Limitations and future work
The underlying benefit to biocontrol is mitigating the higher economic and environmental costs of conventional control programs, such as chemical control of snail hosts and human mass drug administration (MDA). For example, MDA, such as administering praziquantel, relies on extensive monitoring to evaluate success (Gurarie et al. 2015), while chemical programs, such as mollusciciding, incur high environmental costs (Coelho and Caldeira 2016) and rely on precise timing and careful interventions (Malishev and Civitello 2020). Further, collecting field data to improve models, such as host demography, can be cheaper than MDA intervention and supplement model forecasts with detailed ecological consequences needed to sustain long-term biocontrol (Louda 2003), bolster existing programs (King et al. 2020), or revisit alternatives, i.e. the lottery model (Myers 1985). Another major barrier is program logistics and scaling biocontrol programs constrained or complicated by the ecology and physiology of the biocontrol agent. For example, prawns farmed for biocontrol may alter diet preferences to meet growth demands, as seen in cichlid fish shifting to species other than snails due to changes in jaw morphology (Slootweg, Malek, and McCullough 1994). In nature, predators may also target narrower snail size cohorts, i.e. 5–10 mm versus 15–20 mm. In our simulations, we chose a wide range of size cohorts that encompasses the entire size distribution of the host lifecycle (Fig. 3) and represents a realistic host mortality rate at the population level. The snail and schistosome relationship also spans many host genera and three major schistosome species, so future models targeting specific snail-schistosome pairs could further tease apart the subtleties of the ecological mechanisms we identify in this study. We encourage future lab and field work to explicitly consider testable ecological mechanisms that will help separate successful and resourceful biocontrol designs from risky and wasteful ones and provide the data to identify economic and ecological red flags.