Neonicotinoids, broad-spectrum systemic insecticides, are the fastest growing class
of insecticides worldwide and are now registered for use on hundreds of field crops
in over 120 different countries. The environmental profile of this class of pesticides
indicate that they are persistent, have high leaching and runoff potential, and are
highly toxic to a wide range of invertebrates. Therefore, neonicotinoids represent
a significant risk to surface waters and the diverse aquatic and terrestrial fauna
that these ecosystems support. This review synthesizes the current state of knowledge
on the reported concentrations of neonicotinoids in surface waters from 29 studies
in 9 countries world-wide in tandem with published data on their acute and chronic
toxicity to 49 species of aquatic insects and crustaceans spanning 12 invertebrate
orders. Strong evidence exists that water-borne neonicotinoid exposures are frequent,
long-term and at levels (geometric means=0.13μg/L (averages) and 0.63μg/L (maxima))
which commonly exceed several existing water quality guidelines. Imidacloprid is by
far the most widely studied neonicotinoid (66% of the 214 toxicity tests reviewed)
with differences in sensitivity among aquatic invertebrate species ranging several
orders of magnitude; other neonicotinoids display analogous modes of action and similar
toxicities, although comparative data are limited. Of the species evaluated, insects
belonging to the orders Ephemeroptera, Trichoptera and Diptera appear to be the most
sensitive, while those of Crustacea (although not universally so) are less sensitive.
In particular, the standard test species Daphnia magna appears to be very tolerant,
with 24-96hour LC50 values exceeding 100,000μg/L (geometric mean>44,000μg/L), which
is at least 2-3 orders of magnitude higher than the geometric mean of all other invertebrate
species tested. Overall, neonicotinoids can exert adverse effects on survival, growth,
emergence, mobility, and behavior of many sensitive aquatic invertebrate taxa at concentrations
at or below 1μg/L under acute exposure and 0.1μg/L for chronic exposure. Using probabilistic
approaches (species sensitivity distributions), we recommend here that ecological
thresholds for neonicotinoid water concentrations need to be below 0.2μg/L (short-term
acute) or 0.035μg/L (long-term chronic) to avoid lasting effects on aquatic invertebrate
communities. The application of safety factors may still be warranted considering
potential issues of slow recovery, additive or synergistic effects and multiple stressors
that can occur in the field. Our analysis revealed that 81% (22/27) and 74% (14/19)
of global surface water studies reporting maximum and average individual neonicotinoid
concentrations respectively, exceeded these thresholds of 0.2 and 0.035μg/L. Therefore,
it appears that environmentally relevant concentrations of neonicotinoids in surface
waters worldwide are well within the range where both short- and long-term impacts
on aquatic invertebrate species are possible over broad spatial scales.