Neonicotinoid Pesticides and Bees - The Food and Environment ...

data. The LD50 for imidacloprid is higher than those of Nauen et al., (2001) and Schmuck et
al., (2003) however, there is some overlap between the ranges of the two sets of data. The
variation observed for clothianidin and thiamethoxam is less.
Table 4: Mean oral LD50 values for three neonicotinoid insecticides. Data taken from (Laurino et
al., 2010)
Duration (hours) Clothianidin Imidacloprid Thiamethoxam
Mean 95% CI Mean 95% CI Mean 95% CI
24 4.48 3.96 – 4.90 183.78 174.5 -190.7 3.55 2.82 – 4.43
48 4.32 3.86 – 4.65 104.12 99.53 – 108.99 3.35 2.68 – 4.25
72 4.21 3.81 – 4.50 72.94 49.55 – 95.15 2.88 2.59 – 3.13
Suchail et al., (2000) identified differences in LD50 response to imidacloprid between two
subspecies of honeybees Apis mellifera mellifera (the dark European Honeybee) and Apis
mellifera caucasica (the Caucasian Honeybee). Similar values were obtained for oral
exposure of 5.4 and 6.6 ng/bee LD50 (24 hour) and 4.8 and 6.5 ng/bee LD50 (48 hour) A.
m. mellifera and A. m. caucasica respectively. However, there were marked differences in
responses between the subspecies after contact exposure. At low doses (between 1 and 7
ng/bee) A. m. mellifera mortality rates apparently increased, between 7 to 15 ng/bee
mortality rates decreased, and at concentrations above 15ng/bee mortality rates increased in
a dose dependent manner (Error! Reference source not found.A). As a result, two 24 hour
LD50 values, 6.7ng/bee and 23.8 ng/bee, were calculated for A. m. mellifera based on the
two ascending parts of the dose response curve. A similar drop in mortality was seen with A.
m. caucasica (Error! Reference source not found.B), but this was less marked, and a
single LD50 value of 15.1 ng/bee was calculated. A similar increase in mortality was seen in
A. m. mellifera over 48 hours at low doses; however this was not followed by a fall below that
expected. Notwithstanding this, 48 hour LD50 values were again calculated from the two
ascending parts of the dose response curve with values of 6.7 and 24.3 ng/bee for A. m.
mellifera; 48 hour contact LD50 for A. m. caucasica was 12.8 ng/bee.
Interestingly, the deviation between replicates is small, and experiments were repeated at
least three times. Suchail et al., (2000) suggested this was a result of biphasic mortality at
low concentrations, particularly via contact exposure. Howeever the result is predicated
primarily on the mortality observed at one dose level around 7 ng/bee. Unfortunately none
of the other reported studies tested imidacloprid at doses this low; the minimum dose in the
reported literature was 40 ng/bee (Nauen et al., 2001; Schmuck et al., 2003) and at Fera the
lowest dose tested in Defra study PS2368 was 12.5 ng/bee. No other studies have reported
to replicate this phenomenon.
Studies by Nauen et al., (2001), Schmuck et al., (2003), and Suchail et al., (2001a) have
looked at the acute toxicity of imidacloprid and the five main metabolites to A. mellifera. Two
metabolites were found to have insecticidal properties; these are olefin and 5-OH
imidacloprid with 48 hour oral LD50 values of >39 and 159 ng/bee respectively (Nauen et al.,
2001; Schmuck et al., 2003) and 28 and 258 ng/bee (Suchail et al., 2001a). Nauen et al.,
(2001) obtained an LD50 for di-hydroxy-imidacloprid of >49 ng/bee; its insecticidal
significance was identified as low due to its weak receptor affinity and lack of receptor
activation despite retaining the nitro guanidine pharmocophore, (Schmuck et al., 2003).
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