Reply to: Issues with combining incompatible and sterile insect techniques

Y. Li, L. A. Baton, D. Zhang, J. Bouyer, A. G. Parker, A. A. Hoffmann, L. C. Ng, C. H. Tan and Z. Xi,  Nature,  590:E3-E5. 2021.

In the accompanying Comment1, the authors advocate for the use of a bidirectional cytoplasmic incompatibility (Bi-CI) system; however, Bi-CI does not necessarily protect against population replacement2,3,4. Released residual females may be more likely to mate with the males with which they are released, rather than males of the target population (as the former are held together overnight before release, and, afterwards, released males vastly outnumber those of the target population)5. Regardless of Bi-CI or unidirectional cytoplasmic incompatibility (Uni-CI), released females have the same probability of mating with released males with the same Wolbachia infection, resulting in Wolbachia-infected offspring in the field. However, in the combined incompatible and sterile insect technique (IIT–SIT), in addition to the sterilization of residual females, irradiation also reduces the fertility of the released males by more than 80% when they mate with residual females5, which provides a second protection against population replacement. Furthermore, when population suppression is high and approaches the elimination of the population, any accidentally released fertile females will inevitably constitute the majority—if not the entirety—of the field population, and so any invasion threshold is irrelevant and will be exceeded6,7. Therefore, when the aim is elimination of the target population, Uni-CI and Bi-CI do not have appreciably different risks of population replacement. The small-scale field trial8 cited by Moretti and Calvitti in their Comment1 does not provide evidence that Bi-CI protects against population replacement during IIT, as the occurrence of the released Wolbachia strain in the field was not determined and the level of population suppression was insufficient to facilitate surpassing of the invasion threshold.

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