Toward implementation of combined incompatible and sterile insect techniques for mosquito control: Optimized chilling conditions for handlingAedes albopictusmale adults prior to release

Combined incompatible and sterile insect technique (IIT-SIT) has been considered to be an effective and safe approach to control mosquito populations. Immobilization of male adults by chilling is a crucial process required for the packing, transportation and release of the mosquitoes during the implementation of IIT-SIT for mosquito control. In this study, effects of chilling on theAedes albopictusmales with tripleWolbachiainfections (HC line), a powerful weapon to fight against the wild typeAe.albopictuspopulation via IIT-SIT, were evaluated under both laboratory and field conditions. Irradiated HC (IHC) males were exposed to 1, 5 and 10 degrees C for 1, 2, 3, 6 and 24 h. The survival rate of the post-chilled IHC males was then monitored. Longevity of post-chilled IHC males was compared to non-chilled males under laboratory and semi-field conditions. Mating competitiveness of IHC/HC males after exposure to 5 or 10 degrees C for 0, 3 and 24 h was then evaluated. Effects of compaction and transportation under chilled conditions on the survival rate of IHC males were also monitored. The optimal chilling conditions for handling IHC males were temperatures between 5 and 10 degrees C for a duration of less than 3 h with no negative impacts on survival rate, longevity and mating competitiveness when compared to non-chilled males. However, the overall quality of post-chilled IHC/HC males decreased when exposed to low temperatures for 24 h. Reduced survival was observed when IHC males were stored at 5 degrees C under a compaction height of 8 cm. Transportation with chilling temperatures fluctuating from 8 to 12 degrees C has no negative impact on the survival of IHC males. This study identified the optimal chilling temperature and duration for the handling and transportation ofAe.albopictusIHC male adults without any detrimental effect on their survival, longevity and mating competitiveness. Further studies are required to develop drone release systems specific for chilled mosquitoes to improve release efficiency, as well as to compare the population suppression efficiency between release of post-chilled and non-chilled males in the field. Author summary Combining the incompatible insect technique (IIT) and sterile insect technique (SIT) is considered to be a promising approach to controlAedes albopictus, the world's most invasive mosquito species which can transmit dengue and zika. The combined IIT-SIT technique depends on the release of overflooding numbers of mass-reared male mosquitoes to sterilize female mosquitoes through mating, resulting in the reduction of the wild mosquito population. In existing SIT programs against agricultural pests, male insects are often immobilized by chilling to pack, transport and release them and thereby increase handling efficacy. Chilling for the immobilization of male mosquitoes may also improve program efficiency in the IIT-SIT against mosquitoes. Thus, it is essential to ascertain the optimal chilling conditions for handlingAe.albopictusmale mosquitoes. In this study, we found thatAe.albopictusmales could be chilled at 10 degrees C for 3 h without negative impacts on the biological qualities such as survival, longevity and mating competitiveness. Our results show that male mosquitoes could preserve a high level of quality under optimal chilling conditions, which indicates that immobilization by chilling is an important element for the combined IIT-SIT for mosquito control.