Modeling Remobilization of Neonicotinoid Residues from Tree Foliage in StreamsA Relevant Exposure Pathway in Risk Assessment?

Systemic
neonicotinoid insecticides are increasingly used as a
crop protection measure to suppress insect pests on trees. However,
senescent foliage falling from treated trees represents a rarely studied
pathway through which neonicotinoids may enter nontarget environments,
e.g., surface waters. To estimate risk posed by this pathway, neonicotinoid
residues were analyzed in foliage from black alder trees treated with
one of three neonicotinoid insecticides (imidacloprid, thiacloprid,
or acetamiprid) at five concentrations, each ranging from 0.0375–9.6
g active ingredient/cm trunk diameter at breast height (n = 3). Foliar residues measured at the time of leaf fall were used
as input parameters for a model predicting imidacloprid water concentrations
over a 100-m-long stream stretch as a consequence of remobilization
from introduced foliage (input: 600 g foliage/m2 containing
80 μg imidacloprid/g). The water concentration (up to ∼250
ng/L) predicted by the model exceeded the recently proposed Maximum
Permissible Concentration of 8.3 ng/L for ∼6.5 days. Moreover,
dietary uptake was identified as an additional exposure route for
aquatic organisms. The alternative pathway (i.e., introduction via
leaf fall) and exposure route (i.e., dietary uptake) associated with
the systemic nature of neonicotinoids should be accounted for during
their registration process in order to safeguard ecosystem integrity.