The 2019 survey highlighted a conspicuous difference between intervention and non-intervention municipalities in the seasonal distribution of numbers of both Ae. albopictus eggs and adult females. In the non-intervention municipalities, there was a steady increase from June in the number of eggs and adults with a peak in August, followed by a decrease in September and October, indicating the end of the mosquito reproductive season. In the intervention municipalities, the increase in the number of eggs and adults was much more contained, without an evident peak. We believe this to be a first major indicator of the efficacy of the IVM programme implemented in Ticino, helping in keeping the numbers of Ae. albopictus almost stable during the reproductive season of the mosquito.
In 2019, the number of Ae. albopictus eggs in urban environment was 3.8 times higher in non-intervention sites than in intervention sites. Compared to the situation in 2012/2013, with a proportion of 2.26 (14), the divergence between non-intervention and intervention areas has increased. In the Italian communities, not following an IVM programme, there was a striking increase in the number of eggs per ovitrap, with an average of about 50 eggs in 2012/2013 to more than 200 eggs in 2019. The fitted model showed a peak in eggs per ovitrap from less than 150 in 2012/2013 to about 330 in 2019. In comparison, in the Swiss sites following an IVM programme, the increase was much more moderate, with an average of about 20 eggs per ovitrap in 2012/2013, to between 57 and 80 eggs in 2019, while the fitted model showed a moderate increase in the peak of eggs from about 65 in 2012/2013 to about 100 eggs per ovitrap in 2019. These observations strongly suggest that the IVM programme implemented in Ticino helps keeping the numbers of Ae. albopictus almost stable also over the years in the urban environment. On the contrary, the absence of an IVM programme in the Italian communities across the border seems to lead to a deterioration of the situation, with an increase in numbers of Ae. albopictus. Further evaluations of the control system in the coming years will allow confirming whether this is a consolidated trend or if it is just due to a particular 2019.
In 2013, Suter et al. (14) observed that few ovitraps (30%) in Ticino were positive for Ae. albopictus earlier in the warm season, in early/mid-June, while in Italy many traps (62%) were already positive. A possible explanation was the positive impact of control treatments at the end of the 2012 season and before the start of the 2013 season in lowering mosquito reproduction in Ticino. Another suggested possibility was that mosquito populations in Ticino, rather than being stable overwintering populations, were annually re-introduced from Italy, so that their numbers managed to pick up only later in the season. In early/mid-June 2019, in contrast, half of the ovitraps (50%) in Ticino were already positive for Ae. albopictus, and most ovitraps (80%) were positive in the non-intervention areas. It seems therefore that in 2019, the numbers of tiger mosquito in Ticino picked up faster compared to 2013. This could be due to the presence of more stable overwintering populations in 2019, compared to 2013. However, at the beginning of the 2019 survey, between end of May and beginning of June, very few ovitraps (7%) were positive in Ticino, while half of the traps (58%) were already positive in the non-intervention municipalities. A likely explanation is that the larvicide treatments in Ticino at the end of the 2018 reproduction season reduced the number of mosquitoes laying diapausing eggs. This, in addition to the impact of control treatments at the start of the 2019 season, slowed down the annual reconstitution of mosquito populations in the intervention areas, differently from the non-intervention areas.
A concern in the 2012/2013 survey was the use of egg counts from ovitraps to estimate and compare Ae. albopictus densities. Ovitrap data are considered appropriate to assess presence/absence of Ae. albopictus in a given site but not adult population estimation, since the relationship between the two parameters might be affected by several factors. For example, a single female mosquito might lay its eggs in multiple breeding sites or the ovitraps may compete with nearby sites (13). In 2019, gravid female traps (GATs) were deployed in parallel to the ovitraps, in order to compare the two surveillance methods. Both methods showed a variability among traps in the same municipality and within traps themselves, with a higher variability among GATs. A possible explanation could be a variability in the presence of breeding sites other than traps during the study period, with an effect more accentuated on the GATs, since individuals are captured with this method. Nevertheless, a significant positive correlation was found between eggs in ovitraps and number of Ae. albopictus adult females, in agreement with other studies (9, 26, 27). The numbers of Ae. albopictus adult females per GAT followed the same trend as the numbers of eggs per ovitrap, being significantly higher in the non-intervention sites than in the intervention ones, with a ratio non-intervention/intervention areas varying between two and four over the season. Therefore, we can confirm that both egg and adult data are useful to determine efficacy of intervention methods employed, or lack thereof.
Another concern in using ovitraps to estimate Ae. albopictus numbers might be the increasing presence of other invasive Aedes species, such as Ae. japonicus and Ae. koreicus, whose eggs cannot be morphologically discerned from the eggs of Ae. albopictus and could consequently introduce a bias in the evaluation of egg counts. In order to avoid as much as possible the presence of other Aedes species, the sampling sites in this study were selected in urban environment, where Ae. albopictus appears to predominate over the other two Aedes species. The identification of randomly picked eggs through MALDI-TOF MS, combined with the morphological species determination of adults, confirmed that the large majority of mosquitoes found at the sampling sites were tiger mosquitoes. Therefore, the influence of other Aedes species on the results was negligible. We are currently developing a cost- and time-effective method for differentiation of eggs of different Aedes species. Regardless, the containment measures adopted for Ae. albopictus also apply to other container-breeding Aedes species. Therefore, by lowering the density of the other depositing mosquitoes we were not introducing a bias in the data.
In terms of public health risk, a main concern related to the presence and abundance of the tiger mosquito is its role as a vector of arboviruses. Epidemics of chikungunya and dengue viruses have already occurred in Italy and France (8). In Switzerland, autochthonous cases of chikungunya and dengue viruses have not been reported so far and we are not aware of autochthonous cases in the Italian communities included in the present study. However, the number of imported cases in Switzerland, including Ticino, increases regularly (https://www.bag.admin.ch) as it does in the neighbouring Italian regions (https://www.epicentro.iss.it/arbovirosi/bollettini) and in other European countries. In 2008, after the 2007 chikungunya epidemic in the Emilia-Romagna region of northern Italy, Carrieri and collaborators (28) calculated the epidemic risk threshold in terms of numbers of eggs per ovitrap above which an arbovirus epidemic may initiate, in presence of imported human cases. A threshold of 250–450 and 451–750 eggs per ovitrap in 14 days was calculated for an epidemic of E1-A226V mutated and non-mutated form of the chikungunya virus, respectively (29). In Ticino intervention areas, the mean number of eggs per ovitrap in 14 days in 2019 was between 57 and 80, while in non-intervention areas across the Italian border it was about 200. Maximum number of eggs per ovitrap in Ticino was between 400 and 500, while in non-intervention areas across the Italian border it was about 1,000. Although the geographical characteristics of the Emilia-Romagna region are different from the area monitored in the present work, we can still perceive that the risk of an arbovirus epidemic is much more concrete in the non-intervention areas. Moreover, even with the lower number of Ae. albopictus in Ticino, a study carried out in 2018 in six municipalities of the canton estimated that the risk of outbreak in the case of the introduction of chikungunya, dengue or Zika viruses was present in all the municipalities investigated (30). Consequently, a strategy for preventing and managing potential arbovirus outbreaks, as well as the surveillance and control activities of Ae. albopictus according to the situation and level of epidemic risks, has been recently elaborated for Ticino (31).
The scope of this work was to evaluate the effectiveness of integrated control on the field, where not all variables can be controlled. As an observational study, we must be aware that differences between intervention sites and non-intervention sites could be explained by other yet unknown factors that differ between Switzerland and Italy. However, following an experimental approach, namely setting up untreated control sites in Ticino, where all municipalities follow an IVM programme, would be ethically unfeasible. The non-intervention control sites selected in Italy were very similar and geographically close to the intervention sites in Ticino. Moreover, some variables (i.e. municipality and trap identity) were included in the models to control for possible differences.
Our results are in accordance with previous studies on integrated control strategies (e.g. (32–34)). The IVM plays an essential role in reducing the nuisance for the human population. In addition, from a public health point of view, it might limit both the risk of autochthonous transmission and the size of potential epidemics, as showed by Guzzetta and colleagues (35). According to Baldacchino et al. (9), the most effective integrated control includes door-to-door education. Door-to-door education and treatment actions were included in the Ticino IVM between 2008 and 2010 (5). In this period, breeding sites in private domains were removed directly by GLZ, Civil Protection Units or municipality workers after agreement with the residents. With the gradual spread of the mosquito to larger areas, this fine-scale approach became less and less sustainable. Therefore, the part of the IVM regarding private domains currently focuses on a less fine but constant approach over the years with extensive information campaigns carried out every year, including for example information events and door-to-door delivery of education material (5). In addition, the municipalities can issue a specific ordinance not permitting uncared breeding sites for the tiger mosquito on the municipality territory. Consequently, the GLZ and the municipality workers are allowed to conduct inspections to verify the presence of breeding sites in private domains and report violations of the ordinance.
Reintroductions of mosquitoes in Ticino from across the border are most probably occurring every year. From our data, it is not possible to tell the effect of these reintroductions on the quantities of Ae. albopictus in Ticino. However, we have demonstrated here that, with the implementation of an IVM, it is possible to contain the numbers of Ae. albopictus at a manageable level, independently of the likely constant reintroduction of individuals from outside the intervention areas. Although it would certainly be desirable to undertake concerted actions across the Swiss-Italian Insubria region, with the development and implementation of a transnational action plan for the surveillance and control of Ae. albopictus, it is possible to achieve containment of the vector also without cross-border concerted measures.