Wayne A. Rowley

The effect of age on the flight performance of female Aedes aegypti mosquitoes

Abstract The paucity of information concerning mosquito flight ability resulted in experiments designed to estimate some aspects of mosquito flight potential under laboratory conditions. Each day for 6 weeks, eight female Aedes aegypti mosquitoes were flown on flight mills, and the distance flown, duration of flights, flight speed, and weight lost during exhaustive flights were recorded. Comparisons of the performances of mosquitoes of different ages indicated that no statistical differences existed in flight ability as measured between consecutive days. Differences did occur between weekly age groups. Flight performance was maximal during the first 14 days of adlut life. Flight performance (distance flown) decreased by 42 per cent at the beginning of the third week (day 15) and again, sharply, in the fourth week. Exhausted mosquitoes were analysed for glycogen content (the primary source of flight energy) and comparisons were made with glycogen levels of non-flown controls of identical physiological age. Mosquitoes aged from 1 to 12 days utilized almost all available glycogen in exhaustive flights. During the third and fourth weeks, glycogen in exhausted mosquitoes increased concomitantly with a reduction in flight ability.

West Nile Virus Viremia in Eastern Chipmunks (Tamias striatus) Sufficient for Infecting Different Mosquitoes

Chipmunks might play a role in enzootic WNV cycles and be an amplifying host for mosquitoes that infect humans.

The effect of temperature and relative humidity on the flight performance of female Aedes aegypti.

Abstract The influence of temperature and r.h. on the flight performance of tethered virgin female Aedes aegypti was investigated. Mosquitoes of similar age were flown to exhaustion on flight mills at various temperatures and relative humidities. Parameters measured were distance flown, duration of flight, speed of flight, initial weight of mosquitoes, and live weight lost during exhaustive flight. The temperature range at which sustained tethered flight occurred was 15 to 32°C. Temperature extremes at which flight was possible were 10 and 35°C; however, performance was minimal in both duration and distance at these temperatures. The optimal temperature for flight was 21°C. Mosquitoes flew farther at 15°C than at the commonly reported optimal temperature of 27°C. In general, flight performance was greater below 27°C. Relative humidity within 30 to 90 per cent had no demonstrable influence on flight performance at any temperature studied except at 32°C, where 30 per cent had a marked limiting effect on mosquito flight.

Attraction of Anopheles (Diptera: Culicidae) to Volatile Chemicals in Western Kenya

Abstract Anopheles gambiae s.l. and Anopheles funestus Giles are the primary vectors of malaria in East Africa. Identification of host-location olfactory cues may increase trap sensitivity for vector control and surveillance programs. Solid-state army miniature light traps were operated near sleeping humans in huts at night without lights and augmented with the potential attractants: L-lactic acid, Limburger cheese volatiles, hexanoic acid, and carbon dioxide. Mosquito response varied between species and gender. Female An. funestus exhibited a greater response to traps baited with L-lactic acid in combination with carbon dioxide than carbon dioxide alone in two different experiments.

Fox squirrels (Sciurus niger) develop West Nile virus viremias sufficient for infecting select mosquito species.

The West Nile virus (WNV) viremia and shedding profiles of 11 adult fox squirrels (Sciurus niger) infected by needle inoculation or mosquito bite were characterized. Daily mean titers (95% confidence intervals) for all squirrels on days 1 through 6 postexposure (p.e.) were: 10(1.7 (1.32.1)), 10(4.4 (4.04.8)), 10(5.3 (5.05.6)), 10(4.4 (3.94.9)), 10(2.7 (2.03.4)), and 10(1.1 (0.52.1)) plaque-forming units (PFU)/mL. The highest WNV serum titers of individual squirrels infected by needle inoculation or mosquito bite ranged from 10(4.5) to 10(6.1) and from 10(5.1) to 10(5.3) PFU/mL, respectively. Nine (82%) squirrels, including all 4 squirrels infected by mosquito bite, had WNV serum titers > or =10(5.1) PFU/mL that persisted on average for 1.6 +/- 0.3 days. Infection and dissemination rates of Culex pipiens (L.) that fed on squirrels with serum titers of 10(4.4 +/- 0.1) PFU/mL were 56% and 13%, respectively. Both of these rates increased to over 80% when fed on squirrels with a mean WNV titer of 10(5.5 +/- 0.1) PFU/mL. Infection and dissemination also occurred in Aedes triseriatus (Say) but at a much lower rate. WNV was isolated from the oral and rectal cavities of all squirrels and from urine that was opportunistically collected from 5 squirrels. The largest quantity of WNV recovered from swabs of the oral cavity and urine was 10(3.1) PFU. The longest periods after exposure that WNV was isolated from the oral cavity and urine from a squirrel were 22 and 17 days p.e., respectively. WNV RNA was also detected in kidney tissue in 1 squirrel 29 days p.e., suggesting that fox squirrels can be persistently infected. Collectively these observations provide further evidence that squirrels can contribute to the natural history and epidemiology of WNV, especially in peridomestic environments.

Detection of dengue virus neutralizing antibodies in bats from Costa Rica and Ecuador.

Abstract Neutralizing antibodies for dengue virus serotypes 1 and 2 and serotypes 2 and 3 were detected in 1998 in 12 of 53 (22.6%) and 3 of 10 (30.0%) bats sampled in Costa Rica and Ecuador, respectively. Dengue is a consistent health problem in the two Costa Rican communities in which bats were sampled. The high percentage of bats with neutralizing antibodies to dengue virus in these two Costa Rican communities suggests that bats may become infected with dengue virus. This appears to be the case in Costa Rica and Ecuador.

A Device for Sampling Aquatic Insects Associated with Carrion in Water

A device that permits the sampling of aquatic insects associated with carcasses in water with minimal disturbance of the natural environment is described. A field study conducted in early summer documented strictly aquatic insects recovered from decaying pig carcasses in an Iowa freshwater lake using this sampling method. Forty carcasses of newborn pigs were placed in the devices and removed at two-day intervals to simulate a complete decomposition sequence. Twenty species of insects from twelve families in five orders were documented, along with water mites (Arachnida:Acari) and amphipods (Crustacea:Amphipoda).

Vector Competence of Aedes vexans (Diptera: Culicidae) for West Nile Virus and Potential as an Enzootic Vector

Abstract Vector competence of Aedes vexans (Meigen) and Culex pipiens pipiens L. (Diptera: Culicidae) for West Nile virus (family Flaviviridae, genus Flavivirus, WNV) was compared. Infection rates of both species were similar 14 d after feeding on chickens, with WNV titers ranging from 104.2 to 108.7 plaque-forming units (PFU)/ml. Median infectious doses and 95% confidence intervals (CI) were 106.0(5.8,6.3) and 105.7(5.4,5.9) PFU for Ae. vexans and Cx. p. pipiens, respectively. WNV transmission was not observed in Ae. vexans that fed on chickens with WNV titers <105.0 PFU/ml, in contrast to a mean (95% CI) transmission rate of 7(2,18)% for Cx. p. pipiens. Mean WNV transmission rates for Ae. vexans and Cx. p. pipiens were 13(7,21)% and 10(5,19)%, respectively, after feeding on chickens with WNV titers of 105.3 ± 0.1 and 105.7 ± 0.1 PFU/ml, and 31(25,37)% and 41(30,53)% after feeding on chickens with WNV titers ≥106.1 ± 0.1 PFU/ml. Time postinfection (p.i.) significantly influenced WNV transmission by Ae. vexans as indicated by a nearly 10-fold increase in transmission rate between days 7 and 14 p.i. Mean WNV load expectorated with saliva of Ae. vexans was 102.4(2.1,2.7) PFU, and it was not significantly affected by the titer of chickens on which they originally fed or time p.i. These data indicate that vector competence of the primarily mammalophilic Ae. vexans, which also feeds on birds, approaches that of Cx. p. pipiens for WNV. Because peridomestic mammals, such as cottontail rabbits, squirrels, and chipmunks, develop WNV titers infective for Ae. vexans, this species may play a significant role in WNV enzootic cycles.

Longevity and spontaneous flight activity of Culex tarsalis (Diptera: Culicidae) infected with western equine encephalomyelitis virus.

Abstract The longevity of an Iowa strain of Culex tarsalis Coquillett fed blood meals containing 2 concentrations of western equine encephalomyelitis virus from Iowa (WEE-7738) was compared with that of Cx. tarsalis fed blood without virus. Females exposed to 4.7–5.0 log TCID50 per mosquito of WEE-7738 did not live as long as mosquitoes exposed to 2.7–3.0 log TCID50 per mosquito or controls. Only 1% of mosquitoes fed blood containing the higher virus concentration survived to day 18 after exposure. However, 13% of mosquitoes fed blood with the lower virus titer and 19.5% of the controls were still alive on day 18 after exposure. Flight activity scores of Cx. tarsalis infected with 4.7–5.0 log TCID50 per mosquito of WEE-7738 were 27.5% lower, and there were 26.1% fewer spontaneous flights than noninfected controls from days 6–11 after infection. After day 8 after infection, infected Cx. tarsalis had 37.1% lower activity scores and 40.0% fewer spontaneous flights than noninfected controls. Virus infection did not affect how long a mosquito flew in a 24-h period (the daily flying time) or the duration of individual flights. The spontaneous flight activity pattern (circadian rhythm) of infected mosquitoes was identical to those of controls. Both infected and noninfected mosquitoes began spontaneous flight activity at 2000–2100 hours (CST) and were active throughout the entire dark phase of the 24-h cycle. Although mosquitoes were active throughout the night, there was a burst or peak of activity between 2200 and 2300 hours when the complete dark cycle began. These results indicate that the adverse effect of WEE infection on longevity and spontaneous flight activity of Cx. tarsalis may decrease vectorial capacity of Cx. tarsalis for WEE.

Age-related ultrastructural changes in the flight muscle of the mosquito, Culex tarsalis☆

The flight muscle of Culex tarsalis and other Diptera is asynchronous fibrillar muscle. Ultrastructurally, we found that it resembles flight muscle described in other insects, except that it contains more glycogen than has been reported for other species. Profound ultrastructural changes occur in relation to age. Newly emerged mosquitoes have fewer glycogen deposits than older individuals. Glycogen in the young mosquito is in the form of beta particles, 25 nm in diameter, and flight muscle mitochondria of newly emerged individuals are smaller and have fewer cristae than those in older mosquitoes. Three-day-old mosquitoes (the age of all mosquitoes is expressed in days after eclosion) exhibit an abundance of glycogen, some of which exists as a dense aggregation along the M-line (intrafibrillar glycogen). Mitochondria in 3-day-old mosquitoes (i.e. 3 days after eclosion) are more dense than in newly emerged individuals, and about twice as large. In mosquitoes aged 42 days, intrafibrillar glycogen still exists as beta particles, but extrafibrillar glycogen is primarily of the alpha or rosette type. Mitochondria in individuals of this age group are present in two morphological forms. Most are similar in morphology to mitochondria of 3-day-old mosquitoes. A few, however, exhibit fewer cristae, some of which may be tubular, and a less dense matrix. Most glycogen has disappeared from both the intrafibrillar and extrafibrillar locations, in 84-day-old (after eclosion) mosquitoes. Mitochondria are present in two morphological types, as they were in insects, 42 days after eclosion.