January 24, 2013

expert reaction to pesticide impact on amphibians

A paper published in Scientific Reports studied the effects of exposing young frogs to pesticides, and said that existing risk assessments for pesticide regulation may require re-examining.

Prof Sir Colin Berry, Emeritus Professor of Pathology, Queen Mary University of London, said:

“This study is interesting but there are some significant points to consider. There are ways in which some of these pesticides might have a mechanism of action that would affect amphibians, these will differ between compounds and so would need examining carefully.

“The main problem is how the experimental levels of exposure compare to the likely exposures that amphibians might receive in the field. In the test system used, the frogs were subjected to very different kinds of exposure than one would expect in the field. This means that it is very difficult to draw conclusions about the implications of the work.

“In the field, if the amount of pesticide applied is ‘x’ then the actual exposure experienced by amphibians will be significantly less than ‘x’ due to dilution in water and water masses, fixation to the target crop and to soil etc. Of the findings in this study, the ones for exposure levels of 10 x the maximum use amount are much less interesting than those around the normal levels of application.

“Of the compounds that are tested, a number have highly restricted uses and some of them are specific in their action (affecting particular processes in fungi, for example). A number of the compounds used are only used in restricted circumstances (notably the organophosphate compounds).

“It is interesting to note that Fenoxaprop is a compound which mimics juvenile hormone activity and which might be thought to affect development in amphibians. It would be necessary to determine whether levels that produce this type of effect would be reached in the field.”

Dr Mick Hamer, Environmental Safety, Syngenta, said:

“Bruhl et al (2013), in their paper ‘Terrestrial pesticide exposure of amphibians: An underestimated cause of global decline?’ have exposed juvenile common frogs to a number of pesticides. The study was designed to fill a knowledge gap with respect to the effects of pesticides on terrestrial stages of amphibians and as such, followed a similar methodology, and found similar results, to Belden et al (2010), referenced within this study. As with Belden (2010), the scenario was direct overspray of juvenile amphibians, in this case, confined to a cage above bare soil.

“The study was limited, particularly with respect to numbers of animals and this is to be applauded as the sacrifice of animals in such studies seems unnecessary, as the relevance to exposure of amphibians in the field has not been established. The claim is that the exposure is ‘realistic worst-case’, however the only thing realistic are the application rates. It has yet to be established how and the extent to which amphibians are exposed to pesticides, particularly for terrestrial life-stages in agricultural environments.

“The extrapolation from laboratory mortalities following overspray to effects in the field and the subsequent link to amphibian decline is not justified.  Consideration needs to be given to application scenarios and timing, crop interception (these pesticides are not applied to bare soil) and the co-occurrence of amphibians in the field at application.

“Extrapolating from these simplistic lab exposures to effects in the field, with the use of language such as ‘alarming’ and ‘astonishing’, whilst perhaps journalistic, is not sound science.  Indeed, the counter argument can be made, that if amphibian mortality is as portrayed in this paper, it is ‘astonishing’ that there are not more reports of dead amphibia from the field.”

None of the pesticides used in the study are Syngenta products.

Prof Richard Shore, Centre for Ecology & Hydrology, Lancaster Environment Centre, said:

“This study is useful in that it highlights the toxic hazard posed by terrestrial pesticides to amphibians that might be present in cropped areas; such risks are not routinely assessed. 

“However, the exposure scenarios in this experiment are not realistic of field situations and might be expected to result in higher exposures (and associated effects) for the given spray rates. Therefore, while highlighting the hazard posed by these terrestrial pesticides to common frogs and potentially other amphibians, the authors’ statement that their results suggest ‘large-scale negative effects of terrestrial pesticide exposure on amphibian populations seems likely’ are difficult to justify.

“However, the importance of cropped areas for amphibians is uncertain and if such areas are a significant habitat then consideration of likely risk to amphibians within crop may well be needed.”

Further info from Prof Shore:

Toxicity

“The paper raises some valid points about whether current testing regimes are adequately protective of adult amphibians that might be present in agricultural crops. I believe the authors to be correct when arguing there are no direct tests for this and so there is uncertainty as to whether current risk assessment is protective here, although it is not known what percentage of amphibians are likely to be in fields during  spraying periods… and therefore how important any lack of direct assessment may be.

“The authors note some toxicity with all products at x1 normal maximum application and with the higher rate (x10 application rate), and some mortality with three products at 10 times lower than the maximum application rate. The study therefore demonstrates hazard to common frogs from these formulated products. The data suggest that compounds used in the formulation of the product may also affect toxicity which indeed might be expected.”

Exposure

“Exposure in this study appears to be through direct over-spraying both of the soil substrate and, as far as I can tell from the methods (although it is not crystal clear from the text), of the frogs. I am assuming that the frogs were over-sprayed as the text refers to the animals being confined to the substrate and in the bottom part of the containers that were sprayed. 

If we consider the x1 maximum application rate treatment, exposure in the study is most likely worse than in the field for various reasons, such as:

a)      Frogs were directly over-sprayed (this would occur in the field only if animals were present on the soil surface when spraying occurred),

b)      There was 0% interception of spray by crops whereas interception would be expected by crops in the field.  The amount of interception varies with a bunch of factors but could easily be more than 60% and quite possibly much higher. 

c)       Frog exposure to subsequently volatilised pesticide from soil and surface residues on the soil may well be higher than in the field as animals had no ability to move away from contact with sprayed soil, and the way that moisture was added to the soil in the experiment was designed to prevent pesticide leaching.  

“Overall therefore, the experiment is not realistic of exposure in the field.  The x1 maximum application rate in this study might be expected to result in an exposure in frogs greater than the equivalent spray rate in the field, the 10X dose rate obviously more so.  The relative exposure associated with the 0.1 dose rate relative to what may occur in the field when pesticide is applied at normal rate is uncertain.”

Sample size

“The study is mostly well explained but the numbers of animals used per test is small, albeit for good reasons.  Because of small sample numbers, mortality could only be recorded in steps of 20% at best (maximum of five frogs were used per dose).  There was no replication of tests per compound and full dose response curves are not described.  Thus, there will be considerable uncertainty around the given % toxicity values, although the tests are clear in showing that some toxicity does occur in frogs that were exposed.” 

Should we be doing anything differently with pesticide use?

“I don’t think this is the right question.  I think the question is should we be specifically evaluating the potential risk to amphibians from terrestrial pesticides?  This depends on whether there is good evidence that within-field cropped areas are utilised significantly as habitat by amphibians. If there is such evidence, consideration of likely risk to amphibians within crop may well be needed.”

Reference: 

‘Terrestrial pesticide exposure of amphibians: An underestimated cause of global decline?’ by Bruhl et al., published in Scientific Reports on Thursday 24th January.