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Summary In ideal conditions (static assay with long exposure time) several species of submerged weeds (Myriophyllum propinquum A. Cunn, Potamogeton tricarinatus F. Muell and A. Benn, and Vallisneria gigantea Graebner) were controlled by terbutryne at initial concentrations of <0·2 g m−3. However, since water management practices in irrigation systems limit the contact time which can be imposed, the availance (concentration‐time integral required for effectiveness) was further investigated, using Elodea canadensis Rich., which is dominant and widespread in the irrigation systems of south‐eastern Australia. In many experiments in irrigation channels, where control was poor, contact times were probably inadequate. Other factors may also contribute to inefficiency, including poor penetration of the weed beds after surface application, protection by high loadings of aufwuchs (periphyton, bacteria, detritus and associated particulate matter) on the leaves in the autumn, and inhibition of photosynthesis in an unfavourable light climate. Weed control was improved by injecting the herbicide into Mowing water, before ponding the treated water in the channels in spring or summer for at least 11 days. Terbutryne residues in the water decreased with a first order half‐life of about 9–20 days. Therefore, terbutryne is unsuitable for pulse injection treatment of flowing water in summer. Instead, effective control might be achieved by ponding the treated water in the channels in spring before the period of great water demand for summer crops. Contaminated water would be discarded onto tolerant crops or fallow land or retained in the system for about 7 weeks to ensure degradation of terbutryne before reuse.

Host heterogeneity can impact parasite transmission, but determining underlying traits and incorporating them into transmission models remains challenging. Body size is easily measured and influences numerous ecological interactions, including transmission. In the snail-schistosome system, larger snails have a higher exposure to parasites but lower susceptibility to infection per parasite. We quantified the impact of size-based heterogeneity on population-level transmission by conducting transmission trials in differently size-structured snail populations and competing size-dependent transmission models. Populations with greater proportions of large snails had lower prevalence, and small snails were shielded from infection by co-occurring large conspecifics. Using the winning size-explicit model, we then estimated that schistosome transmission potential varies dramatically across time due to seasonal changes in snail population size structure. Thus, incorporating traits such as body size, which are impacted by and directly affect host ecology, into transmission models could yield insights for natural dynamics and disease mitigation in many systems.