Annex VI to Directive 91/414 requires the calculation of a short-term toxicity/exposure ratio to assess pesticide risks to non-target terrestrial vertebrates. In most cases this requires knowledge of the daily food intake of one or more birds or mammals. The scientific literature on the diets of wild birds and mammals often contains detailed information on the proportion of different food items taken, but says very little on how much food might eaten in a day. Some data exists on the food intake of captive animals but it is unlikely that the requirements of animals confined in cages match those of free-living individuals. In the absence of empirical data from the field, the approach recommended by the EPPO/CoE guidelines is to use equations developed by Nagy (1987) relating bodyweight to daily energy requirement. By capturing animals in the wild, administering a doubly labelled water isotope, and recapturing at a later date, Nagy found he could calculate the energy expenditure between capture and recapture. He discovered that the metabolic rate of animals in the field (FMR) was typically 2-3 times the equivalent basal metabolic rate (BMR) in the laboratory. Nagy's equations allow one to estimate the daily energy requirement for animals of different bodyweights. Given additional knowledge of gross energy content of an animal's diet and the efficiency with which it digests it, it is possible to estimate its daily food requirement. Nagy also supplies appropriate equations for this calculation, which are cited in the EPPO/CoE guidelines.
Nagy's 1987 monograph summarised of all the published data using the doubly labelled water technique. At the time, the limited number of published studies obliged Nagy to derive his equations from relatively broad species groupings (e.g. passerines, desert birds and seabirds). No data were available for birds weighing over 500g. Furthermore, the different studies used individuals of different ages, at different times of year, in different breeding conditions, in different habitats, feeding on different food types. The end result is that all passerines of a certain bodyweight are assumed to eat the same fixed ration measured in g/day dry weight of some unspecified food source.
haveIn the 10 years or so since Nagy's review, one would hope to improve on this. FMRs have now been measured for more species under a wider variety of conditions, so it would be timely to update Nagy's equations. More data is also available now on species differences in energy requirements at times when pesticides are being used, on digestive efficiencies and the energy and moisture content of food items they take from agricultural environments. It would be particularly useful if the equations could be tailored as closely as possible to fit circumstances in which animals are exposed to pesticides in the UK, particularly for combinations of species and crop which recur frequently in risk assessment. For example, we
should be able to produce much improved estimates of the weight of foliage a hare niight graze in a sprayed crop; the amount of pesticide treated seeds a pigeon or field mouse might take from a newly sown cereal field; the insects eaten by a bluetit in an orchard; or the worms caught by a hedgehog or blackbird. The data and equations required for these calculations could be compiled into a computer database so that tailored estimates can be produced easily for other species/crop combinations, as and when required. Finally, we can assess the uncertainty in the resulting estimates and consider whether it can be taken into account using safety factors.
The project will begin by identifying a list of key species/crop combinations, in consultation
with PSD. This will be used to focus the following stages on those combinations most frequently considered in risk assessment. A search will be made for published and unpublished data on FMRs, energy efficiencies and gross energy contents additional to those used by Nagy, and for data on moisture content of different foods. These data will be used to calculate updated equations for estimating food intake, sirnilar to Nagy's but tailored to the species and diets considered in risk assessment. Where direct measurements of daily intake in captivity or the wild exist for a species (e.g. geese), their consistency with the estimated values will be evaluated. An assessment will be made of the contribution of each element of the estimation process to uncertainty in the results, and of the scale of safety factors which would be required to limit the chance of underestimating exposure.