Background The invasive species Aedes albopictus, commonly known as the Asian tiger mosquito, has undergone extreme range expansion by means of steady introductions as blind passengers in vehicles traveling from the Mediterranean to south-west Germany. The more than 25 established populations in the State of Baden-Württemberg, Palatine and Hesse (south-west Germany) have become a major nuisance and public health threat. Aedes albopictus deserves special attention as a vector of arboviruses, including dengue, chikungunya and Zika viruses. In Germany, Ae. albopictus control programs are implemented by local communities under the auspices of health departments and regulatory offices. Methods The control strategy comprised three pillars: (i) community participation (CP) based on the elimination of breeding sites or improved environmental sanitation, using fizzy tablets based on Bacillus thuringiensis israelensis (fizzy Bti tablets; Culinex® Tab plus); (ii) door-to-door (DtD) control by trained staff through the application of high doses of a water-dispersible Bti granular formulation (Vectobac® WG) aimed at achieving a long-lasting killing effect; and (iii) implementation of the sterile insect technique (SIT) to eliminate remaining Ae. albopictus populations. Prior to initiating large-scale city-wide treatments on a routine basis, the efficacy of the three elements was evaluated in laboratory and semi-field trials. Special emphasis was given to the mass release of sterile Ae. albopictus males. Results More than 60% of the local residents actively participated in the first pillar (CP) of the large-scale control program. The most effective element of the program was found to be the DtD intervention, including the application of Vectobac® WG (3000 ITU/mg) to potential breeding sites (10 g per rainwater container, maximum of 200 l = maximum of approx. 150,000 ITU/l, and 2.5 g per container < 50 l) with a persistence of at least 3 weeks. In Ludwigshafen, larval source management resulted in a Container Index for Ae. albopictus of < 1% in 2020 compared to 10.9% in 2019. The mean number of Aedes eggs per ovitrap per 2 weeks was 4.4 in Ludwigshafen, 18.2 in Metzgergrün (Freiburg) (SIT area) and 22.4 in the control area in Gartenstadt (Freiburg). The strong reduction of the Ae. albopictus population by Bti application was followed by weekly releases of 1013 (Ludwigshafen) and 2320 (Freiburg) sterile Ae. albopictus males per hectare from May until October, resulting in a high percentage of sterile eggs. In the trial areas of Ludwigshafen and Frieburg, egg sterility reached 84.7 ± 12.5% and 62.7 ± 25.8%, respectively; in comparison, the natural sterility in the control area was 14.6 ± 7.3%. The field results were in line with data obtained in cage tests under laboratory conditions where sterility rates were 87.5 ± 9.2% after wild females mated with sterile males; in comparison, the sterility of eggs laid by females mated with unirradiated males was only 3.3 ± 2.8%. The overall egg sterility of about 84% in Ludwigshafen indicates that our goal to almost eradicate the Ae. albopictus population could be achieved. The time for inspection and treatment of a single property ranged from 19 to 26 min depending on the experience of the team and costs 6–8 euros per property. Conclusions It is shown that an integrated control program based on a strict monitoring scheme can be most effective when it comprises three components, namely CP, DtD intervention that includes long-lasting Bti-larviciding to strongly reduce Ae. albopictus populations and SIT to reduce the remaining Ae. albopictus population to a minimum or even to eradicate it. The combined use of Bti and SIT is the most effective and selective tool against Ae. albopictus, one of the most dangerous mosquito vector species. Graphical Abstract

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BackgroundThe invasive species Aedes albopictus, the Asian tiger mosquito, has undergone an extreme expansion by steady introductions as blind passengers in vehicles from the Mediterranean to South-West Germany. The more than 15 established populations in the State of Baden-Württemberg and Palatine (South-West Germany) have become a major nuisance and public health threat. Aedes albopictus deserves special attention as vector of arboviruses like dengue, chikungunya or Zika virus. In Germany, control of Ae. albopictus is implemented under the auspice of health departments and regulatory offices. MethodsThe control strategy comprised three components or pillars: a) community participation (CP) based on the elimination or sanitation of breeding sites with the use of fizzy Bti-tablets (Culinex Tab plus); b) Door-to-Door (DtD) control by trained staff applying high doses of a Bti-water-dispersible granular formulation (Vectobac WG) aimed for a long-lasting killing effect; and c) the Sterile Insect Technique (SIT) to eliminate remaining Ae. albopictus populations. Prior to large scale routine city-wide treatments, the efficacy of the three elements was evaluated in laboratory and semi-field trials. Special emphasis was given to the mass release of Ae. albopictus sterile males.ResultsMore than 60% of the local residents joined the Community Participation within the large-scale control program. It was shown that the most effective element was the DtD intervention including the application of Vectobac WG (2700 ITU/mg after radiation with 25 kGy) to potential breeding sites (10 g/rainwater container, max. 200L ≙ 13,500ITU/L and 2.5g/container <50L) with a persistence of at least about three weeks. The average time required for the inspection and treatment per property was 27 minutes. In Ludwigshafen the larval source management resulted in a container index for Ae. albopictus below 1% in 2020 compared to 10.9% in 2019. The mean number of Aedes eggs/ovitrap were 4.3 in Ludwigshafen and 18.23 in Freiburg-Metzgergrün (SIT areas); while 22.4 in Freiburg-Gartenstadt (Control area). After the strong reduction of the Aedes population by Bti-application, the weekly release of 1,013 (Ludwigshafen) and 2,320 (Freiburg) sterile Ae. albopictus males/ha from May until October resulted in a high percentage of sterile eggs. In the trial area of Ludwigshafen the sterility of eggs reached 82.61% (mean: 60.52%; SD: 42.88%) and in Freiburg 62.68% (SD 28.21%). The natural sterility in the control area was 16.93±13.5%. The field results were in line with data obtained in cage tests under laboratory conditions where wild females mated with sterile males showed sterility rates of 87.53±9.15%. The sterility of eggs laid by females mated with unirradiated males was only 3.3±2.8%. The overall sterility of about 83% in Ludwigshafen indicates that our goal to almost eradicate the Ae. albopictus population could be achieved. ConclusionsIt is shown that an integrated control program based on a strict monitoring scheme is most effective when it comprises three components, namely a) community participation, b) DtD intervention including long-lasting Bti-larviciding to strongly reduce Ae. albopictus populations and c) the release of sterile males to reduce the remaining Ae. albopictus population to a minimum or even to eradicate it. The combination of the use of Bti with SIT are most effective and selective tools against Ae. albopictus, one of the most dangerous mosquito vector species.