Aedes albopictus abundance and phenology along an altitudinal gradient in Lazio region (central Italy)

Abstract

Background: The Asian tiger mosquito Aedes albopictus (Skuse 1894), which is native to Southeast Asia, is among the top 100 invasive species worldwide and one of the most troubling vector species. It has become established in more than 20 European countries. Since its arrival in Italy in the 1990s, the species has colonized all the regions of the country, up to an altitude of 600 m. Nevertheless, no thorough investigation has ever been performed to confirm or extend its elevation limit (EL) in Italy.

Methods: To define the EL of Ae. albopictus and analyse its phenology along an altitudinal gradient, we carried out an investigation by means of ovitraps placed in Lazio region, central Italy. Sampling was performed on a weekly basis in 13 villages within five 200-m altitudinal ranges [0-1000 m above sea level (asl)], with the addition of higher localities to the species range whenever the species was recorded in the highest range.

Results: Aedes albopictus has colonized sites well beyond its known EL, with established populations at 900 m asl and positive ovitraps recorded at 1193 m asl. The relationship between egg abundance and elevation was described by an exponential decay regression, which predicted an EL for oviposition at 1015 m asl. In the active season, egg-laying started earlier at low altitude and ended earlier within the highest altitudinal range. Aedes albopictus abundance and activity period (number of days active) decreased, respectively, by 95% and 34% from the lowest to the highest altitudinal range.

Conclusions: Using data from the present study, the altitudinal limit of Ae. albopictus in central Italy was updated from 600 to 900 m asl. In addition, established populations were predicted to exist up to 1015 m asl. Considering that up to 99.5% of Lazio region's inhabitants could potentially be affected by Aedes-borne virus outbreaks, the surveillance area for Ae. albopictus should be expanded accordingly. However, our results also indicate that Ae. albopictus surveillance programs need to be revised in order to harmonize the resources earmarked for these with the altitudinal changes in the phenology of this species.

Keywords: Culicidae; Disease vector; Distribution; Invasive mosquitoes; Risk map.

Fig. 1

Map of the study area in Lazio region, central Italy, with sampling localities (dots) and transects (lines) highlighted (a). Elevation profile of each transect produced using the Profile Tool in QGIS from the digital elevation model of Italy, to visualize the sectional view as it would appear when connecting one locality to the next by a straight line (b). NT North transect, ET east transect

Fig. 2

a Map of Lazio region showing the municipalities in red (dark grey in the monochrome version) (n = 281) where Aedes albopictus surveillance should be carried out according to the national plan for the surveillance and response to arboviruses (PNA), namely those municipalities with a mean altitude < 600 m above sea level (asl). The boundaries of municipalities with on-going surveillance (n = 24) have been highlighted in orange (light grey in the monochrome version). The remaining municipalities with a mean altitude above 600 m asl (n = 97) are in white. b Relationship between the weekly maximum (max) number of eggs laid (MEggs) recorded in each ovitrap (OT) and altitude range, as described by the best-fitting model (exponential decay). Bias-corrected and accelerated confidence intervals (CIs) calculated with 1000 bootstrap replicates are in light green (white in the monochrome version). The grey shaded areas indicate the decreasing abundance of MEggs at increasing altitude till the highest locality where the species was recorded. c Altitudinal and MEggs ranges (in parentheses) according to the exponential decay results, with municipalities in decreasingly intense shades of red (grey in the monochrome version) at increasing altitude (with decreasing MEggs abundance). max Maximum, min Minimum

Fig. 3

Gaussian model fitted to the seasonal trend of Aedes albopictus mean weekly number of eggs laid (mEggs_w) at different altitudinal ranges. Blue solid lines (black in the monochrome version) show the best-fitting Gaussian model (negative binomial), dashed lines indicate the CIs calculated with the parametric bootstrapping approach. Note the change in scale of the y-axis (0–30 mEggs_w at the highest altitudinal range, and 0–400 mEggs_w at the lowest altitudinal range)