to S. oryzae adults, while the same concentration on maize killed 76.8% of T. confusum adults after 21 days of exposure. Larvae of T. confusum exhibited high mortality levels reaching 99.4% after 21 days of exposure to barley treated with 10 ppm etofenprox. Progeny production of parental R. dominica adults was almost suppressed on all commodities treated with 5 ppm etofenprox. The offspring emergence of S. oryzae ranged between 4.4 and 24.6 adults per vial at 10 ppm. No T. confusum progeny was produced at 10 ppm etofenprox. Our results document that etofenprox is highly effective as grain protectant against several insect species, their developmental stages and their progeny production, but its performance depends on the type of the commodity that it is applied on.

The effect of insecticide-treated mosquito nets (ITMNs) on Japanese encephalitis virus seroconversion in pigs and humans

The effect of insecticide-treated mosquito nets (ITMNs) on Japanese Encephalitis (JE) virus seroconversion in pigs and humans was studied in Assam, Northeast India. A sharp reduction of seroconversion rate in human and pig was found in treated localities after intervention. A marked reduction was achieved in humans (risk ratio [RR] = 0.28, 95% confidence interval [CI] = 0.16-0.49) and pigs (RR = 0.21, CI = 0.11-0.40) in the Kollolua locality where ITMNs were used on both humans and pigs compared with the other two area, Athabari and Rajmai, where ITMNs were covering only either humans or pigs. Monitoring of the mosquito population in and around cattle sheds during dusk revealed no significant decline (P > 0.05) of vector density during the post-intervention period in study localities. In spite of the high preponderance of potential JE vector outdoors during the post-intervention period, an encouraging line of defense against circulation of JE virus through the use of ITMNs can be achieved in endemic areas.

Recent rapid rise of a permethrin knock down resistance allele in Aedes aegypti in México

Background

Aedes aegypti, the ‘yellow fever mosquito’, is the primary vector to humans of dengue and yellow fever flaviviruses (DENV, YFV), and is a known vector of the chikungunya alphavirus (CV). Because vaccines are not yet available for DENV or CV or are inadequately distributed in developing countries (YFV), management of Ae. aegypti remains the primary option to prevent and control outbreaks of the diseases caused by these arboviruses. Permethrin is one of the most widely used active ingredients in insecticides for suppression of adult Ae. aegypti. In 2007, we documented a replacement mutation in codon 1,016 of the voltage-gated sodium channel gene (para) of Ae. aegypti that encodes an isoleucine rather than a valine and confers resistance to permethrin. Ile1,016 segregates as a recessive allele conferring knockdown resistance to homozygous mosquitoes at 5–10 µg of permethrin in bottle bioassays.

Methods and Findings

A total of 81 field collections containing 3,951 Ae. aegypti were made throughout México from 1996 to 2009. These mosquitoes were analyzed for the frequency of the Ile1,016 mutation using a melting-curve PCR assay. Dramatic increases in frequencies of Ile1,016 were recorded from the late 1990's to 2006–2009 in several states including Nuevo León in the north, Veracruz on the central Atlantic coast, and Yucatán, Quintana Roo and Chiapas in the south. From 1996 to 2000, the overall frequency of Ile1,016 was 0.04% (95% confidence interval (CI95) = 0.12%; n = 1,359 mosquitoes examined). The earliest detection of Ile1,016 was in Nuevo Laredo on the U.S. border in 1997. By 2003–2004 the overall frequency of Ile1,016 had increased ∼100-fold to 2.7% (±0.80% CI95; n = 808). When checked again in 2006, the frequency had increased slightly to 3.9% (±1.15% CI95; n = 473). This was followed in 2007–2009 by a sudden jump in Ile1,016 frequency to 33.2% (±1.99% CI95; n = 1,074 mosquitoes). There was spatial heterogeneity in Ile1,016 frequencies among 2007–2008 collections, which ranged from 45.7% (±2.00% CI95) in the state of Veracruz to 51.2% (±4.36% CI95) in the Yucatán peninsula and 14.5% (±2.23% CI95) in and around Tapachula in the state of Chiapas. Spatial heterogeneity was also evident at smaller geographic scales. For example within the city of Chetumal, Quintana Roo, Ile1,016 frequencies varied from 38.3%–88.3%. A linear regression analysis based on seven collections from 2007 revealed that the frequency of Ile1,016 homozygotes accurately predicted knockdown rate for mosquitoes exposed to permethrin in a bioassay (R² = 0.98).

Conclusions

We have recorded a dramatic increase in the frequency of the Ile1,016 mutation in the voltage-gated sodium channel gene of Ae. aegypti in México from 1996 to 2009. This may be related to heavy use of permethrin-based insecticides in mosquito control programs. Spatial heterogeneity in Ile1,016 frequencies in 2007 and 2008 collections may reflect differences in selection pressure or in the initial frequency of Ile1,016. The rapid recent increase in Ile1,016 is predicted by a simple model of positive directional selection on a recessive allele. Unfortunately this model also predicts rapid fixation of Ile1,016 unless there is negative fitness associated with Ile1,016 in the absence of permethrin. If so, then spatial refugia of susceptible Ae. aegypti or rotational schedules of different classes of adulticides could be established to slow or prevent fixation of Ile1,016.

Pyrethroid insecticides prolong the opening of voltage-dependent sodium channels in insect nerves to produce instant paralysis and “knock-down.” Many insects have evolved knock-down resistance through nonsynonymous mutations that reduce pyrethroid binding in the channels. In 2006 we discovered one such mutation in the arbovirus mosquito vector Aedes aegypti, called Ile1,016, that confers very high knockdown resistance to the pyrethroid insecticide permethrin in mosquitoes homozygous for this mutation. We examined collections of Ae. aegypti from México during 1996–2009 and found that the overall Ile1,016 frequency increased from <0.1% in 1996–2000, to 2%–5% in 2003–2006, to 38.3%–88.3% in 2007–2009 depending upon collection location. We also demonstrate a strong linear relationship between the frequency of Ile1,016 homozygotes and knockdown rate in bioassays and speculate that widespread use of permethrin-based insecticides in México may be impacting the frequency of Ile1,016. Such a rapid increase is predicted by a simple model of positive directional selection acting on a recessive allele. Unfortunately this model also predicts rapid fixation of Ile1,016 unless there is a negative fitness associated with Ile1,016 in the absence of permethrin and if insecticidal pressure can be reduced.

Wash resistance and bioefficacy of Olyset net--a long-lasting insecticide-treated mosquito net against malaria vectors and nontarget household pests

During recent years, long-lasting insecticide-treated nets (LLINs) have been developed to overcome the problems of low retreatment rates, washing, and erratic dose of the insecticide resulting in the dilution of efficacy of the conventional insecticide-treated mosquito nets. These nets are treated at factory level with insecticide either incorporated into or coated around fibers. Olyset net, a polyethylene net with 2% permethrin incorporated within fibers, is one type of LLIN. Therefore, these nets were evaluated for their wash resistance and bioefficacy against malaria vectors Anopheles culicifacies Giles and Anopheles fluviatilis James (Diptera: Culicidae) and other nontarget species. Cone bioassay tests produced 100% mortality in these two vector species with 3-min exposure. Results of the bioassays on washed nets showed 100% mortality in An. fluviatilis even after 20 washings, whereas in An. culicifacies 100% mortality up to 11 washings and 80% mortality up to 20 washings were observed. Cone bioassay tests also were performed on nontarget mosquito species Culex quinquefasciatus Say; house fly, Musca domestica L.; American cockroach, Periplaneta americana (L.); head louse, Pediculus humanus capitis De Geer; and bed bug, Cimex lectularius L. with 30-min exposure. Except for bed bugs, 100% mortality was observed in these nontarget species after 24-h recovery period. In bed bugs, only 25% mortality was observed. The density of An. culicifacies and An. fluviatilis was significantly reduced in houses with Olyset nets compared with those with untreated nets or no nets. Thus, it may be concluded that Olyset nets are highly effective against malaria vectors and moderately against other nontarget household insects.

Bioefficacy of Olyset nets against mosquitoes in India

Olyset nets are highly effective in killing anopheline mosquitoes within 3 min of exposure, as evidenced in cone bioassay tests. These nets also showed their efficacy on other vector mosquitoes (Aedes aegypti and Cx. quinquefasciatus) at higher exposure periods. The efficacy remained at approximately 90% even after 20 washings at an interval of 24 h against Anopheles culicifacies and Cx. quinquefasciatus. Use of the nets inside the house also resulted in drastic reduction of daytime resting density of mosquitoes because of high repellency, excitorepellency, and killing action of the nets. Pilot studies are indicated to evaluate the impact of use of Olyset nets on vector-borne diseases, particularly malaria, and its cost-effectiveness in comparison to conventional indoor residual spraying.