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.