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Aerial applications of asulam: A bioassay technique for assessing buffer zones to protect sensitive sites in upland Britain
Biological Conservation 1992, 59,19-23
Aerial applications of asulam: A bioassay technique for assessing buffer zones to protect sensitive sites in upland Britain R. H. Marrs, a* A. J. Frost, b R. A. Plant b & P. Lunnis a aEcological Processes and bConservation Management Sections, NERC, Institute of Terrestrial Ecology, Monks Wood Experimental Station, Abbots Ripton, Huntingdon PEI7 2LS, UK (Received 13 December 1990; revised version received 14 March 1991; accepted 27 March 1991) Aerial spraying with asulam is commonly used to control bracken in upland Britain, and there is a possibility that drift from these applications may reach sites containing other sensitive species. Here a field-based bioassay experiment is reported where a sensitive species (Rumex acetosa) was exposed downwind of an area being sprayed commercially with asulam (4.4 kg a.i. ha-1 in 44 litres spray ha -1) by helicopter in the North Peak Environmentally Sensitive Area in Derbyshire. Although drift of large droplets decreased rapidly, with most deposition within 35m downwind, damage to the test plants was detected at distances of greater than 100 m. We predicted that the no-effect distance was 161 m and that a minimum buffer zone of > 160m was, therefore, required to protect sensitive sites.
Gymnocarpium dryopteris, lemon-scented fern Oreopteris limbosperma, and beech-fern Phegopteris connectilis (nomenclature follows Clapham et al.,
INTRODUCTION There has been increasing concern for many years that herbicide spray drift from agricultural, forestry and other treated land can affect plant species growing on adjacent nature reserves. Two situations pose particularly severe problems: where nature reserves are part of a mosaic of patches within a landscape where large areas of 'weeds' are to be treated, and where aerial applications are made. In Britain the control of bracken Pteridium aquilinum (L.) Kuhn in upland areas causes concern for nature reserves on both these counts. Large areas of bracken are often sprayed from the air as part of a region-wide campaign to reduce the infestation on moorland. Often within the bracken-moorland matrix, there are areas of high conservation interest where spray drift might cause damage to rare species of fern such as lady-fern A thyriumfilix-femina, hardfern Blechnurn spicant, scaly male-fern Dryopteris affinis, broad buckler-fern D. dilatata, oak-fern
1987). Spray from aircraft drifts further than. from most ground sprayers (Elliott & Wilson, 1983; Williams et al., 1987), with drift being detected between 400 and 1000 m away from the application point (reviewed by Davis & Williams, 1990). There is, however, very little information on the likely biological effects of this drift. Asulam is the main herbicide used to control bracken and was originally developed for the control of docks Rumex spp. Other ferns are susceptible (Horrill et al., 1978; Marrs & Griffiths, 1986) and are the most likely candidates to be affected by asulam drift in upland spraying campaigns. Because bracken control is a priority in many parts of upland Britain for effective management of sheep and grouse, and for moorland and landscape conservation, it is unlikely that the use of asulam will decline in the near future. Indeed asulam applications from the air have increased greatly during the 1980s from 838ha in 1981 to 5292ha in 1990 (Sly & Neale, 1983; MAFF, pers. comm.). Asulam is by far the most commonly used herbicide in aerial applications, accounting for more than
* Present address: University of Liverpool Botanic Gardens, Ness, Neston, South Wirral L64 4AY, UK. Biological Conservation 0006-3207/92/$05.00 © 1992 Elsevier Science Publishers Ltd.
19
20
R. H. Marrs, A. J. Frost, R. A. Plant, P. Lunnis
98% of the total herbicide applied by air in 1990 (MAFF, pers. comm.). It is therefore essential that a safe 'buffer zone' distance is estimated, so that sensitive sites can be protected during spraying. Here, we describe an experimental field bioassay approach, using plants sensitive to asulam, to estimate the size of the buffer zone needed.
METHODS Plant propagation Common sorrell R u m e x acetosa L. was the test plant used in this study as it had already been shown to be susceptible to asulam drift in pilot studies (Marrs et al., 1990). After 2-3 weeks the leaves of affected plants show severe chlorosis, followed in some instances by necrosis and death. Seeds of R u m e x acetosa were sown in January 1990 and then potted individually into 7 cm x 7 cm × 8 cm pots containing SAI GP compost. The plants were maintained outside and any developing flowering stems were cut to prevent premature dormancy. Six pots were placed in seed trays for bioassay use in the field. At the time of spraying the plants had between 10 and 40 mature leaves.
Experimental layout A map of the experimental area at Bamford Edge in Derbyshire (National grid reference SK 214842) is shown in Fig. 1. The site is a steep escarpment
reaching a gently rising plateau at an altitude of approximately 400m. In July 1990, dense bracken ( > 2 0 fronds m-2; 1.5-2.0m tall)covered the steep slope, and the vegetation changed abruptly into moorland at the edge of the plateau. At the study site, eight transects, each 2 m apart, were laid out on 22 July at right angles downwind of the bracken front into the moorland (Fig. 1). On each transect, trays were placed 10m inside the bracken patch within a small cleared area (designated - 10 m), at the boundary between the bracken patch and moorland (0m), and thereafter at 2.5, 5, 10, 20, and at subsequent 20-m intervals until 240m downwind into the moorland. Water-sensitive papers were attached horizontally to short canes on two transects to provide a crude assessment of spray drift deposition, using the image analysis technique of Sinha et al. (1990). Twelve trays of R u m e x acetosa were kept as untreated 'controls', these plants being treated in an identical manner to treated ones except that they were not exposed to asulam on the transects. The spraying was done by a commercial operator as part of a wider bracken control programme within the North Peak Environmentally Sensitive Area (ESA). Asulam was applied by a Bell 47G3B1 helicopter at a height of between 5 and 10m and a rate of 4"4 kg a.i. h a - 1 (11 litres Asulox ha - 1) in 44 litres ha -1 spray containing a 0-1% non-ionic wetter (Agral). A 20m boom was used fitted with 72 Raindrop nozzles and the tank pressure was 2 bar. The upper edge of the bracken patch was sprayed in
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Wind direction Fig. 1. Schematicmap of the studyarea showingthe brackenarea (hatched),and the approximateposition of(l) the fivetransectsand (2) the sprayed area (double hatched).
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Aerial applications of asulam
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Fig. 2. Mean deposition of visible spray on pairs of water-sensitive papers, expressed as a % of deposition at the 0 m position, and a logistic curve fitted to these data, in the bioassay study at Bamford Edge, Derbyshire.
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Distance downwind (m) Fig. 3. Mean values _ standard errors (n = 8) for leaf damage on test plants (Rumex acetosa)caused by asulam downwind of a sprayer, and a logistic curve fitted to these data, in the bioassay study at Bamford Edge, Derbyshire. The mean values +_ standard errors (n = 12) for untreated controls (u) are also presented for comparison.
22
R. H. Marrs, A. J. Frost, R. A. Plant, P. Lunnis
three swaths flying perpendicular to the transects (Fig. 1), starting at the upper edge and moving down the slope. Wind speed and direction were monitored continuously during the spraying using a Vector Instruments R500 recording anemometer. The wind direction was constant, and was from the south-east. Wind speed varied between 6 and 1 0 m s -x at a height of 2 m. There was no rain during the spraying period. After spraying, the plants were left in situ for 2 h to dry, before transfer to Monks Wood Experimental Station. The R u m e x plants were placed on a sand bed outside and given an appropriate horticultural watering regime. After three weeks each individual plant was assessed visually for damage by counting (1) the total number of leaves, and (2) the number of these leaves which were showing chlorotic or necrotic symptoms. The percentage of leaves damaged at each transect point was then calculated based on the six replicate individuals in each tray.
RESULTS Drift deposition and damage to the R u m e x acetosa test plants declined with distance downwind of the sprayer (Figs 2, 3). Logistic equations fitted to these data using the Maximum Likelihood Program (Ross, 1980) gave the following equations, which were used to estimate safe buffer zone distances: Yaeposi,o, = 0"44206 -~ 1 +
116.085 68 e 0 ' 0 9 5 0 2 ( x - 8.48837)
(n = 17; d.f. = 13; r 2
----
0-71)
95.32509 Yaamage = 3"41984 -~ 1 q- e 0"05693(x-84"01972) (n= 136; d.f. = 132; r z = 0-99) Deposition of drift measured with the watersensitive papers declined rapidly downwind of the sprayer, with only 10% of the applied rate reaching 33 m. This technique underestimates the deposition of small droplets, but even so 0.2% of the amount detected at 0 m was found at 220 m. Damage to R u m e x acetosa extended to much greater distances than suggested by the deposition data, with 10% of leaves damaged at 131m. Untreated 'control' plants had a mean value (__+SE) for leaf damage of 3-5 ___0.4%. The no-effect level was estimated at 5% leaf damage (NEL = untreated m e a n + 1.96 × SD), and the predicted distance required to achieve this level was 161 m downwind.
DISCUSSION As expected, there was a rapid fallout of large spray droplets, with few droplets detectable using watersensitive papers extending past 33 m. However, a few large droplets, and smaller ones which were not detected using our simple monitoring system, caused damage up to an estimated distance of 161 m from the sprayed zone. Thus this bioassay technique gives a buffer zone of > 160 m to protect sensitive upland plants during aerial applications of asulam. This buffer zone requirement is much greater than for herbicide applications from ground sprayers. A buffer zone of 6 - 1 0 m was considered adequate to protect a range of established perennial species from four herbicides applied by tractor-mounted hydraulic sprayers (Marrs et aL, 1989), 20-30m was suggested for the protection of fish from glyphosate applied from the air (Payne et al., 1990), while > 200m was needed to protect three-spine sticklebacks Gasterosteus aeuleatus from applications of endosutfan applied from aircraft (Ernst et aL, 1991). There are several additional points to make about this study. Native fern species may be more or less sensitive than R u m e x acetosa. We chose this species because it is very sensitive to asulam drift, showing obvious symptoms of damage a few weeks after treatment, and because it can be propagated easily in large numbers. It would obviously be impractical to consider collecting rare fern species from nature reserves in the numbers required for bioassay studies of this type. However, a small number of commercially available ferns were included in this, and an earlier trial. The results were generally similar, but were more difficult to interpret, partly because the damage takes much longer to appear (sometimes over winter), and because herbicide damage can be confounded with damage from other sources (frost, temperature and water supply). It is of course possible that sub-lethal effects on species with extreme sensitivity to asulam extend beyond the distance detected here. Moreover, only leaf damage was assessed, no account being taken of subtle damage which may influence population performance, for example fecundity and survivorship in communities where the competitive balance between species has potentially been altered by asulam drift. To investigate this aspect, detailed population studies are required in sprayed and unsprayed situations. In this study only damage downwind of the sprayer was assessed. No account was taken of additional risks associated with the helicopter
Aerial applications of asulam turning at the end of a swath. When the helicopter reaches the end of a swath the operator switches off the spray, turns, and then switches on again at the start of the next swath. An error at this point could cause the drift to start outside the target zone, and where this might occur the buffer zone may have to be extended. Clearly, the results obtained apply only to the test situation. The experimental site was at the upper edge of the bracken patch on a sloping plateau site with the wind speed at 6 - 1 0 m - 1 during spraying. The results may be less severe where the bracken edge is on steep slopes, on flatter ground and at different wind speeds. The wind speed was higher than the limit recommended by M A F F / H S E (1989) for aerial spraying ( < 10 knots or < 5 m s- 1), but the spraying was done by a commercial operator as part of a large bracken control campaign within the North Peak ESA. T h u s , even if we consider our buffer zone estimate to be a 'worst case' scenario, it is one that may not be atypical in practice. If wind speeds are lower, then added protection will be achieved. It is also worth noting that the > 160 m zone is 90 m less than the 250m buffer currently used in the North Peak ESA scheme for the protection of sensitive sites. The main need for accurately determining adequate buffer zones is because of the statutory requirement under the FEPA for aerial spray operators to consult the Nature Conservancy Council when spraying within three-quarters of a nautical mile of a Site of Special Scientific Interest (FEPA, 1989). There is, therefore, an opportunity to require at this consultation stage that such sites are surrounded by a buffer zone. An alternative approach may be to spray bracken patches only to the leeward of sensitive sites. Before this suggestion is implemented, however, it is essential to determine the upwind drift effects caused by turbulence from the helicopter. ACKNOWLEDGEMENTS We would like to thank R. Starbuck, N. Taylor, F. Darwent for considerable help in planning this experiment in Derbyshire; East Midlands Region of the Nature Conservancy Council for the use of their
23
Argocat; and Drs A. Cooke, B. N. K. Davis and T. M. Roberts for helpful criticism of the manuscript. This work was funded by the Nature Conservancy Council as part of their research programme in nature conservation.
REFERENCES Clapham, A. R., Tutin, T. G. & Moore, D. M. (1987). Flora of the British Isles, 3rd edn. Cambridge University Press, Cambridge. Davis, B. N. K. & Williams, C. T. 0990). Buffer zone widths for honeybees from ground and aerial spraying of insecticides. Environ. Pollut., 63, 247-59. Elliott, J. G. & Wilson, B. J. (eds.) 0983). The Influence of Weather on the Efficiency and Safety of Pesticide Application: the DriJ? of Herbicides. BCPC Occ. Publs, No. 3. BCPC, London. Ernst, W. H., Jonah, P., Doe, K., Julien, G. & Hennigar, P. (1991). Toxicity of aquatic organisms of off-target deposition of endosulfan applied by aircraft. Environ. Toxicol. & Chem., 10, 103-14. FEPA (1989). FEPA, 1985: Control of Pesticide Regulations 198~-Consent C(ii): Aerial Application of Pesticides. London Gazette, 20 January, 784-5. Horrill, A. D., Dale, J. & Thomson, A. (1978). Asulam--its effects on selected plants, plant communities and animals. NERC contract report to Nature Conservancy Council. Institute of Terrestrial Ecology, Grange-over-Sands. MAFF/HSE (1989). Pesticides 1989. MAFF Reference Book, No. 500. HMSO, London. Marrs, R. H., Frost, A. J. & Plant, R. A. (1989). Assessment of the effects of herbicide spray drift on a range of plant species of conservation interest. Environ. Pollut., 59, 71-86. Marts, R, H. & Griffiths, F. M. 0986). Terrestrial herbicides. In The Use of Herbicides on Nature Reserves, ed. A. S. Cooke. Focus on Nature Conservation, No. 14, Nature Conservancy Council, Peterborough, pp. 13-64. Marrs, R. H., Frost, A. J. & Plant, R. A. (1990). Effects of herbicide drift on higher plants. In Pesticide Drift and Impact, ed. B. N. K. Davis. NERC contract report to NCC/DOE. Institute of Terrestrial Ecology, Huntingdon, pp. 3-50. Payne, N. J., Feng, J. C. & Reynolds, P. E. (1990). Off-target deposits and buffer zones required around water for aerial glyphosate applications. Pestic. Sci., 30, 183-98. Ross, G. J. S. (1980). MLP: Maximum Likelihood Program. Rothamsted Experimental Station, Harpenden. Sinha, S. N., Lakhani, L. K. & Davis, B. N. K. (1990). Studies on the toxicity of insecticidal drift to the first instar larvae of the large white butterfly Pieris brassicae L. (Lepidoptera: Pieridae). Ann. Appl. Biol., 116, 27-41. Sly, J. M. A. & Neale, M. C. (1983). Aerial Spraying, Great Britain 1981. A D A S Survey Report, No. 24. MAFF, Alnwick. Williams, C. T., Davis, B. N. K., Marrs, R. H. & Osborn, D. (1987). Impact of Pesticide Drift. NERC contract report to NCC, Institute of Terrestrial Ecology, Huntingdon.
Aerial applications of asulam: A bioassay technique for assessing buffer zones to protect sensitive sites in upland Britain R. H. Marrs, a* A. J. Frost, b R. A. Plant b & P. Lunnis a aEcological Processes and bConservation Management Sections, NERC, Institute of Terrestrial Ecology, Monks Wood Experimental Station, Abbots Ripton, Huntingdon PEI7 2LS, UK (Received 13 December 1990; revised version received 14 March 1991; accepted 27 March 1991) Aerial spraying with asulam is commonly used to control bracken in upland Britain, and there is a possibility that drift from these applications may reach sites containing other sensitive species. Here a field-based bioassay experiment is reported where a sensitive species (Rumex acetosa) was exposed downwind of an area being sprayed commercially with asulam (4.4 kg a.i. ha-1 in 44 litres spray ha -1) by helicopter in the North Peak Environmentally Sensitive Area in Derbyshire. Although drift of large droplets decreased rapidly, with most deposition within 35m downwind, damage to the test plants was detected at distances of greater than 100 m. We predicted that the no-effect distance was 161 m and that a minimum buffer zone of > 160m was, therefore, required to protect sensitive sites.
Gymnocarpium dryopteris, lemon-scented fern Oreopteris limbosperma, and beech-fern Phegopteris connectilis (nomenclature follows Clapham et al.,
INTRODUCTION There has been increasing concern for many years that herbicide spray drift from agricultural, forestry and other treated land can affect plant species growing on adjacent nature reserves. Two situations pose particularly severe problems: where nature reserves are part of a mosaic of patches within a landscape where large areas of 'weeds' are to be treated, and where aerial applications are made. In Britain the control of bracken Pteridium aquilinum (L.) Kuhn in upland areas causes concern for nature reserves on both these counts. Large areas of bracken are often sprayed from the air as part of a region-wide campaign to reduce the infestation on moorland. Often within the bracken-moorland matrix, there are areas of high conservation interest where spray drift might cause damage to rare species of fern such as lady-fern A thyriumfilix-femina, hardfern Blechnurn spicant, scaly male-fern Dryopteris affinis, broad buckler-fern D. dilatata, oak-fern
1987). Spray from aircraft drifts further than. from most ground sprayers (Elliott & Wilson, 1983; Williams et al., 1987), with drift being detected between 400 and 1000 m away from the application point (reviewed by Davis & Williams, 1990). There is, however, very little information on the likely biological effects of this drift. Asulam is the main herbicide used to control bracken and was originally developed for the control of docks Rumex spp. Other ferns are susceptible (Horrill et al., 1978; Marrs & Griffiths, 1986) and are the most likely candidates to be affected by asulam drift in upland spraying campaigns. Because bracken control is a priority in many parts of upland Britain for effective management of sheep and grouse, and for moorland and landscape conservation, it is unlikely that the use of asulam will decline in the near future. Indeed asulam applications from the air have increased greatly during the 1980s from 838ha in 1981 to 5292ha in 1990 (Sly & Neale, 1983; MAFF, pers. comm.). Asulam is by far the most commonly used herbicide in aerial applications, accounting for more than
* Present address: University of Liverpool Botanic Gardens, Ness, Neston, South Wirral L64 4AY, UK. Biological Conservation 0006-3207/92/$05.00 © 1992 Elsevier Science Publishers Ltd.
19
20
R. H. Marrs, A. J. Frost, R. A. Plant, P. Lunnis
98% of the total herbicide applied by air in 1990 (MAFF, pers. comm.). It is therefore essential that a safe 'buffer zone' distance is estimated, so that sensitive sites can be protected during spraying. Here, we describe an experimental field bioassay approach, using plants sensitive to asulam, to estimate the size of the buffer zone needed.
METHODS Plant propagation Common sorrell R u m e x acetosa L. was the test plant used in this study as it had already been shown to be susceptible to asulam drift in pilot studies (Marrs et al., 1990). After 2-3 weeks the leaves of affected plants show severe chlorosis, followed in some instances by necrosis and death. Seeds of R u m e x acetosa were sown in January 1990 and then potted individually into 7 cm x 7 cm × 8 cm pots containing SAI GP compost. The plants were maintained outside and any developing flowering stems were cut to prevent premature dormancy. Six pots were placed in seed trays for bioassay use in the field. At the time of spraying the plants had between 10 and 40 mature leaves.
Experimental layout A map of the experimental area at Bamford Edge in Derbyshire (National grid reference SK 214842) is shown in Fig. 1. The site is a steep escarpment
reaching a gently rising plateau at an altitude of approximately 400m. In July 1990, dense bracken ( > 2 0 fronds m-2; 1.5-2.0m tall)covered the steep slope, and the vegetation changed abruptly into moorland at the edge of the plateau. At the study site, eight transects, each 2 m apart, were laid out on 22 July at right angles downwind of the bracken front into the moorland (Fig. 1). On each transect, trays were placed 10m inside the bracken patch within a small cleared area (designated - 10 m), at the boundary between the bracken patch and moorland (0m), and thereafter at 2.5, 5, 10, 20, and at subsequent 20-m intervals until 240m downwind into the moorland. Water-sensitive papers were attached horizontally to short canes on two transects to provide a crude assessment of spray drift deposition, using the image analysis technique of Sinha et al. (1990). Twelve trays of R u m e x acetosa were kept as untreated 'controls', these plants being treated in an identical manner to treated ones except that they were not exposed to asulam on the transects. The spraying was done by a commercial operator as part of a wider bracken control programme within the North Peak Environmentally Sensitive Area (ESA). Asulam was applied by a Bell 47G3B1 helicopter at a height of between 5 and 10m and a rate of 4"4 kg a.i. h a - 1 (11 litres Asulox ha - 1) in 44 litres ha -1 spray containing a 0-1% non-ionic wetter (Agral). A 20m boom was used fitted with 72 Raindrop nozzles and the tank pressure was 2 bar. The upper edge of the bracken patch was sprayed in
/o425m k/ // X / / /
,S / / /
/ / z /
o I
Mov/////~/////orl~nO
Eightrr ansect
"-~
500 i
i
i
i
I
Metres
Wind direction Fig. 1. Schematicmap of the studyarea showingthe brackenarea (hatched),and the approximateposition of(l) the fivetransectsand (2) the sprayed area (double hatched).
loo
Aerial applications of asulam
21
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.
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100 150 Distance downwind (m)
I
200
250
Fig. 2. Mean deposition of visible spray on pairs of water-sensitive papers, expressed as a % of deposition at the 0 m position, and a logistic curve fitted to these data, in the bioassay study at Bamford Edge, Derbyshire.
100 ,e
75Damage(%)
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r-~ .
.
-50
.
.
.
.
.
.
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.
,
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.
.
,
150
.
.
.
.
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.
.
.
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.
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Distance downwind (m) Fig. 3. Mean values _ standard errors (n = 8) for leaf damage on test plants (Rumex acetosa)caused by asulam downwind of a sprayer, and a logistic curve fitted to these data, in the bioassay study at Bamford Edge, Derbyshire. The mean values +_ standard errors (n = 12) for untreated controls (u) are also presented for comparison.
22
R. H. Marrs, A. J. Frost, R. A. Plant, P. Lunnis
three swaths flying perpendicular to the transects (Fig. 1), starting at the upper edge and moving down the slope. Wind speed and direction were monitored continuously during the spraying using a Vector Instruments R500 recording anemometer. The wind direction was constant, and was from the south-east. Wind speed varied between 6 and 1 0 m s -x at a height of 2 m. There was no rain during the spraying period. After spraying, the plants were left in situ for 2 h to dry, before transfer to Monks Wood Experimental Station. The R u m e x plants were placed on a sand bed outside and given an appropriate horticultural watering regime. After three weeks each individual plant was assessed visually for damage by counting (1) the total number of leaves, and (2) the number of these leaves which were showing chlorotic or necrotic symptoms. The percentage of leaves damaged at each transect point was then calculated based on the six replicate individuals in each tray.
RESULTS Drift deposition and damage to the R u m e x acetosa test plants declined with distance downwind of the sprayer (Figs 2, 3). Logistic equations fitted to these data using the Maximum Likelihood Program (Ross, 1980) gave the following equations, which were used to estimate safe buffer zone distances: Yaeposi,o, = 0"44206 -~ 1 +
116.085 68 e 0 ' 0 9 5 0 2 ( x - 8.48837)
(n = 17; d.f. = 13; r 2
----
0-71)
95.32509 Yaamage = 3"41984 -~ 1 q- e 0"05693(x-84"01972) (n= 136; d.f. = 132; r z = 0-99) Deposition of drift measured with the watersensitive papers declined rapidly downwind of the sprayer, with only 10% of the applied rate reaching 33 m. This technique underestimates the deposition of small droplets, but even so 0.2% of the amount detected at 0 m was found at 220 m. Damage to R u m e x acetosa extended to much greater distances than suggested by the deposition data, with 10% of leaves damaged at 131m. Untreated 'control' plants had a mean value (__+SE) for leaf damage of 3-5 ___0.4%. The no-effect level was estimated at 5% leaf damage (NEL = untreated m e a n + 1.96 × SD), and the predicted distance required to achieve this level was 161 m downwind.
DISCUSSION As expected, there was a rapid fallout of large spray droplets, with few droplets detectable using watersensitive papers extending past 33 m. However, a few large droplets, and smaller ones which were not detected using our simple monitoring system, caused damage up to an estimated distance of 161 m from the sprayed zone. Thus this bioassay technique gives a buffer zone of > 160 m to protect sensitive upland plants during aerial applications of asulam. This buffer zone requirement is much greater than for herbicide applications from ground sprayers. A buffer zone of 6 - 1 0 m was considered adequate to protect a range of established perennial species from four herbicides applied by tractor-mounted hydraulic sprayers (Marrs et aL, 1989), 20-30m was suggested for the protection of fish from glyphosate applied from the air (Payne et al., 1990), while > 200m was needed to protect three-spine sticklebacks Gasterosteus aeuleatus from applications of endosutfan applied from aircraft (Ernst et aL, 1991). There are several additional points to make about this study. Native fern species may be more or less sensitive than R u m e x acetosa. We chose this species because it is very sensitive to asulam drift, showing obvious symptoms of damage a few weeks after treatment, and because it can be propagated easily in large numbers. It would obviously be impractical to consider collecting rare fern species from nature reserves in the numbers required for bioassay studies of this type. However, a small number of commercially available ferns were included in this, and an earlier trial. The results were generally similar, but were more difficult to interpret, partly because the damage takes much longer to appear (sometimes over winter), and because herbicide damage can be confounded with damage from other sources (frost, temperature and water supply). It is of course possible that sub-lethal effects on species with extreme sensitivity to asulam extend beyond the distance detected here. Moreover, only leaf damage was assessed, no account being taken of subtle damage which may influence population performance, for example fecundity and survivorship in communities where the competitive balance between species has potentially been altered by asulam drift. To investigate this aspect, detailed population studies are required in sprayed and unsprayed situations. In this study only damage downwind of the sprayer was assessed. No account was taken of additional risks associated with the helicopter
Aerial applications of asulam turning at the end of a swath. When the helicopter reaches the end of a swath the operator switches off the spray, turns, and then switches on again at the start of the next swath. An error at this point could cause the drift to start outside the target zone, and where this might occur the buffer zone may have to be extended. Clearly, the results obtained apply only to the test situation. The experimental site was at the upper edge of the bracken patch on a sloping plateau site with the wind speed at 6 - 1 0 m - 1 during spraying. The results may be less severe where the bracken edge is on steep slopes, on flatter ground and at different wind speeds. The wind speed was higher than the limit recommended by M A F F / H S E (1989) for aerial spraying ( < 10 knots or < 5 m s- 1), but the spraying was done by a commercial operator as part of a large bracken control campaign within the North Peak ESA. T h u s , even if we consider our buffer zone estimate to be a 'worst case' scenario, it is one that may not be atypical in practice. If wind speeds are lower, then added protection will be achieved. It is also worth noting that the > 160 m zone is 90 m less than the 250m buffer currently used in the North Peak ESA scheme for the protection of sensitive sites. The main need for accurately determining adequate buffer zones is because of the statutory requirement under the FEPA for aerial spray operators to consult the Nature Conservancy Council when spraying within three-quarters of a nautical mile of a Site of Special Scientific Interest (FEPA, 1989). There is, therefore, an opportunity to require at this consultation stage that such sites are surrounded by a buffer zone. An alternative approach may be to spray bracken patches only to the leeward of sensitive sites. Before this suggestion is implemented, however, it is essential to determine the upwind drift effects caused by turbulence from the helicopter. ACKNOWLEDGEMENTS We would like to thank R. Starbuck, N. Taylor, F. Darwent for considerable help in planning this experiment in Derbyshire; East Midlands Region of the Nature Conservancy Council for the use of their
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Argocat; and Drs A. Cooke, B. N. K. Davis and T. M. Roberts for helpful criticism of the manuscript. This work was funded by the Nature Conservancy Council as part of their research programme in nature conservation.
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