Studying Aedes albopictus dispersal in Italy by mark-release-recapture experiments.

Knowledge about movements of adult mosquito vectors in endemic or epidemic areas is essential for understanding the disease transmission dynamics and for determining appropriate control limits necessary to interrupt pathogen transmission. Dispersal of the endophilic Dengue vector Aedes aegypti has been extensively studied in the field by Mark-Release-Recapture (MRR) experiments. However, very few data on this subject are available on Aedes albopictus, mostly due to the lack of an effective recapture method in the case of this exophilic species. We here present the results of Ae. albopictus MRR recapture experiments in which sticky-traps (ST, Facchinelli L et al, 2007, Med Vet Entomol, 21: 183-95) were used to recapture released blood-fed females. Three MRRs were carried out in a 250 m radius area within the campus of Sapienza University in Rome (central Italy) in summer 2008, releasing about 500 females and employing 55 STs (≈1 ST/3,600 m2) in each replicate. Other 3 MRRs were carried out in a 500 m radius area in a rural/periurban site in the Province of Padova (northern Italy) in summer 2009, releasing about 1.000 females and employing 94 STs (≈1 ST/8,400 m2) in each replicate. Recaptures were carried out for 16 days after releases. The recapture rates obtained were 4.5% (CI=3.0-6.8%), 5.1% (CI=3.6-7.3%) and 3.3% (CI=2.1-5.1%) in Rome and 8,8% (CI=7.3-10.6%), 13.4% (CI=11.8-15.1%) and 3.4% (CI=2.5-4.6%) in Padova. Our recapture rates are of the same order of magnitude as those obtained in the only MRR study exploiting sticky-ovitraps to re-capture Ae. albopictus (6.1% in Brasil, Maciel-de-Freitas R et al, 2006, J Med Entomol, 43: 1112-20) and in most MRR studies on Ae. aegypti females. Most re-captured females were collected at the gravid stage in the first 8 days after releases. This observation - coupled with the results of single oviposition experiments carried out in Padova simultaneously to the releases - allows to conclude that our results mainly refer to the dispersal of females looking for an oviposition site after having completed a single gonotrophic cycle triggered by the blood-meal provided before releases. The females were mostly re-captured at 50-200 m and 0-150 m from the release sites, in Rome and Padova, respectively. The lack of recaptures below 50 m in Rome is probably due to a lower suitability for resting of the area around the release site in Rome compared to Padova. In both sites females reached the limit of the study areas. Single females flew up to 230 and 464 m in 4 days in Rome and Padova, respectively. The cumulative mean distance travelled (Morris CD et al., 1991, J Am Mosq Control Assoc, 7: 608-15) was 105, 121, 139 m in the 3 replicates in Rome and 110, 77, 68 m in Padova. Overall, our study represents the first evaluation of the movements of Ae. albopictus female in European area. We also showed that ST represents an useful tool in the frame of MRR experiments focused on exophilic species such as Ae. albopictus. Our populations rapidly dispersed in a 150 m-radius area, reaching up to 500 m, even under relaxed pressures (i.e. a relatively high abundance of oviposition sites and hosts) and we hypothesize that the species flight range could increase under stronger pressures (e.g. low abundance of oviposition sites or of host for blood-meals). This conclusion is particularly instrumental to plan control activities in Italy, as well as other European Countries, and to determine appropriate control limits necessary to interrupt pathogen transmission in case of possible arbovirus epidemics, such the Chikungunya outbreak occurred in Ravenna in 2007. The information obtained may also be relevant for the planning of strategies aimed to reduce Ae. albopictus densities by mean of mass releases of sterile males, an approach currently under consideration for high infested areas in our Country (Alphey L et al, 2009, Vector Borne Zoonotic Dis, doi: 10.1089/vbz.2009.0014).