Figure 2. Imidacloprid is transformed in the environment into a number of different compounds, several of which are more toxic to honey bees than imidacloprid itself.4
havioral abnormalities. However, several
of the imidacloprid metabolites are equally
or more toxic to honey bees. Imidacloprid
(Compound A in Figure 2) is transformed
through a Type 1 process to 5-hydroxyimidacloprid (Compound B), followed by transfor(Compound C).2 While the hydroxy metabolite B is less toxic than the parent imidacloC is 1.6 times as
toxic as imidacloprid to honey bees.3
Toxic transformation products are also a
concern with the other neonicotinoids. The
insecticide thiamethoxam, for example, is
readily converted to clothianidin through a
Type 2 process in the environment (Figure
3).5, 6 Also a registered pesticide active ingredient, the clothianidin degradate is more
acutely toxic to honey bees than the thiamethoxam parent based on LD50 values.
When considering how long a compound
lasts in the hive, soil or water, it is important
to account for the persistence of both the
parent and the degradates. If both the parent
and the degradate are highly persistent, the
exposure risks remain high for longer than
one would predict based on the persistence
of the parent compound alone. Unfortunately, information on persistence and toxicity of degradates is not available for many
pesticides. US EPA only recently started
requiring pesticide manufacturers to submit
information on the toxicity of degradates to
honey bees, and still does not require submission of persistence data.
clothianidin.
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Figure 4. Keep in mind the message of
-
When it comes to assessing the scope of
a pesticide problem, analysis of plants, pollen, nectar, bees, brood, wax, honey, soil,
and water can provide useful information.
However, if a full accounting of toxicity is
desired, it is important that the analysis also
include the transformation products of pesticides that degrade to form more toxic chemicals. Unfortunately, inclusion of degradates
is not routine in the “multi-residue screen”
provided by most analytical labs. Even the
labs that do include the degradates often cannot detect low levels.7
What this means for beekeepers is that a
lab analysis report with “ND” (no detection)
for the parent compound does not necessarily mean that there were no transformation
compound” is a different message than “We
didn’t look for it,” so it is important to know
what the lab is capable of before sending a
sample out for analysis.
A recent study shows the pitfalls of analyzing for only the parent compound. Researchers measured the spatial and temporal
distribution of imidacloprid after injection
into apple trees, focusing only on imidacloprid and not including the toxic metabolites.8
While the study did provide useful data on
imidacloprid distribution in apple trees over
time, the authors cannot draw any conclusions about the overall toxicity of the apple
trees to both pest and pollinating insects be-
mean it is not present in the sample.
cause they did not measure concentrations of
the toxic metabolites.
In contrast, another study focused on
measuring levels of imidiacloprid and its
metabolites over time in forests treated for
the hemlock wooly adelgid.9 The researchers found that imidicloprid concentrations
peaked 12 months after treatment, but imidaincreased over time and were the highest at
the end of the study, 36 months after treatment. The study author also found that the
than imidacloprid to the hemlock woolly
adelgid.
Conclusions
A pesticide active ingredient is not the
only source of toxicity and, as it degrades
over time, the transformation products can
contribute to adverse effects. As shown
for the neonicotinoids imidacloprid, thiamethoxam and clothianidin, some degradation products are even more toxic to honey
bees than the parent active ingredient. As
a result, degradation of pesticides in contaminated plants, water and hive materials
may not necessarily decrease the risks associated with honey bee exposure to these
substances. To gain a more complete picture
of the pesticide impacts on bees, it is equally
important to measure the concentrations of
active ingredients and toxic transformation
products.
Footnotes
1
Ministry of Agriculture. Environmental
Protection – Environmental Fate. British
Columbia Ministry of Agriculture. http://
www.agf.gov.bc.ca/pesticides/c_2.htm.
2
Nauen R, Tietjen K, Wagner K, Elbert
A. 1998.
Imidacloprid against Myzus persicae and
Aphis gossypii (Homoptera: Aphididae).
Pestic. Sci. 52: 53–57.
3
Suchail S, Guez D, Belzunces LP. 2001.
American Bee Journal
4
5
Discrepancy Between Acute and Chronic
Toxicity Induced by Imidacloprid and its
Metabolites in Apis Mellifera. Environ.
Toxicol. Chem. 20: 2482–2486.
Reported LD50 values for a single pesticide may vary among multiple studies
of bees used as well as other experimental
factors.
US EPA. 2011. Registration Review:
Problem Formulation for the Environmental Fate, Ecological Risk, Endangered
Species and Drinking Water Exposure
Assessments for Thiamethoxam. US Environmental Protection Agency, December 13, 2011. http://www.regulations.
June 2014
6
7
8
gov/#!documentDetail;D=EPA-HQOPP-2011-0581-0003.
US EPA. 2011. Registration Review:
Problem Formulation for the Environmental Fate and Ecological Risk, Endangered Species, and Drinking Water
Exposure Assessments of Clothianidin.
US Environmental Protection Agency,
December 13, 2011. http://www.regulations.gov/#!documentDetail;D=EPAHQ-OPP-2011-0865-0003.
to obtain in pure form for the lab to create
standards of known concentration.
Reeb PD, Vandervoort C, Garavaglia
9
T, Cregg BM, et al. 2014. Spatial and
temporal distribution of trunk-injected
imidacloprid in apple tree canopies: Distribution of trunk-injected imidacloprid in
apple tree canopy. Pest Management Science n/a–n/a; doi:10.1002/ps.3747.
Coots CI, Spatial, temporal, and tritrophic distribution of imidacloprid,
on hemlock woolly adelgid, Adelges
tsugae Annand, (Hemiptera: Adelgidae). PhD Thesis, Univer- sity of Tennessee, Knoxville, TN (2012). http://
trace.tennessee.edu/cgi/viewcontent.
cgi?article=2772&context=utk_
graddiss
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