Increased biting rate and decreased Wolbachia density in irradiated mosquitoes: safety issues in the framework of genetic control strategies

Releasing considerable numbers of radiation-sterilized males is a promising strategy to suppress mosquito vectors. However, under large-scale settings, small percentages of biting females can also be released in the field. Currently, the effects of the irradiation on host-seeking and host-biting behaviors have not been exhaustively investigated. Information is also lacking regarding the effects of a sterilizing treatment on the endosymbiotic bacterium Wolbachia, that is known to attenuate the vector competence of infected mosquito species. To ascertain the risks associated with the release of irradiated females, the pupae of two Aedes albopictus strains, differing by their natural or artificial Wolbachia infection type, were X-ray-treated using various doses to monitor key fitness parameters and biting behavior over two weeks. The effect of the radiation on Wolbachia was investigated by qPCR and FISH analysis. Radiation doses of 35 and 45 Gy completely inhibited progeny production but did not significantly affect Ae. albopictus female survival and flight ability, and caused a tripling of the number of bites per female per week compared to untreated controls. These results were also confirmed in Ae. aegypti after treatment at 50 Gy. A partial Ae. albopictus female sterility was achieved at 28 Gy but the number of weekly bites more than doubled compared to the controls. The Wolbachia density showed a significant downward trend in 45-Gy-irradiated females and decreased with adult aging and in younger treated pupae. This trend was confirmed in ovaries but not in extraovaric tissues. FISH analysis evidenced a strongly reduced Wolbachia-specific fluorescence in the ovaries of 13{+/-}1-day-old females. These findings suggested that irradiation could markedly increase the vectorial capacity of female mosquitoes, raising concerns about open field programs involving the release of substantial numbers of irradiated mosquitoes in the absence of perfect sexing methods. Author summaryTo reduce the reproductive potential of a target wild population of mosquitoes, X- or gamma-ray-based irradiation is a common procedure used to induce sterility in males prior to their release in the field. A small percentage of females escaping the sexing procedure may be also unintentionally released. Herein, we tested the effect of the radiation treatment on Aedes albopictus females and found that doses commonly used in SIT programs were not sufficient to significantly reduce survival and flight abilities, and the treated females continued to seek hosts even after engorgement, tripling the weekly number of bites compared to the controls. This phenomenon was confirmed in irradiated Ae. aegypti females. Moreover, the radiation treatment reduced the density of the endosymbiotic Wolbachia bacteria and markedly affected their distribution in the tissues of Ae. albopictus possibly reducing their ability to affect the vector competence, which is common in infected Aedes species. This was more evident when radiation had been applied on younger pupae and it increased with female aging. Results suggest that large-scale control programs based on mosquito irradiation should include rigorous methods for perfect sexing to avoid the risk of release of a myriad enhanced vectors.