Abstract
Background: Large amounts of insecticide-containing dusts produced from abrasion of the seed dressing can be
released into the atmosphere during sowing operations. Neonicotinoid pesticides, introduced in the 1990s for several
crops, are the leading products for seed-coating treatments in many countries. Neonicotinoid containing dusts can
be efectively intercepted by bees in fight over the sowing feld, inducing lethal acute efects, so that restrictions in
the use of the main neonicotinoids have been adopted in the European Union. This led to the consequent introduction of replacement insecticides for seed-coating, i.e. methiocarb and thiacloprid, despite the lack of information on
both the toxicity and the exposure scenarios for honeybees.
Results: In this study, a laboratory apparatus was developed in order to quantify the toxicity of the dusts produced
from the abrasion of the seed coating. This quantifcation is based on (i) an airstream transporting coating particles
into an exposure chamber; (ii) exposure of bees to reproducible and measurable concentrations of insecticide, and
(iii) direct measurement of the exposure dose on single bees. The method allowed us to perform in vivo experiments
of honeybee exposure to provide toxicity data in more realistic exposure conditions. In fact, the formulation rather
than the active principle alone can be tested, and the exposure is through dusts rather than a solution so that specifc
absorption behavior can be studied in representative environmental conditions. The method was used to quantify
the acute toxicity (LD50) of dusts obtained from the abrasion of corn seeds coated with clothianidin, thiacloprid and
methiocarb.
Conclusions: Our results show that, surprisingly, the replacement insecticide methiocarb has a toxicity (LD50=421–
693 ng/bee) in the same order of magnitude as clothianidin (LD50=113–451 ng/bee) through this specifc exposure
route, while thiacloprid (LD50=16.9·103 ng/bee) has a signifcantly lower acute toxicity. Moreover, dusts containing
methiocarb and clothianidin show a signifcant increase in toxicity when, after exposure, bees are kept under high
humidity conditions. This suggests that the method here presented can be used to obtain complementary toxicity
data in the risk assessment procedure for the authorization of new seed-coating insecticides or new formulations.
Keywords: Systemic insecticides, Neonicotinoids, Methiocarb, Seed coating, Abrasion dusts, Honeybees, Pollinator
insects, Acute toxicity, Exposure measuremen