Model-Like Implementation of Integrated Control Strategies Targeting Aedes Albopictus (Diptera: Culicidae) In Southwest Germany With Special Consideration of the Sterile Insect Technique

Abstract

BackgroundIn recent years the invasive species Aedes albopictus, known as the Asian tiger mosquito, has undergone an extreme expansion by steady introductions as blind passengers in vehicles from the Mediterranean to South-West Germany. Nowadays, more than 15 established populations are known in the State of Baden-Württemberg and Palatine (South-West Germany) which locally constitute a significant nuisance and public health threat. Therefore, the species deserves special attention as vector of several arboviruses like dengue, Chikungunya or Zika virus. As a consequence, immediate surveillance and control activities against Aedes albopictus have been implemented in the infested areas under the auspice of health departments and regulatory offices. MethodsThe control strategy comprises 3 columns: a) community participation (CP) based on detailed information for the elimination and sanitation of breeding sites as well as the use of fizzy Bti-tablets containing the pro-toxins of Bacillus thuringiensis israelensis (Bti); b) Door-to-Door (DtD) activities by trained staff including the application of Bti (Vectobac WG) at high dosages to achieve a sustained killing effect of the mosquito larvae; and c) the Sterile Insect Technique (SIT) to almost wipe out or eliminate the remaining Aedes albopictus population after intensive Bti-applications. In the laboratory and in semi-field tests, the different elements of the control strategy are evaluated and the efficacy of the integrated control strategy is assessed in routine treatments in the cities of Ludwigshafen (Palatine) and Freiburg (Baden-Württemberg) with special emphasis on the release of sterile Aedes/males. ResultsFollowing our information campaigns, more than 60% of the residents were practicing CP focusing on environmental sanitation and the use of fizzy Bti tablets. Although CP is an essential element of the integrated control strategy, it was shown that the strongest asset in our programme was the DtD activity and the application of a water suspension of Vectobac WG (2700 ITU/mg) to all potential breeding sites at dosages of 10g/rainwater container and 2.5g/smaller container which provided an effect for more than a month. The mean time for the inspection and treatment of a property was 27 minutes. As a result of the larval source management, the container index for Aedes albopictus achieved no more than 0.5% in Ludwigshafen. The mean number of Aedes eggs/ovitrap in Ludwigshafen was 4,3 and in Freiburg -Metzgergrün (SIT area) 18.23 and Freiburg-Gartenstadt-Freiburg (Control area) 22,4 eggs/trap. After the strong reduction of the Aedes population by Bti-application, the weekly release of 1013 (Ludwigshafen) and 2320 (Freiburg) sterile Aedes albopictus males/ha from May until October resulted in an overall sterility by egg counts 82.61% ( (Ø60.52±42.88%) in Ludwigshafen and 62.68±28.21% in Freiburg compared to a natural sterility of 16.93±13.5% in the SIT untreated area. The field results are proven by the data achieved in cage tests in the laboratory. The mating of wild females with sterile males showed sterility rates of 87.53±9.15% whereas the sterility of eggs laid by females mated with unirradiated males was only 3.3±2.8%. The most effective ratio of wild to sterile males is 1:5. The overall sterility of about 83% in Ludwigshafen indicates that our goal to almost eradicate the Aedes albopictus population could be achieved. The hydrogen-peroxide bleaching technique allowed a quick assessment of embryogenesis. ConclusionsIn this study, we clearly prove that an integrated control program based on a strict monitoring scheme is most effective when it comprises three columns, namely a) community participation, b) DtD activities including long-lasting Bti-larviciding of all possible breeding sites to strongly reduce the wild Aedes albopictus population as a basis for the successful application of SIT, and c) SIT to almost wipe out the Aedes albopictus population. The combination of Bti and SIT guarantees that two of the most selective, save and effective tools are employed against one of the most dangerous mosquito vector species Aedes albopictus, the Asian tiger mosquito.