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Journal of Medical Entomology Saul Lozano-Fuentes 1
SAMPLING, DISTRIBUTION, Department of Microbiology, Immunology 2
DISPERSAL and Pathology, 3
Colorado State University, 4
LOZANO-FUENTES ET AL.: 1690 Campus Delivery 5
TEMPORAL CHANGES IN Fort Collins, CO 80523 6
Aedes aegypti ABUNDANCE Phone: (970) 491 8745 7
E-mail: [email protected] 8
9
Intra-Annual Changes in Abundance of Aedes (Stegomyia) aegypti and Aedes (Ochlerotatus) 10
epactius (Diptera: Culicidae) in High-Elevation Communities in México 11
12
SAUL LOZANO-FUENTES1,4
, CARLOS WELSH-RODRIGUEZ2, ANDREW J. 13
MONAGHAN3, DANIEL F. STEINHOFF
3, CAROLINA OCHOA-MARTINEZ
2, BERENICE 14
TAPIA-SANTOS2, MARY H. HAYDEN
3, AND LARS EISEN
1 15
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1 Department of Microbiology, Immunology and Pathology, Colorado State University, 3185 17
Rampart Road, Fort Collins, CO 80523. 18
2 Centro de Ciencias de la Tierra, Universidad Veracruzana, Calle Francisco J. Moreno 207, 19
Colonia Emiliano Zapata, Xalapa, Veracruz, Mexico C.P. 91090.
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3 National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307. 21
4 Corresponding author, e-mail: [email protected] 22
23
2
Abstract 24
We examined temporal changes in the abundance of the mosquitoes Aedes (Stegomyia) aegypti 25
(L.) and Aedes (Ochlerotatus) epactius Dyar & Knab from June – October 2012 in one reference 26
community at lower elevation (Rio Blanco; ~1,270 m) and three high-elevation communities 27
(Acultzingo, Maltrata, and Puebla City; 1,670-2,150 m) in Veracruz and Puebla States, México. 28
The combination of surveys for pupae in water-filled containers and trapping of adults, using 29
BG-Sentinel traps baited with the BG-Lure, corroborated previous data from 2011 showing that 30
Ae. aegypti is present at low abundance up to 2,150 m in this part of México. Data for Ae. 31
aegypti adults captured through repeated trapping in fixed sites in Acultzingo – the highest 32
elevation community (~1,670 m) from which the temporal intra-annual abundance pattern for Ae. 33
aegypti has been described – showed a gradual increase from low numbers in June to a peak 34
occurring in late August and thereafter declining numbers in September. Aedes epactius adults 35
were collected repeatedly in BG-Sentinel traps in all four study communities; this is the first 36
recorded collection of this species with a trap aiming specifically to collect human-biting 37
mosquitoes. We also present the first description of the temporal abundance pattern for Ae. 38
epactius across an elevation gradient: peak abundance was reached in mid-July in the lowest 39
elevation community (Rio Blanco) but not until mid-September in the highest elevation one 40
(Puebla City). Finally, we present data for meteorological conditions (mean temperature and 41
rainfall) in the examined communities during the study period, and for a cumulative measure of 42
the abundance of adults over the full sampling period. 43
Keywords: Aedes aegypti, Aedes epactius, abundance, temporal changes, high elevation, México 44
3
Introduction 45
The mosquito Aedes (Stegomyia) aegypti (L.) is a primary vector of dengue, yellow fever and 46
chikungunya viruses (Gratz 1999, Gubler 2002, Weaver and Reisen 2010). We reported 47
previously on the collection of Ae. aegypti at high elevation (1,600-2,100 m above sea level) in 48
Veracruz State and Puebla State, México, based on recovery of immatures from water-holding 49
containers on residential premises and species identification conducted after rearing to the adult 50
stage (Lozano-Fuentes et al. 2012a). Those collection records extended the known elevation 51
range for Ae. aegypti in México by >300 m, as previously published studies had not reported this 52
mosquito species, or local dengue virus transmission, above 1,750 m (Ibanez-Bernal 1987, 53
Herrera-Basto et al. 1992). Our collections from high-elevation communities also commonly 54
included another mosquito species: Aedes (Ochlerotatus) epactius Dyar & Knab (Lozano-55
Fuentes et al. 2012b). The females of Ae. epactius reportedly are aggressive blood feeders 56
(O’Meara and Craig 1970, Farajollahi and Price 2013), and a laboratory strain of this mosquito 57
was shown to be capable of transmitting Jamestown Canyon virus (Heard et al. 1991). 58
Because our previous study, with fieldwork conducted from July – August 2011, included 59
as many as 12 communities along an elevation/climate gradient from Veracruz City (sea level) to 60
Puebla City (above 2,100 m), it was restricted to a single sampling occasion per community. 61
Herein, we report on a follow-up study with repeated sampling, conducted from June – October 62
2012, that focused on a subset of four communities, of the 12 examined in 2011, located at 63
elevations ranging from 1,200 – 2,100 m. These communities represented areas where the 64
abundance of Ae. aegypti in the previous year ranged from moderate (Rio Blanco; ~1,270 m; 65
estimated proportion of premises with Ae. aegypti present in 2011 of 0.62) to low (Acultzingo; 66
4
~1,670 m; 0.26) and very low (Maltrata; ~1,720 m; 0.06; and Puebla City; ~2,150 m; 0.05) 67
(Lozano-Fuentes et al. 2012a). 68
The primary aim of the present study was to determine temporal changes in the 69
abundance of Ae. aegypti adults in high-elevation communities, through bi-weekly trapping of 70
mosquitoes using the BG-Sentinel trap, during the wet and warm part of the year (June to late 71
September / early October). Secondary aims included: (1) corroborate, through a combination of 72
trapping of adults and surveys for pupae, the presence of Ae. aegypti in two high-elevation 73
communities (Maltrata and Puebla City) where it was collected and identified conclusively to 74
species in very low numbers (7 and 3 specimens, respectively) in the previous year (Lozano-75
Fuentes et al. 2012a); and (2) determine if Ae. epactius adults can be collected with the BG-76
Sentinel trap and, if so, examine temporal changes in the abundance of this species along the 77
elevation gradient from Rio Blanco to Puebla City. 78
79
Materials and Methods 80
Study environment. Studies were conducted in four communities in Veracruz and 81
Puebla States (Figure 1). Population size, elevation, and basic meteorological characteristics of 82
the study communities are given in Table 1 and Figure 2. The community of Rio Blanco, at 83
~1,270 m, was included as a lower elevation reference for comparison with the high elevation 84
communities located near the elevational range margin for Ae. aegypti (Acultzingo, Maltrata, and 85
Puebla City; 1,670-2,150 m). 86
To facilitate comparison among communities, the study focused on neighborhoods 87
dominated by middle-income homes with small to medium-sized yards. Neighborhoods 88
dominated by the following premises types were excluded from the study: high income premises 89
5
and low income “fraccionamiento” style premises, which typically are small homes clustered 90
closely together and with very small yards. Based on a survey of the characteristics of the 91
premises that were included in the study (data not shown), the typical home was a one-story 92
house constructed from concrete, brick, or cinder blocks, and with a roof made of concrete. The 93
vast majority of the study homes (> 95%) had piped water (albeit with varying water outage 94
schedules) and regular trash removal services but lacked air conditioning. The average number 95
of rooms per house was 4.8 and the average lot size was ~400 m2. Shrubs and trees were 96
common in the yards, and potential water-holding containers were frequently observed outdoors 97
on the premises (averages of 56 potentially water-holding containers and 8.2 actual water-98
holding containers per premises, with no distinction made between containers filled by rain 99
versus human action was made per premises). 100
Imagery available through Google Earth (Google, Mountain View, CA), typically <3 yr 101
old, was used to select four clusters within each of the four communities. A cluster was defined 102
as an area of ~1 km2 including blocks (groups of houses surrounded by streets or roads) 103
considered suitable for inclusion in the study. When possible, with the exception of the small 104
communities of Acultzingo and Maltrata, clusters were separated by a distance of at least 1 km, 105
which exceeds the typical flight range (<100 m) of Ae. aegypti (Harrington et al. 2005). Adult 106
trapping and pupal surveys were both performed within the selected clusters, but pupal surveys 107
were not done on the specific premises where adults were trapped. 108
Trapping of adults. Trapping of adult mosquitoes was done with battery-operated BG-109
Sentinel (BGS) traps equipped with the BG-Lure (a combination of lactic acid, ammonia, and 110
fatty acids, especially caproic acid; Biogents AG, Regensburg, Germany). The battery runs a fan 111
located inside the trap that circulates air between the trap and the environment. The air coming 112
6
out of the trap disperses the lure chemicals while the air coming in forces attracted insects into a 113
collection net. The rationale for using this particular trap is that it has emerged as a standard for 114
collection of adult Ae. aegypti, our target species (Krockel et al. 2006, Maciel-de-Freitas et al. 115
2006, Williams et al. 2006, Barrera et al. 2011, Johnson et al. 2012, Salazar et al. 2012). Each of 116
the four examined communities held 10 fixed trap locations on 10 different residential premises 117
(1 trap per premises). No single cluster within a community had more than four fixed trap 118
locations. Specific trap locations were under roof cover to prevent rain from damaging the traps 119
or the mosquito catches, and in the backyard to minimize the risk of traps or batteries being 120
stolen or vandalized. To maximize catches of Ae. aegypti adults as they enter or exit homes, 121
traps were placed in proximity to windows or doors. 122
Trapping was conducted every two weeks from early June to late September / early 123
October 2012, for a total of nine sampling occasions for each of the 40 fixed trap locations 124
(Table 1). This time period typically includes the warmest and wettest months in the targeted 125
communities (e.g., Fernandez-Eguiarte et al. 2014). For each sampling occasion, the traps were 126
operated over a 48-hr period, with the battery and mosquito catch bag replaced after the first 24 127
hr. The total number of trap-days during the study was 720 (9 sampling occasions x 40 trap 128
locations x 2 days of trapping per sampling occasion). 129
Mosquitoes recovered from the traps were stored dry in tubes together with a desiccant 130
(t.h.e. Desiccant 100% Indicating; EMD Chemicals, Waltham, MA) prior to identification. The 131
adults were identified, using the key of Darsie and Ward (2005), to the following taxonomic 132
entities: (1) Ae. aegypti, (2) Ae. epactius, or (3) a grouping consisting of any other mosquito 133
species (hereinafter referred to as other mosquito species). 134
7
Pupal surveys. We focused solely on pupae in the surveys because the pupal stage has 135
lower mortality than the larval stage and pupal abundance therefore is a better proxy for the 136
abundance of emerging adults compared to abundance of both immature stages combined (Tun-137
Lin et al. 1996, Focks and Chadee 1997, Knox et al. 2010). The minimum target number of 138
premises to examine per community and sampling occasion was 50; these fell within 3 – 4 139
different clusters per community. A larger number of premises, >100, were examined per 140
sampling occasion in Puebla City because of the very low abundance of Ae. aegypti in that 141
community in our previous survey for immatures (Lozano-Fuentes et al. 2012a). No more than 5 142
premises were examined within a single block. Survey teams started at the northeastern corner 143
of a block and then proceeded in a clockwise direction, sampling every household for which 144
someone was present to permit entry. Only homes within a targeted block that presented obvious 145
safety concerns for the survey teams were excluded. 146
Pupal surveys were conducted on two occasions in each community, between early July 147
and mid-August (Table 1). As shown in Table 1, pupal surveys proceeded from the lowest 148
elevation community (Rio Blanco) to the highest one (Puebla City) within each of the two 149
sampling rounds; this was designed to minimize the potential confounding effect of increasing 150
mosquito numbers over time on the comparison of presence and abundance of Ae. aegypti among 151
communities. Out of the total of 550 premises visits to conduct pupal surveys, only 10 152
individual premises were surveyed during both sampling occasions. 153
Water-holding containers located outdoors on the study premises were examined for 154
presence of mosquito pupae. Container types were classified following the scheme described 155
previously by Lozano-Fuentes et al. (2012a, b). The following container types were excluded 156
from the examination based on safety concerns or difficulty of access: plastic roof water tanks, 157
8
rain gutters, and septic tanks. All mosquito pupae were collected from small (≤5 liter) to 158
medium-sized (5-200 liters) containers, whereas we followed the methodology described by 159
Romero-Vivas et al. (2007) for sampling of larger ones (≥ 200 liters), such as barrels/drums or 160
cement tanks, with a sweep net mounted on a pole. This methodology also estimates the total 161
number of pupae in a large container based on those collected with a single sweep of the net and 162
a multiplication factor determined by the container water capacity (less or more than 1,000 liters) 163
and the water fill level (1/3 full, 2/3 full or full) (Romero-Vivas et al. 2007). 164
Collected pupae, separated by premises and container type, were transferred to plastic 165
bags with water and placed inside coolers for transport to the laboratory. The pupae were then 166
placed in emergence chambers (Mini Mosquito Breeder; Bioquip, Rancho Dominguez, CA) and 167
allowed to emerge to adults. Adults were stored and identified as described previously for those 168
recovered from BGS traps. 169
Estimation of mosquito abundance based on trapping of adults. We calculated the 170
mean daily abundance of Ae. aegypti and Ae. epactius adults per community based on the BGS 171
trap catches. The mean daily abundance of a given species for a community and sampling 172
occasion is calculated as the total number of adults collected in the 10 BGS traps during the 48 hr 173
collecting period divided by the total number of trap-days (n=20). We also calculated the 174
geometric mean daily abundance, but those data are not shown herein because they resulted in 175
similar bi-weekly abundance patterns as for the arithmetic mean. 176
In addition, we calculated the area under the curve (AUC) for the mean daily abundance 177
plots of Ae. aegypti and Ae. epactius adults over the full sampling period (i.e., from the first to 178
the last sampling date) in a given community. The overall AUC was calculated by first 179
determining and then summing AUCs between subsequent trapping occasions (i.e., from 5 to 19 180
9
June, from 19 June to 3 July, etc.). As an example, Figure 3 shows the AUCs for the 14 August 181
to 11 September period for Ae. aegypti in Acultzingo. Estimates for the days within the 182
sampling periods when we did not conduct mosquito trapping were based on a linear 183
relationship. The AUC combines mosquito abundance and time into an expression that describes 184
the intensity and duration of a mosquito infestation, similar to the concept of insect-days used in 185
economic entomology (Ruppel 1983, Dittrich et al. 1985, Archer and Bynum 1992, Beckendorf 186
et al. 2008, Ohnesorg et al. 2009). 187
The AUC between sampling occasions is calculated from the definitive integration of a 188
linear function (f(x) = mx + b) that describes the relationship between collection days. Using the 189
fundamental theorem of calculus we can say that: 190
And the indefinite integration of , where m and b are constants, is: 191
Thus: 192
where ti is the day-of-year for which the mosquito density estimation was performed. 193
Summation of AUCs is calculated as: 194
where: 195
10
196
Estimation of mosquito abundance based on pupal surveys. Of the 2,500 pupae 197
recovered in the surveys, 1,110 (44%) were successfully reared to the adult stage and identified. 198
To account for the remaining non-identified pupae we proportionally allocated them to Ae. 199
aegypti versus Ae. epactius or other mosquito species based on the identified adults from the 200
same container type and premises. Twenty five premises (with a total of 262 non-identifiable 201
pupae) out of the 550 premises surveyed were excluded from further analyses because 202
collections from these premises failed to produce any identifiable adults. 203
To account for complete sampling of small and medium-sized containers versus partial 204
sampling of very large containers (barrels/drums, water tanks and water cisterns) a multiplication 205
factor was applied. Container types with complete sampling were uniformly assigned a neutral 206
multiplication factor of 1, whereas multiplication factors ranging from 1.9 – 3.5 were used for 207
very large container types with partial sampling based on their water volume and water fill level 208
(Romero-Vivas et al. 2007). The final step in the estimation of abundance of pupae for a given 209
premises was to sum the estimated number of pupae across the encountered container types. 210
Meteorological data. Temperature and relative humidity (RH) data for the study period 211
(May – October 2012) were obtained from HOBO® (Onset Computer Corporation, Bourne, 212
MA) data loggers set up in each examined community. Rainfall data for the locations where the 213
HOBO loggers were placed were obtained from the 0.07° gridded Climate Prediction Center 214
Morphing technique (CMORPH) version 1.0 dataset (Joyce et al. 2004), which uses precipitation 215
estimates derived exclusively from low orbiter satellite microwave observations and features 216
11
transported via spatial propagation information obtained from geostationary satellite IR data. 217
CMORPH provides some of the most reliable estimates for tropical summer rainfall compared to 218
other satellite- and model-based rainfall products (Ebert et al. 2007). Weekly averages for mean 219
temperature and rainfall are shown in Figure 2 for the period from 1 May to 31 October 2012 in 220
the four study communities. 221
222
Results 223
Meteorological data. As expected, the mean temperature for the study communities was 224
negatively associated with their elevation. From 1 May to 31 October 2012, the seasonal mean 225
temperature was 20.3 °C for the lowest elevation community of Rio Blanco (just below 1,300 226
m), 18.6 and 19.1 °C, respectively, for the mid-range elevation communities of Acultzingo and 227
Maltrata (~1,700 m), and 17.8 °C for the highest elevation community of Puebla City (above 228
2,100 m). The 1 May – 31 October 2012 seasonal mean temperature departures from the 1951 – 229
2010 averages (Table 2) were -0.7 °C, -0.3 °C, +1.6 °C, and -0.8 °C for Rio Blanco, Acultzingo, 230
Maltrata, and Puebla City, respectively, indicating slightly cooler than normal conditions in all 231
communities except for Maltrata. The highest mean temperatures in the study communities were 232
recorded during early May and early June (Figure 2). Mean temperatures in a given community 233
then were relatively stable from late June to early September, albeit varying by 0.9–2.4 °C 234
among weeks in a given community, before decreasing in late September and October (Figure 2). 235
Rainfall patterns were variable among the study communities (Figure 2). The total 236
rainfall from 1 May to 31 October 2012 was higher in Rio Blanco and Puebla City (784 mm in 237
both communities) compared to Acultzingo and Maltrata (463 and 501 mm, respectively). The 1 238
May – 31 October 2012 total rainfall departures from the 1951 – 2010 averages (Table 2) were -239
12
883 mm, -44 mm, -161 mm, and -97 mm for Rio Blanco, Acultzingo, Maltrata, and Puebla City, 240
respectively, indicating drier-than-normal conditions, especially in Rio Blanco. There were 241
distinct peaks in weekly rainfall during July for Puebla City and from late July to early 242
September for Rio Blanco, whereas weekly rainfall from late June to early September was less 243
variable in Acultzingo and Maltrata (Figure 2). 244
Summary of mosquito collections. The overall numbers of Ae. aegypti and Ae. epactius 245
collected and conclusively identified to species during the study are summarized in Table 2. 246
Trapping of adults – based on 180 trap-days per community – produced 245 Ae. aegypti in the 247
lowest elevation community of Rio Blanco, and 28 and 1, respectively, for the mid-range 248
elevation communities of Acultzingo and Maltrata (Table 2). No Ae. aegypti adults were 249
collected by trapping in the highest elevation community of Puebla City. Pupal surveys 250
produced specimens later identified as Ae. aegypti in the adult stage from three of the four study 251
communities, with the greatest numbers from Rio Blanco (n=133), followed by Acultzingo 252
(n=21) and Puebla City (n=1) (Table 2). None of the conclusively identified mosquitoes from 253
Maltrata that originated from pupal surveys were Ae. aegypti. Our estimates for the mean 254
abundance of Ae. aegypti pupae per examined premises – including allocation of non-identified 255
pupae as described in the Materials and Methods – in the study communities (Table 2) further 256
underscore the trend of this species occurring most commonly in the lowest elevation community 257
of Rio Blanco, in lower numbers in the mid-range elevation community of Acultzingo, and in 258
very low numbers in the mid-range elevation community of Maltrata and the highest elevation 259
community of Puebla City. Aedes epactius was collected through both trapping of adults and 260
pupal surveys in all four communities (Table 2). 261
13
Temporal patterns of abundance, and cumulative abundance over the study period, 262
of Ae. aegypti and Ae. epactius based on repeated trapping of adults. The bi-weekly patterns 263
of abundance for adults over the study period in 2012 could be examined only for Rio Blanco 264
and Acultzingo for Ae. aegypti (due to collection of only zero or a single specimen in Maltrata 265
and Puebla City) but for all four communities for Ae. epactius. Aedes aegypti was collected on 266
all (Rio Blanco) or nearly all (Acultzingo) sampling occasions from early June to late September, 267
with the peak recorded abundance occurring in late August in both communities (Table 3, Figure 268
3). However, the bi-weekly abundance pattern differed between the two communities in that 269
abundances near the peak recorded value occurred already in early June for the lower elevation 270
community of Rio Blanco, whereas the abundance increased gradually from June to late August 271
in Acultzingo (Figure 3). In both communities, the abundance of Ae. aegypti declined over the 272
month of September (Figure 3). The bi-weekly patterns of abundance for Ae. aegypti adults, in 273
relation to weekly averages for mean temperatures and rainfall, are shown for Rio Blanco and 274
Acultzingo in Figure 4. Notably, an unusual rainfall event in early May, combined with warm 275
weather in the latter part of that month, was associated with a substantial but short-lived spike in 276
abundance of Ae. aegypti adults in early June in Rio Blanco and a minor spike in abundance in 277
Acultzingo. The abundance then declined in Rio Blanco and did not peak until late August, 278
following substantial rainfall in late July and early August. Rainfall occurring later, in early 279
September, had no similar association with increased abundance of Ae. aegypti adults as their 280
numbers declined and remained very low until late September in both Rio Blanco and 281
Acultzingo. 282
Aedes epactius displayed variable patterns of bi-weekly abundance across the study 283
communities (Table 3, Figure 3). The greatest contrast is seen when comparing the lowest and 284
14
highest elevation communities: Rio Blanco and Puebla City. For example, peak abundance 285
occurred in mid-July in Rio Blanco but not until mid-September in Puebla City (Figure 3). One 286
of the mid-range elevation communities – Acultzingo – displayed an intermediate pattern with 287
peak abundance recorded in early August, whereas the temporal abundance pattern in the other 288
mid-range elevation community – Maltrata – was more similar to that for Puebla City (Figure 3). 289
By the last sampling occasion in late September / early October, the abundance of Ae. epactius 290
had decreased to low levels in all four study communities (Figure 3). 291
To provide a quantitative estimate for total adults over the full study period, which is 292
more informative for human risk of encountering adult mosquitoes compared to the peak value 293
recorded during the study period, we calculated the area under the curve (AUC) for Ae. aegypti 294
and Ae. epactius adults in the study communities when this was possible for a given mosquito 295
species (Table 3). Calculations based on mean daily abundances per trap-day, calculated from 296
20 trap-days per sampling occasion, resulted in an AUC estimate of 75.8 for Ae. aegypti for the 297
full sampling period in Rio Blanco (20 x 75.8 = 1,516 total estimated adults over the full 298
sampling period based on our level of trapping effort) compared to only 8.0 for Acultzingo (160 299
total estimated adults) (Table 3). This represented a 9.4-fold difference between the estimates 300
for the two communities (75.8/8.0) and differentials between the two communities of 67.8 for the 301
AUC estimate (75.8-8.0) and 1,356 for total estimated adults (1,516-160). For Ae. epactius, the 302
corresponding AUC estimates for the full sampling period were more similar across the four 303
communities, ranging from 6.8 (136 total estimated adults) in Puebla City to 14.2 (284 total 304
estimated adults) in Rio Blanco (Table 3). Figure 3 appears to show a similar pattern, where the 305
highest abundance of Ae. epactius adults can be seen to shift among communities for different 306
15
sampling occasions rather than consistently being higher in one community (as seen in Figure 3 307
for Ae. aegypti adults in Rio Blanco versus Acultzingo). 308
309
Discussion 310
Our most notable findings are: (1) the description of the temporal abundance pattern for 311
Ae. aegypti adults at high elevation (~1,670 m); (2) the corroboration of data from Lozano-312
Fuentes et al. (2012a) showing that Ae. aegypti occurs in low numbers at elevations up to 2,150 313
m in México; (3) the first recorded collection of Ae. epactius with a trap designed to collect 314
human-biting mosquitoes; and (4) the first description of the temporal abundance pattern for Ae. 315
epactius across an elevation gradient. The main weaknesses of the study were that we were able 316
to examine the temporal abundance patterns only for a single year, and that the sampling did not 317
adequately capture the start of the active season in the lower elevation reference community of 318
Rio Blanco. Use of a greater number of traps in each study community also would be beneficial 319
in future studies targeting locations near the cool range margin where Ae. aegypti is present but 320
occurs only in low numbers. 321
In a previous study (Lozano-Fuentes et al. 2012a), we reported the collection of Ae. 322
aegypti immatures in 2011 in high-elevation communities (1,670–2,150 m) in México based on 323
surveys for larvae and pupae on residential premises. However, many of the specimens were 324
collected in the larval stage and then raised to adults for identification in the laboratory. This 325
transfer of larvae to potentially more favorable temperature conditions for continued 326
development raised the question of whether they could have progressed to the pupal and adult 327
stages in the examined high-elevation communities. The present study combined trapping of Ae. 328
aegypti adults with surveys of pupae, which are far more likely to emerge as adults compared to 329
16
larvae (Knox et al. 2010) and can complete the emergence to adults in water temperatures as low 330
as 13–16 °C (Bar-Zeev 1958). These collection efforts confirmed the presence of Ae. aegypti for 331
a second consecutive year in the high-elevation communities of Acultzingo (~1,670 m; 332
collection of 28 Ae. aegypti adults and 21 pupae in 2012), Maltrata (~1,720 m; collection of a 333
single adult), and Puebla City (~2,150 m; collection of a single pupa). The observed abundance 334
patterns in 2012 among these high-elevation communities and the lower elevation, warmer 335
reference community of Rio Blanco (Tables 1–2; Figure 2) was similar to that seen in 2011 336
(Lozano-Fuentes et al. 2012a), with large numbers of Ae. aegypti collected in Rio Blanco, 337
moderate numbers in Acultzingo, and very low numbers in Maltrata and Puebla City. 338
Perhaps the most intriguing finding is the discrepancy between Acultzingo and Maltrata, 339
which are located within 11 km of each other, have similar elevations (~1,670 and ~1,720 m), 340
and experienced similar meteorological conditions with regards to mean temperature and rainfall 341
during the study period in 2012 (Figure 2). Pursuing the question of why Acultzingo appears to 342
consistently support a larger Ae. aegypti population compared to nearby Maltata could be 343
fruitful. Both communities have an abundance of water-holding containers (data not shown) but 344
there could be differences in the likelihood of annual introductions of Ae. aegypti immatures as 345
Acultzingo is an often used rest stop just below a mountain pass used by commercial traffic to 346
and from Puebla City, whereas Maltrata is bypassed by the major road leading from Veracruz to 347
Puebla City. Moreover, there may be some subtle climatic differences of which we are still 348
unaware that negatively impact the bionomics of Ae. aegypti in Maltrata; for example, 349
temperatures during part of the year falling just above a critical biological threshold in 350
Acultzingo and just below the threshold in Maltrata. This line of thinking is supported by the 351
fact that BGS traps located in the lower altitude (1,623–1,630 m) portion of Acultzingo yielded 352
17
more Ae. aegypti adults compared with traps in the higher altitude (1,678–1,700 m) portion of 353
the community (17 total adults collected over 54 trap-days for lower altitude traps versus 11 total 354
adults over 126 trap-days for higher altitude traps. Wilcoxon rank sum test W = 3891; df = 54, 355
126; P = 0.002). The small community of Acultzingo has homogenous housing but is located 356
on a slope and encompasses an elevation range of nearly 100 m. We speculate that this 357
community includes microclimates with temperatures near important biological development or 358
survival thresholds for Ae. aegypti, and that the observed decrease in abundance of Ae. aegypti 359
adults in the higher altitude portion of the community result from the negative effects of 360
meteorological factors. 361
The observed temporal pattern for abundance of Ae. aegypti adults in the high-elevation 362
community of Acultzingo – unimodal with a gradual increase from late spring to a summer peak 363
followed by a decline in late summer and early fall – is commonly seen for mosquitoes near the 364
cool margin of their ranges, and a similar temporal abundance pattern was reported previously 365
for Ae. aegypti from Buenos Aires City in Argentina (Vezzani et al. 2004, de Majo et al. 2013). 366
To provide quantitative estimates for total adults over the full study period, we calculated 367
cumulative mean abundances of Ae. aegypti in the lower elevation reference community of Rio 368
Blanco and the high-elevation community of Acultzingo. We estimate that the cumulative 369
abundance of adults over the ~17 wk study period was 9.4-fold greater in Rio Blanco versus 370
Acultzingo. This type of estimate – similar to the concept of insect-days used in agricultural 371
entomology (Ruppel 1983, Dittrich et al. 1985, Archer and Bynum 1992, Beckendorf et al. 2008, 372
Ohnesorg et al. 2009) – can be useful when comparing different geographical locations over the 373
same time period or the same location in different years, or when exploring potential climate-374
18
driven change in mosquito populations in a given location, which could impact both the number 375
of days within the year during which adults can be active and the population size. 376
In our previous paper on Ae. epactius (Lozano-Fuentes et al. 2012b), we presented data 377
for immatures collected from water-holding containers and reviewed what little is known about 378
the biology of this mosquito. The present study adds new knowledge about the seasonality of the 379
adult stage. The females of Ae. epactius reportedly are aggressive blood feeders (O’Meara and 380
Craig 1970, Farajollahi and Price 2013), and here we present the first evidence that they can be 381
collected from residential premises with the BG-Sentinel equipped with the BG-Lure, designed 382
specifically to collect human-biting mosquitoes. It therefore would be interesting to determine in 383
future studies how commonly Ae. epactius females feed on humans versus other vertebrate 384
animals in the study area. 385
To the best of our knowledge, we present the first description of the temporal abundance 386
pattern for Ae. epactius across an elevation/climatic gradient. Our previous study (Lozano-387
Fuentes et al. 2012b) showed that Ae. epactius is encountered at elevations ranging from near sea 388
level in Veracruz City on the Gulf of México to above 2,100 m in Puebla City, and that the 389
mosquito is most abundant at elevations from 1,250–1,750 m and then decreases in abundance 390
above 1,800 m. This geographical abundance pattern was related to meteorological conditions, 391
as we found statistically significant parabolic relationships between the percentage of premises in 392
a community with Ae. epactius pupae present (peaking at mid-range elevations) and temperature-393
related factors (Lozano-Fuentes et al. 2012b). Here we present additional information showing a 394
linkage between the timing of peak recorded abundance of adults and elevation-dependent 395
meteorological conditions: the peak occurred by mid-July in the warmest study community (Rio 396
Blanco) but not until mid-September in the coolest one (Puebla City) (Figures 2–3). Similar to 397
19
the curious geographical pattern for abundance of Ae. aegypti, for which Maltrata is more similar 398
to the distant Puebla City than the proximate Acultzingo, the peak recorded abundance of Ae. 399
epactius adults occurred later in Maltrata and Puebla City (mid-September) than in Acultzingo 400
(early August). This finding strengthens our speculation that some yet-to-be-determined aspects 401
of the local climatic conditions in Acultzingo versus Maltrata may have important impacts on 402
mosquito biology. 403
404
Acknowledgments 405
We thank Eric Hubron, Elena Rustrian, Selene Tejeda, Marco Aurelio Morales, Selene 406
Janitzio Perez and Jesus Yair Zamora of Universidad Veracruzana for field and laboratory 407
assistance. We also thank the Ministries of Public Health, Education and Civil Protection of 408
Veracruz Government for their support. Finally, we are grateful to the involved home owners 409
for granting us access to collect mosquitoes from their properties. This study was funded by a 410
grant from the National Science Foundation to the University Corporation for Atmospheric 411
Research (GEO-1010204). 412
413
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24
Table 1. Characteristics of study communities in Veracruz and Puebla States, México, and 508
overview of mosquito sampling efforts from June–October 2012. 509
Community
Rio Blanco Acultzingo Maltrata Puebla City
Characteristics of study communities
Mean elevation (m)a 1,275 1,677 1,719 2,150
Mean annual temperature (C)b 19.7 17.6 16.7 17.2
Mean max. / min. July temp. (C)b 25.6 / 15.6 24.8 / 12.4 22.5 / 11.1 25.3 / 11.6
Mean max. / min. Jan. temp. (C)b 23.1 / 10.2 20.5 / 6.9 21.5 / 6.9 23.0 / 4.9
Mean annual rainfall (mm)b 1,903 579 783 961
Mean May–Oct. temp. (C) /
rainfall (mm)b
21.0 / 1,667 25.3 / 507 17.5 / 662 18.6 / 881
Population estimatec 40,000 7,040 11,840 1,434,000
Trapping of adults
First / last trapping date 5 June / 26 Sept. 5 June / 26 Sept. 12 June / 3 Oct. 12 June / 3 Oct.
No. fixed trap locations 10 10 10 10
No. sampling occasions 9 9 9 9
Total no. trap-days 180 180 180 180
Pupal surveys – 1st occasion
Survey dates 9-10 July 16-17 July 10-11 July 23-24 July
No. examined premises 56 48 50 129
Pupal surveys – 2nd
occasion
Survey dates 30 July 31 July - 1 Aug. 30-31 July 13-15 Aug.
No. examined premises 52 50 54 111 aFor the premises included in pupal surveys or trapping of adults;
bBased on 1951-2010 climate 510
normals obtained from Mexico’s Servicio Meteorológico Nacional; cBased on data for 2010 511
obtained from Mexico’s Instituto Nacional de Estadística y Geografía; 512
25
Table 2. Summary of collections of Ae. aegypti and Ae. epactius from June–October 2012. 513
Community
(elevation in
meters)
Total no. mosquitoes collected and
conclusively identified to species
Estimated mean abundance of pupae per examined
premises (standard error of the mean)
Ae. aegypti Ae. epactius Ae. aegypti Ae. epactius
Adult
trapsa
Pupal
surveysb,c
Adult
trapsa
Pupal
surveysb,c
1
st survey
d 2
nd survey
d 1
st survey
d 2
nd survey
d
Rio Blanco (1,275) 245 133 44 115 2.2 (0.9) 2.7 (1.6) 3.1 (1.5) 1.5 (0.7)
Acultzingo (1,677) 28 21 25 260 0.7 (0.5) 0.0 10.7 (3.4) 1.8 (0.9)
Maltrata (1,719) 1 0 40 64 0.0 0.0 0.5 (0.2) 4.1 (3.0)
Puebla City (2,150) 0 1 9 58 0.008 (0.008) 0.0 0.9 (0.3) 0.4 (0.2) aBased on 180 trap-days per community;
bOnly 44% of collected pupae emerged as adults and could be conclusively identified to taxa; 514
cThe total number of premises examined ranged from 98 in Acultzingo to 104 in Maltrata, 108 in Rio Blanco, and 240 in Puebla City; 515
dCommunity-specific survey dates are given in Table 1.516
26
Table 3. Aspects of the temporal occurrence of Ae. aegypti and Ae. epactius adults in the study communities based on trapping of 517
adults from early June to late September / early October 2012. 518
Community
(elevation in
meters)
Full extent
of 2012
sampling
period
Sampling occasion with
first collection
Sampling occasion with
peak recorded
abundance
Sampling occasion with
last collection
AUCb (Total estimated
adults)c
Ae. aegypti Ae. epactius Ae. aegypti Ae. epactius Ae. aegypti Ae. epactius Ae. aegypti Ae. epactius
Rio Blanco
(1,275)
5 June to 26
Sept. 5-6 June 5-6 June 28-29 Aug. 17-18 July 25-26 Sept. 25-26 Sept.
75.8
(1,516)
14.2
(284)
Acultzingo
(1,677)
5 June to 26
Sept. 5-6 June 5-6 June 28-29 Aug.
31 July-1
Aug. 25-26 Sept. 25-26 Sept.
8.0
(160)
8.0
(160)
Maltrata
(1,719)
12 June to 3
Oct. ND
a 12-13 June ND
a 18-19 Sept. ND
a 2-3 Oct. ND
a
12.8
(256)
Puebla City
(2,150)
12 June to 3
Oct. ND
a 26-27 June ND
a 18-19 Sept. ND
a 2-3 Oct. ND
a
6.8
(136) aNot determined, due to no or a single specimen trapped over the full sampling period;
bArea under the curve for the full sampling 519
period based on mean daily abundances per trap. See Materials and Methods for a description of how this was calculated. cBased on 520
overall effort with 20 trap-days per sampling occasion. 521
27
Figure legends 522
523
Figure 1. Locations of study communities in Veracruz State and Puebla State, México. 524
525
Figure 2. Weekly averages for selected meteorological variables – mean temperature (oC; left) 526
and mean rainfall (mm/day; right) – for May – October 2012 in the four study communities. 527
528
Figure 3. Temporal abundance pattern for Ae. aegypti (left) and Ae. epactius (right) adults from 529
early June to late September / early October 2012 in the study communities. The shading 530
illustrates the area under the curve (AUC) for the mean daily abundance for Ae. aegypti in 531
Acultzingo for the 14 August to 11 September period. 532
533
Figure 4. Temporal abundance pattern for Ae. aegypti adults from early June to late 534
September 2012 in Rio Blanco (left) and Acultzingo (right), in relation to weekly averages of 535
mean temperature (oC) and mean rainfall (mm/day). The break interval for the secondary Y-536
axis is set from 12 to 16 537
538
28
Figure 1.
29
Figure 2.
30
Figure 3.
31
Figure 4.
