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This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License
Article
Author(s)
Marco Pompeo Candolfi, Holger Bargen, Sigrun Bocksch, Olaf Klein, Marco Kleinhenz, Silvio Knaebe and Bronislawa Szczesniak
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DOI:10.17265/2161-6256/2018.03.003
Affiliation(s)
Eurofins Agroscience Services Ecotox GmbH, Eutinger Str. 24, Niefern-Öschelbronn75223, Germany
ABSTRACT
Statistical power, number of replicates and experiment complexity of
semi-field and field studies on Apis and non-Apis bee species has become a
major issue after publication of the draft European
Food Safety Authority (EFSA) Guidance on risk
assessment of plant protection products (PPP) on bees (Apis mellifera, Bombus spp. and solitary bees).
According to this guidance document, field studies have to be designed to be
able to detect significance differences as low as 7% for certain endpoints such
as reduction in colony size. This will require an immense number of replicates
which is obviously not feasible. In the present study, key endpoints such as
mortality, termination rate and number of brood cells in honeybee studies,
cocoon production and flight activity in solitary bee studies and number of
gynes in bumble bee studies (just to mention some of the endpoints considered)
in semi-field studies were analyzed, with Apis mellifera, Bombus terrestris and Osmia bicornis during the past five
years (2013-2017). The results indicate huge differences in the percentage
minimal detectable differences (%MDDs) (MDD expressed as
median of control value of the
endpoint in percent) depending on endpoint and species tested. For honeybee
semi-field studies, the lowest %MDDs recorded were between 10% and 15% for the
endpoints foraging, number of brood cells and colony strength. The highest
%MDDs were observed for the endpoint termination rate, with a %MDD of almost
50%. For the endpoints in bumble bee semi-field studies the %MDDs varied
between 17% for bumble bee colony weight and 53% for average mortality during
the exposure period in the tunnel. The %MDD for the number of gynes (young
queens) was slightly below 25%. For the semi-field solitary bee test system,
the %MDDs for the measured endpoints seem to be lower than those for the other
two species tested. The %MDDs for the endpoints hatching of offspring, nest
occupation and number of cocoons were 8%, 13% and 14%, respectively. Most of
the %MDDs were between 10% and 30% indicating clearly that the currently
performed experimental design for the semi-field pollinator studies allowed to
determine relatively small effects on key study endpoints. The analysis
indicated that for all the three bee species tested, the semi-field test design
detected low %MDDs for most of the endpoints. It was also observed that detectable
differences between the control and PPP treatments were much lower in semi-field test designs than in field
studies with these bee species. The “perfect sample size” really does not exist
but test design and statistical analysis can be adapted to lower the %MDDs.
Measured and simulated (according to Student’s t-test-distribution) data
and results showed that statistical evaluations parameter selection (e.g., alpha value), data
transformation (log10) and the number of replicates had a direct effect on the
ability of the test design to detect lower or higher %MDD values. It could show
that a change of alpha value from 0.05 to 0.1, increases the ability of the studies to detect lower %MDDs. For most of
the measured endpoints, increasing the number of replicates e.g., from four to eight, improved the power
of the test design by decreasing the %MDD. The reduction magnitude of the %MDD
is dependent on the endpoint and selection of statistical parameters such as
the alpha value. Parameters that display effects at a biologically relevant
scale will be a better indicator for effects than parameters that are able to
detect minor differences that are not biologically relevant.
KEYWORDS
Apis mellifera, Bombus terrestris, Osmia bicornis, OECD75, minimal detectable difference (MDD), statistical power.
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