Coquillettidia perturbans (Walker), a competent bridge vector of eastern equine encephalomyelitis virus (EEEV), is found throughout North America where suitable habitat exists. Fatalities in humans due to EEEV are rare, yet the virus regularly circulates at high levels in native bird populations in late summer and early fall. A number of possibilities exist that may explain the low transmission of EEEV from viremic birds to mosquitoes and ultimately to dead end hosts, including differences in bird/mosquito habitat preference, mosquito avoidance behavior by birds, mosquito host-seeking behavior and mosquito/human interaction.
Our study examined two hypotheses relating to bird/mosquito interaction: 1.) Cq. perturbans females host-seek preferentially in sylvan habitats where birds most typically roost versus open field and marsh habitats and 2.) Cq. perturbans females exhibit changes in both temporal and vertical distribution while host-seeking in a sylvan habitat.
Field studies were conducted at Colliers Mills Wildlife Management Area in Ocean County New Jersey. For the first part of the study three ABC (American Biophysics Corp. Jamestown RI) traps supplemented with carbon dioxide [CO2 ] and 1-octen-3-ol [octenol] were placed at a height of 1m in each of the three habitats: forest, open field and marsh . This part of the study was replicated five times (14, 30 July and 4, 6, 19 August 1998). Traps were operated from late afternoon until late morning the following day. Trap catches were returned to the lab, where they were sorted, counted and pooled within habitat. Mean Cq. perturbans was calculated for each habitat and each collection date. To test the first hypothesis of habitat preference, we compared the mean number of Cq. perturbans caught in the three habitats throughout the study period (5 dates) with a 2-way ANOVA (Zar, 1984, SPSS GLM version 7.5.1, Norusis, 1995). For the second part of the study ABC traps were equipped as described above. Three traps were placed at each of three heights (1.0, 2.5, and 6.0 m) for a total of nine traps per collection date. Traps were placed randomly throughout the forest and those at 2.5 and 6.0 m were operated on a pulley system. Trap collections were taken hourly from one hour before sunset to one hour after sunrise. Mosquitoes were sorted and counted on site. To simplify the analysis, we pooled the hourly data for each height into three time periods (evening, night, and morning), each containing approximately 4 hours. Three replicates of this study were conducted (July 14/15, July 22/23 and August 12/13). To test the second hypothesis of temporal and vertical distribution, we compared the mean number of Cq. perturbans caught during the three pooled time periods (evening, night and morning) at the different trap heights using a 2-way ANOVA, with Bonferroni as the post-hoc test. We analyzed the data by date since seasonal effects were not under consideration.
For the first hypothesis of habitat preference, we found no significant interaction between the mean number of host-seeking Cq. perturbans for date and habitat type (mosquitoes tended to behave similarly on each date, F=1.628,30, p=0.160) and therefore we examined simple main effects. The mean number of mosquitoes significantly differed among the three habitat types of forest, marsh and field (F=3.792,30, p=0.034). We found more host-seeking mosquitoes in the woods than in the marsh (Bonferroni = 23.9, p=0.038). Field and marsh did not significantly differ in the number of mosquitoes (Bonferroni = 17.8, p=0.174) as neither did forest and field (Bonferroni = 6.13, p=1.0.). These findings indicate that Cq. perturbans leave the marsh from which they emerge and preferentially host-seek in the forest and to a lesser extent the field. Their host-seeking behavior places them in habitats that are roosting sites of birds such as Blue Jay (Cyanocitta cristata), Wood Thrush (Hylocichla mustelina) and Tufted Titmouse (Parus bicolor), known to have the highest percentages of viremic individuals respectively (Crans et al. 1994).
For the second hypothesis of temporal and vertical distribution, we found a significant 3-way interaction among date, height, and period (F=2.1788,54, p=0.044) and therefore we examined the relationship between height and period for each date. This interaction probably reflects the difference in the total number of mosquitoes caught per night, with July 22-23 trap catches as much as 6 times the number of mosquitoes caught as the other dates. Despite this disparity in the total numbers caught, the same trend was seen for all trap nights: more mosquitoes were caught during the night period. For each night, we found a significant interaction between height and period (July 14-15: F=5.8134,18, p=0.003; July 22-23: F=3.2134,18, p=0.037; August 12-13: F=6.6464,18, p=0.002) and therefore we looked for differences among the means of both height and period while controlling for the presence of each other variable. In general, a trap placed at 1m attracted significantly more mosquitoes than either a 2.5m or a 6m trap. The latter two trap counts did not differ significantly from each other in the number of mosquitoes caught within any period or date.
Our results indicate that Cq. perturbans host-seek equally
in sylvan and field habitats. Additionally, we did not observe
any vertical change in host-seeking temporally as reported by
Snow and Pickard (1957) and Downe (1962). These findings are important
because we found host-seeking mosquitoes together with roosting
and/or nesting birds in the same habitat, and more host-seeking
mosquitoes were collected nearer the ground in the sylvan habitat
as compared to higher up in the canopy. Therefore Cq. perturbans
host-seeking behavior would spatially remove them from the forest
canopy where birds are typically thought to roost, limiting bird
to mosquito EEEV transmission.
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Some aspects of host selection by Mansonia perturbans (Walk.)
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Norusis, M. 1995
SPSS6.1 Guide to Data Analysis. Prentice-Hall: Upper Saddle River,
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Biostatistical Analysis. Prentice-Hall, Englewood Cliffs, NJ.