Foliar Photodegradation in Pesticide Fate Modeling: Development and Evaluation of the Pesticide Dissipation from Agricultural Land (PeDAL) Model.

Pesticide dissipation from plant surfaces depends on a variety of factors including meteorological conditions, the pesticide's physicochemical properties, and plant characteristics. Models already exist for describing pesticide behavior in agriculture fields; however, they do not account for pesticide-specific, condition-specific foliar photodegradation and the importance of this component in such models has not yet been investigated. We describe here the Pesticide Dissipation from Agricultural Land (PeDAL) model, which combines (a) multiphase partitioning to predict volatilization, (b) a new kinetics module for predicting photodegradation on leaf surfaces under varying light conditions based on location and timing, and (c) a generic foliar penetration component. The PeDAL model was evaluated by comparing measured pesticide dissipation rates from field experiments, described as the time for the pesticide concentration on leaves to decrease by half (DT50), to ones generated by the model when using the reported field conditions. A sensitivity analysis of the newly developed foliar photodegradation component was conducted. We also showed how the PeDAL could be used by applicators and regulatory agencies. First, we used the model to examine how pesticide application timing affects dissipation rates. Second, we demonstrated how the model can be used to produce emission flux values for use in atmospheric dispersion and transport models.

Understanding Trends in Pesticide Volatilization from Agricultural Fields Using the Pesticide Loss via Volatilization Model.

The Pesticide Loss via Volatilization model was developed to predict and understand pesticide volatilization rates from a planted agricultural field. The model allows the user to adjust the properties of the pesticide, various soil and plant descriptors, and climatic conditions. A useful output from the model is the 24 h cumulative percentage volatilization (CPV24h) loss. The model was validated by comparing modeled CPV24h values to measured ones compiled from the literature. Sensitivity analysis showed that the plant intercept fraction (%I), leaf area index (LAI), and leaf height (hleaf) strongly affect volatilization rates of plant- and soil-sorbed pesticides whereas LAI, hleaf, and the percent of water on the plant surface strongly affect more water-soluble pesticides. The model showed that most pesticides volatilize more readily from plants than from soil and that volatilization rates vary significantly for certain pesticides (but not all) when applied to plants at different growth stages and for different species of plants. Results are displayed on chemical space diagrams to paint a clear picture of how CPV24h varies for chemicals with different properties under different conditions.

Chronic Toxicities of Neonicotinoids to Nymphs of the Common New Zealand Mayfly Deleatidium spp.

Neonicotinoid insecticides have been shown to have high chronic toxicity relative to acute toxicity, and therefore short-term toxicity tests ≤96 h in duration may underestimate their environmental risks. Among nontarget aquatic invertebrates, insects of the orders Diptera and Ephemeroptera have been found to be the most sensitive to neonicotinoids. To undertake a more accurate assessment of the risks posed by neonicotinoids to freshwater ecosystems, more data are needed from long-term tests employing the most sensitive taxa. Using nymphs of the common New Zealand mayfly genus Deleatidium spp., we performed 28-d static-renewal exposures with the widely used neonicotinoids imidacloprid, clothianidin, and thiamethoxam. We monitored survival, immobility, impairment, and mayfly moulting propensity at varying time points throughout the experiment. Imidacloprid and clothianidin exerted strong chronic toxicity effects on Deleatidium nymphs, with 28-d median lethal concentrations (LC50s) of 0.28 and 1.36 µg/L, respectively, whereas thiamethoxam was the least toxic, with a 28-d LC50 > 4 µg/L (highest concentration tested). Mayfly moulting propensity was also negatively affected by clothianidin (during 3 of 4 wk), imidacloprid (2 of 4 wk), and thiamethoxam (1 of 4 wk). Comparisons with published neonicotinoid chronic toxicity data for other mayfly taxa and larvae of the midge genus Chironomus showed similar sensitivities for mayflies and midges, suggesting that experiments using these taxa provide reliable assessments of the threats of neonicotinoids to the most vulnerable freshwater species. Environ Toxicol Chem 2019;38:2459-2471. © 2019 SETAC.