A mass rearing cost calculator for the control of Culex quinquefasciatus in Hawai 'i using the incompatible insect technique

Background: Hawai 'i's native forest avifauna is experiencing drastic declines due to climate change-induced increases in temperature encroaching on their upper-elevation montane rainforest refugia. Higher temperatures support greater avian malaria infection rates due to greater densities of its primary vector, the southern house mosquito Culex quinquefasciatus, and enhance development of the avian malaria parasite Plasmodium relictum. Here we propose the use of the incompatible insect technique (IIT) or the combined IIT/sterile insect technique (SIT) for the landscape-scale (i.e., area-wide) control of Cx. quinquefasciatus, and have developed a calculator to estimate the costs of IIT and IIT/SIT applications at various sites in Hawai 'i. Methods: The overall cost of the infrastructure, personnel, and space necessary to produce incompatible adult males for release is calculated in a unit of similar to 1 million culicid larvae/week. We assessed the rearing costs and need for effective control at various elevations in Hawai 'i using a 10:1 overflooding ratio at each elevation. The calculator uses a rate describing the number of culicids needed to control wild-type mosquitoes at each site/elevation, in relation to the number of larval rearing units. This rate is a constant from which other costs are quantified. With minor modifications, the calculator described here can be applied to other areas, mosquito species, and similar techniques. To test the robustness of our calculator, the Kaua 'i-specific culicid IIT/SIT infrastructure costs were also compared to costs from Singapore, Mexico, and China using the yearly cost of control per hectare, and purchasing power parity between sites for the cost of 1000 IIT/SIT males. Results: As a proof of concept, we have used the calculator to estimate rearing infrastructure costs for an application of IIT in the Alaka 'i Wilderness Reserve on the island of Kaua 'i. Our analysis estimated an initial investment of at least similar to $1.16M with subsequent yearly costs of approximately $376K. Projections of rearing costs for control at lower elevations are similar to 100 times greater than in upper elevation forest bird refugia. These results are relatively comparable to those real-world cost estimates developed for IIT/SIT culicid male production in other countries when inflation and purchasing power parity are considered. We also present supplemental examples of infrastructure costs needed to control Cx. quinquefasciatus in the home range of 'i 'iwi Drepanis coccinea, and the yellow fever vector Aedes aegypti. Conclusions: Our cost calculator can be used to effectively estimate the mass rearing cost of an IIT/SIT program. Therefore, the linear relationship of rearing infrastructure to costs used in this calculator is useful for developing a conservative cost estimate for IIT/SIT culicid mass rearing infrastructure. These mass rearing cost estimates vary based on the density of the targeted organism at the application site.

Automated summary

This study proposes the use of incompatible insect technique (IIT) or combined IIT/sterile insect technique (SIT) for controlling Culex quinquefasciatus, the primary vector of avian malaria in Hawai'i. A calculator has been developed to estimate the costs of implementing IIT and IIT/SIT at various locations in Hawai'i.