Time-to-death approach to reveal chronic and cumulative toxicity of a fungicide for honeybees not revealed with the standard ten-day test.
Synthetic fungicides are pesticides widely used in agriculture to
control phytopathogenic fungi. The systemicity, persistency and intense
application of some of these fungicides, such as boscalid, leads to long
periods of exposure for honeybees via contaminated water, pollen and
nectar. We exposed adult honeybees in the lab to food contaminated with
boscalid for 33 days instead of the standard 10-day test. Most of the
toxic effects were observed after 10 days. The median time to death (LT50)
ranged from 24.9 days (lowest concentration) to 7.1 days (highest
concentration) and was significantly shorter in all cases than with the
control (32.0 days). The concentration and dietary doses of boscalid
inducing 50% mortality (LC50 and LDD50, respectively) decreased strongly with the time of exposure: LC50 = 14,729 and 1,174 mg/l and LDD50 = 0.318 and 0.0301 mg bee−1 day−1
at days 8 and 25, respectively. We found evidence of reinforced
toxicity when exposure is prolonged, but with an unusual pattern: no
cumulative toxicity is observed until 17–18 days, when a point of
inflexion appears that suggests a reduced capacity of bees to deal with
the toxicant. Our results show the importance of time-to-death
experiments rather than fixed-duration studies for evaluating chronic
toxicity.