A Review of the Factors That Influence Pesticide Residues in Pollen and Nectar: Future Research Requirements for Optimising the Estimation of Pollinator Exposure.

This paper is a review of pesticide residues in pollen and nectar, following pesticide applications.

The literature suggests that four primary factors have the greatest influence on residues in pollen and nectar: the crop type; the application method (including timing and dose rate); the physicochemical properties of a compound; and the environmental conditions.

Residue concentrations are highly variable, with differences evident not only between various active ingredients and crop combinations, but also within each of these groups.  Normalized by the application rate, residues may vary up to 750,000 times among different active ingredients and crops. The authors emphasize that only a small proportion of treatment / crop combinations have been taken into account, with the majority of studies focused on neonicotinoids and oilseed rape (Brassica napus L.).

That said, generally, studies find the following (the authors caution that these observations are less relevant for foliar-sprayed or non-systemic pesticides):

• Compartment influence: Higher residues in pollen than nectar (reasons are unknown). Studies do confirm a strong correlation between residue levels in pollen and nectar.
• Sampling method: Higher residues in pollen and nectar when extracted directly from flowers as opposed to from bees.
• Plant characteristics: Differences in residue levels in various plant compartments can be explained by dilution effects with plant biomass, height and age.
• Dose rates: Pesticides applied at higher dose rates tend to result in higher residues in pollen and nectar.
• Application timing: Applications closer to bloom generally result in higher residue levels, however imidacloprid showed an opposite trend; it is hypothesized that the lower solubility and mobility of imidacloprid may result in its delayed accumulation in plant parts.
• Application method: Method of application has a strong influence on pollen and nectar residue levels; soil drench generally results in higher residues than foliar applications.
• Physiochemical properties: Physiochemical properties, including solubility, the octanol/water coefficient Kow, and the dissociation coefficient pKa are important parameters. The optimum uptake and translocation in xylem occurs for non-ionized pesticides with log Kow values of 1-3.
• Environmental conditions: Environmental conditions such as temperature, rainfall, humidity can all affect absorption and degradation of pesticide residues in plants, including within pollen and nectar.
• Plant physiological state: Several studies have documented sudden increases in imidacloprid levels during flowerhead formation in sunflower; observation is consistent with the/ hypothesis that root systems with high uptake capacity can recover more residues contained in soil.