Field cage insecticide resistance tests against Culex quinquefasciatus Say (Diptera: Culicidae) in Harris County, Texas, U.S.A.

The ground adulticiding program in densely populated Harris County has been employed on the basis of virus-positive mosquitoes almost exclusively using the 'rotation of chemicals best practices' stratagem. To evaluate its effectiveness, 15 comparative field cage testing events were conducted from 2011-2015 using seven wild population samples from repeatedly collected locations of Culex quinquefasciatus Say and a laboratory susceptible Sebring strain colony reared to adults. A 3 × 3 plot design was employed for exposure to ultra-low volume applications of malathion and synergized permethrin. No significant differences were found in mortality rate among testing dates, year, row placement, or relative humidity. Mortality was significantly different between adulticides (p<0.001) with mean mortality rates for malathion 96.42% (±7.95%) and permethrin 92.38% (±14.04). There was a significant temperature difference for permethrin (p<0.001) but none for malathion (p=0.644). Mosquito population mortality was statistically different by study operational area (p<0.011) and chemical (p<0.001). Susceptible colony adults used as positive controls downwind strongly aided determination of efficacy and resistance to each adulticide, providing evidence of individual application coverage, though comparative analysis was done with overall mortalities by normal methodology.

Differential Vector Competency of Aedes albopictus Populations from the Americas for Zika Virus.

To evaluate the potential role of Aedes albopictus (Skuse) as a vector of Zika virus (ZIKV), colonized mosquitoes of low generation number (≤ F5) from Brazil, Houston, and the Rio Grande Valley of Texas engorged on viremic mice infected with ZIKV strains originating from Senegal, Cambodia, Mexico, Brazil, or Puerto Rico. Vector competence was established by monitoring infection, dissemination, and transmission potential after 3, 7, and 14 days of extrinsic incubation. Positive saliva samples were assayed for infectious titer. Although all three mosquito populations were susceptible to all ZIKV strains, rates of infection, dissemination, and transmission differed among mosquito and virus strains. Aedes albopictus from Salvador, Brazil, were the least efficient vectors, demonstrating susceptibility to infection to two American strains of ZIKV but failing to shed virus in saliva. Mosquitoes from the Rio Grande Valley were the most efficient vectors and were capable of shedding all three tested ZIKV strains into saliva after 14 days of extrinsic incubation. In particular, ZIKV strain DakAR 41525 (Senegal 1954) was significantly more efficient at dissemination and saliva deposition than the others tested in Rio Grande mosquitoes. Overall, our data indicate that, while Ae. albopictus is capable of transmitting ZIKV, its competence is potentially dependent on geographic origin of both the mosquito population and the viral strain.

Efficacy of Deltagard®, Fyfanon®, and Evoluer™ 31-66 Against Culex quinquefasciatus In Harris County, Texas.

The comparative efficacy of ultra-low volume applications of deltamethrin, permethrin, and malathion against specific adult populations of Culex quinquefasciatus , a primary regional vector of Saint Louis encephalitis and West Nile virus, using field cage tests is described. At labeled application rates, the estimated mass median diameters for Fyfanon®, Evoluer™, and DeltaGard® were 18.32, 19.24, and 17.44 µm, respectively. No significant differences (P ≥ 0.01) in mean percent control were observed across all 3 wild populations and a susceptible Sebring strain tested with Fyfanon, DeltaGard, and Evoluer, providing 98.25%, 98.20%, and 97.24% mean percent control. Mean percent control for Sebring and 3 wild populations was 99.38%, 97.79%, 97.26%, and 97.15%, respectively, which were not significantly different from one another (P ≥ 0.01). Across all tests, highly significant differences (P ≤ 0.01) in mean percent control were obtained at 30.48, 60.96, and 91.44 m distances downwind, with mean percent control at 91.44 m being significantly less than that observed at 30.48 m, but not at 60.96 m. Individually, tests for DeltaGard, Fyfanon, and Evoluer resulted in no significant differences (P ≥ 0.01) in mean percent control between all mosquito populations tested, ranging from 96.47 to 100. However, Evoluer obtained significantly less (P ≤ 0.01) mean percent control at 91.44 m compared to DeltaGard and Fyfanon. Data suggest that the wild Cx. quinquefasciatus specimens collected were as susceptible as the laboratory Sebring strain. The use of Sebring as positive controls downwind strongly supported determination of the efficacy of all adulticides and susceptibility of wild populations tested.

Zika Virus Vector Competency of Mosquitoes, Gulf Coast, United States.

Zika virus has recently spread throughout the Americas. Although Aedes aegypti mosquitoes are considered the primary vector, Culex quinquefasciatus and mosquitoes of other species may also be vectors. We tested Cx. quinquefasciatus and Ae. taeniorhynchus mosquitoes from the US Gulf Coast; both were refractory to infection and incapable of transmission.

A multiunit CO2 anesthetizing system for use in transferring mosquitoes during field cage insectcide efficacy tests.

A multiunit CO2 delivery and anesthetizing system that supports expanded cage testing of mosquitoes is described. This system has proven more efficient than pipetting for conducting multiple tests and has facilitated the tripling of cage testing capabilities.

A multiple cage-holding, wind-sensitive vane design for use in ground adulticiding efficacy testing in Harris County, Texas.

A wind-sensitive vane apparatus was designed and implemented specifically to accommodate the attachment of otherwise unidirectional insecticide treatment cages used in ground ultra-low volume mosquito adulticide field tests. This cage support system is useful in keeping the potential West Nile virus and St. Louis encephalitis vector Culex quinquefasciatus caged mosquitoes oriented into the wind during field efficacy tests. Testing capacity for resistance surveillance was tripled during the 2005 season, and more reliable results were achieved as a consequence.

Estimation of aerosol droplet sizes by using a modified DC-III portable droplet measurement system under laboratory and field conditions.

Modification of the DC-III portable droplet measurement system, permitting its use under field conditions, is described. Under laboratory conditions, the system effectively sampled water droplets from aerosols produced by a dry ice/water generator and high-pressure syringe. Seven droplet sizes, totaling 71,053 droplets within 22 tests (dry ice method), consisted of 1-, 2-, 6-, 11-, 18-, 25-, and 34-microm droplets with individual (rounded) percentages of 45.25, 37.22, 13.85, 3.17, 0.45, 0.02, and 0.005, respectively, for each size. Cumulatively, 1-microm droplets accounted for ca. 45.25% of the droplets sampled; combined with 2-microm (ca. 82.48% together), 6-microm (ca. 96.33% together), and 11-microm droplets, yielded ca. 99.51% of the droplets sampled. The syringe produced 12 droplet sizes, with 4,121 droplets sampled, consisting of 1, 2, 6, 11, 18, 25, 34, 45, 56, 69, 83, and 99 microm with individual percentages of 15.43, 21.91, 24.58, 17.30, 10.62, 4.65, 2.93, 1.33, 0.63, 0.33, 0.16, 0.07, respectively, for each size. The 6-microm droplets contributed the highest individual percentage, and cumulatively, these droplets combined with 1- and 2-microm droplets, yielding 61.93%, whereas 11- to 45-microm droplets contributed 36.83%, for a total of 98.76%. Droplets measuring 56-99 microm accounted for ca. 1.24% of droplets sampled. Hand-fogger oil aerosols produced 12 droplet sizes (1-38 microm) at test distances of 7.6 and 15.2 m, with 1,979 and 268 droplets sampled, respectively, during 10 tests at each distance. With analysis of variance of transformed individual percentages for each size at both distances, no significant differences were observed for 7.6 and 15.2 m. Cumulatively, 1-, 2-, 3-, and 5-microm droplets contributed 82.87 and 80.97%, whereas 8-, 11-, 14-, and 18-microm droplets added 14.55% to totals at both 7.6 and 15.2 m, respectively. Droplets measuring 22, 27, 32, and 38 microm contributed 2.57% and 4.47% to samples obtained at 7.6 and 15.2 m. The average mass median diameter (MMD) of mineral oil aerosols obtained at 7.6 and 15.2 m were 19.55 +/- 1.62 and 15.49 +/- 1.35, respectively. Positioned at 15 m (50 ft) within a field cage test plot during 12 separate tests, the MMD values obtained downrange were less than calibration MMD values in 5 tests (10-93% of calibration values), but 1.1 to 4.4 times greater in 7 remaining tests. With exception of 2 tests, susceptible Sebring strain Culex quinquefasciatus mortality at the 31-m (100-ft) stake, 15 m (50 ft) behind the DC-III, was consistently 100%. The average MMD values for malathion, permethrin, and resmethrin (at 0.003 and 0.007 rates) at this distance were 13.24 +/- 6.08, 7.48 +/- 5.69, 16.64 +/- 5.54, and 15.48 +/- 2.89, respectively.