Effects of Local Anthropogenic Changes on Potential Malaria Vector Anopheles hyrcanus and West Nile Virus Vector Culex modestus, Camargue, France

Sixty years of environmental modifications have led to strong and rapid effects on the abundance of vector populations

Seasonal synchronization of diapause phases in Aedes albopictus (Diptera:Culicidae)

In temperate areas, population dynamics of the invasive Asian tiger mosquito Aedes albopictus are strongly affected by winter. The work we present here analyzes the adaptive synchronization of the diapause process in the wintry generation of A. albopictus, where the egg stage is exposed to adverse winter conditions. The seasonal pattern of egg laying activity of a French Mediterranean population of the Asian tiger mosquito was monitored weekly for 2 years with ovitraps. The field diapause incidence and the critical photoperiod (CPP, i.e. the maternal day length inducing diapause in 50% of the eggs), were determined by hatching experiments on the collected eggs. The period of diapause termination was estimated by a field survey of the first hatchings for both years. The CPP is equal to 13.5 hours of light and occurs in the field on the 25th of August. Thus, it is on September 11th, 17 days after the CPP, that 50% of the eggs are in a prediapause stage in the field. The egg diapause rate increases rapidly during September, whereas the mean number of eggs laid decreases sharply after mid-September. Surprisingly, after having reached a peak of 95% at the end of September, from mid-October the diapause incidence declined and stayed below 50%. Indeed, both years the diapause initiates before the rapid decrease of the environmental temperature. This leaves a sufficient period of time to the complete development of one generation of A. albopictus with effective induction of diapause in the laid eggs. The very first larvae hatched were sampled both years in the first half of March. With 20 to 26 weeks in the egg stage and about 7 weeks in the larval stages, the first annual generation spends a long time in immature stages. On a practical point of view, this long development time represents a wide window for eggs and larvae control in early spring

Experimental area for seasonal egg laying dynamics and first annual egg hatching monitoring of <i>Aedes albopictus</i> in Mediterranean France.

<p>The area monitored by ovitraps from summer solstice 2010 to summer solstice 2012 was computed as the surface of the smallest polygon including all traps with a buffer distance of 50 m (black area). This area was safe from outdoor insecticide spraying for vector control (circles). Ten infested places localized around the ovitraps network were inspected daily in March 2011 and 2012 to detect the first larvae of the year (triangles).</p

Schematic depiction of the seasonal activity and diapause process of <i>Aedes albopictus</i> population in the Mediterranean city of Cagnes-sur-Mer, France.

<p>Horizontal lines represent the mean durations of diapause syndrome and of the long generation of <i>A</i>. <i>albopictus</i>. Vertical lines delineate the moment when diapause is induced (in red) or initiated (in dark) in 50% of eggs, and the start of diapause termination (in green).</p

Protocol for the determination of egg diapause incidence in <i>Aedes albopictus</i>.

<p>The critical photoperiod (CPP) of the population is determined by an experiment including all steps in this order: A, B, C, D, E, B, C and F (black arrows). The survey of diapause incidence in the field follows steps A, B, C and F (green arrows).</p

Photoperiodic analyses of egg diapause induction and initiation in <i>Aedes albopictus</i>.

<p>X axis represents the mean daylight duration during the egg laying week in the field in 2010 (green diamonds, n = 10) and 2011 (red dots, n = 36). Photoperiod inducing diapause was studied at fixed day length in laboratory (blue squares, n = 15). Sigmoid curves modeled for egg diapause incidences are in bold solid line (laboratory) or solid dashed lines (field). Smooth dotted lines represent the 95% confidence intervals. The sigmoid response curves ignore the field data points of decreasing diapause incidence for day length ≤ 11.5 hours, as these outliers are probably the result of a selection trend. The horizontal dot line (light grey) marks 50% of diapause incidence. The difference between blue curve (laboratory data) and green and red curves (field data) highlights the delay between maternal diapause induction and diapause preparation in offspring.</p

Positive traps observed (A,B) and model predictions (C,D) for 2012.

<p>(A,C) First semester; (B,D) Second semester. Values range from probability of 0% to observe <i>Ae</i>. <i>albopictus</i> (blue) to 100% (red).</p

Location and collection year of traps.

<p>Green, yellow, blue, red, orange are the traps collected in 2006, 2007, 2008, 2009, 2010 respectively.</p

Results of statistical modeling.

<p>All variables have a significant effect of the probability to observe <i>Aedes albopictus</i>. The interaction between the distance to colonized area and colonized area positive shows that probability of observing mosquito presence is higher in the middle of the colonized area than on its edges.</p><p>Results of statistical modeling.</p

Relationship between proportion of positive traps and distance to main colonized area (red line) and to area colonized sporadically (“jump”, blue line).

<p>The x-axis is in km. The gradual relationship shown on the red line suggests an invasive wave. Conversely, abrupt drop of the blue line suggests that such “jump” did not result so far in a new front of invasion. Curves have been fitted through loess algorithm.</p