Determination of Key Parameters Influencing Dislodgeable Foliar Pesticide Residues (DFR)

Abstract

Pesticides have a crucial role to play in assuring food security for an increasing world population. Therefore, a risk assessment should be carried out for plant protection products (PPPs), covering both dietary and non-dietary routes and all possible exposure scenarios. Non-dietary risk assessments for workers, residents and bystanders mandate estimating the amount of pesticide residue transferred from plant foliage to the skin or clothes, known as dislodgeable foliar residues (DFR), along with the pesticide's half-lives (DT50). However, both values were considered outdated and conservative by the industry and the public during the consultations and research (EFSA, 2014c; Kluxen et al., 2021). Moreover, the DFR data in the literature are described as insufficiently reliable, limited, and encompass considerable statistical uncertainties. Thus, new data generation would allow for a more reasonable default value that reflects a more realistic exposure estimate and does not compromise human safety. To this end, the findings of this thesis explore some of the factors that affect DFR using a newly DFR developed lab method. This would enable more data generation and possibly refining of the PPP non-dietary risk assessment. Possible correlations between dietary and DFR residue decline were investigated considering data from 177 dietary residue trials along with 56 DFR trials from outdoor studies on the same crops, besides residue decline data available in the Pesticide Properties Database (PPDB). The residue studies followed the non- normal distribution, and the comparison between DT50 of both types of residues for most active substances revealed a statistical higher DT50 mean value of the dietary residue compared to the DFRs. Furthermore, the numerical back-transformed DT50 data for all tested active substances proved to be higher in an average of 5 to 23% in the dietary studies than in the DFR studies. Therefore, a DT50 value from dietary residue studies could act as a conservative surrogate DT50 for DFR, which could help determine the length of DFR studies and benefit both the agrochemical industry and the regulatory bodies in supporting non-dietary pesticide risk assessment. The current work described a newly developed laboratory method for quantifying DFR. The laboratory method reflected the available field DFR methodology. It involved controlled application of droplets to the leaves and validation of the wash-off process used to completely remove the residue from the leaf surface before the analytical quantification using liquid-chromatography mass spectrometry. The aforementioned DFR technique was used to investigate the effect of leaf texture, formulation and co-formulants on the magnitude of DFR using the fungicide difenoconazole (DFZ) 10% (w/v). DFZ emulsifiable concentrate (EC 10%) and a wettable powder (WP 10%) with and without adjuvants on tomato, French bean and oilseed rape were tested. The findings showed that a comparable DFR% was observed from the WP and EC formulations on most sampled crop leaves, ranging from (82-74%) on French beans and (31-74%) on oilseed rape except for tomatoes (60-39%). No significant effect of adjuvants addition was observed for either formulation except when mixing TEHP (0.1% w/v) to the EC 10% on French bean, which resulted in a DFR recovery % decrease from 82 to 74%. Changing the solvent system or the co-formulants in a DFZ EC formulation did not statistically affect the DFR recovery % in all crops. Still, a slight numerical increase in the DFR %, in that case, was observed on tomatoes from 60 to 65% and from 31% to 37% on oilseed rape. It was associated with low dynamic surface tension (DST) of the formulation, which was the reason for this enhanced uptake and low DFR. The findings of this thesis also shed light on the effect of leaf texture on DFR, which showed a significant difference among all tested crops highlighting the importance of this factor. Moreover, grouping different leaves/crops based on their roughness (i.e., hairy, or waxy) proved to be relevant and applicable as hairy leaves were shown to have higher DFZ DFR (ranges from 82% to 52%) than waxy leaves (31%). However, an accurate classification approach would better involve the coverage degree and types of trichomes in different hairy leaves, as hairy leaves may act differently based on these characteristics. In conclusion, this work has demonstrated the importance of the newly laboratory developed DFR method and its application in studying the factors that could influence DFR to allow for refining the PPP non-dietary risk assessment.