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Phytochemical responses to herbicide exposure and effects on herbivorous insects

9. Drift of herbicides, and the importance of droplet size - Field and laboratory studies

Drift of herbicides may potentially cause detrimental effects on non-target organisms, including indirect effects on insects, birds and small mammals through effects on their food sources. Deposition caused by drift is a function of the distance to the spraying device (e.g. Davies and Gilbert, 1985; Nordby and Skuterud, 1975). However, effects of drifting pesticides may also be a function on the droplet size distribution of the spray, since small droplets may be taken up more easily and may be more concentrated due to higher evaporation of water/solvent than of the active ingredient (Nordby and Skuterud, 1975). Furthermore, droplet size distribution may change with distance from spraying device.

The aim of the present experiments was to study effects of herbicide drift under realistic conditions in the field in relation to measured exposure. This was supplemented with laboratory studies of the impact of droplet size distribution on effects of equal herbicide dosages.

9.1 Materials and methods

F. convolvulus plants were grown in the greenhouse for 4 weeks at min. 20° C, 16 h light and 70 % RH, achieving an average initial dry weight of 0.45 ± 0.11 g (5-7 leaves). The plants were sprayed with a mixture of chlorsulfuron and glycine. Spraying was performed with a pot sprayer, using three different nozzles with different droplet size distribution (small, medium, large average droplets). Chlorsulfuron concentrations were 0, 0.03125, 0.0625, 0.125, 0.25 and 0.5 times the recommended field rate (4 g ha^-1), and glycine concentration in the same relative propotions (37.5 g/l at 0.5 times the recommended field rate). There were 4 plants per nozzle size and concentrations, except for 0.125 times the field rate with the medium nozzle (12 plants).

After spraying, the plants were allowed to grow for 7 days. Then they were harvested, and dry mass was determined for above ground plant parts.

The deposition on glass plates (90 cm²) was measured. All measures of spray deposition were done by spectrometric determination of the glycine present in the spraying mixture . A colour reagent consisting of 95 g KH₂PO₄, 43 g NA₂HPO₄, 5 g ninhydrin and 3 g fructose dissolved in 1 l water was prepared, together with a KIO₃ solution made of 4 g KIO₃, 1.2 l water and 0.8 l 96 % ethanol. For calibration purposes, glycine standards of 2.0, 4.0, 8.0 and 16 mg/l were prepared. For analysis, 3 ml sample achieved by washing the glass plates with water was mixed with 2 ml colour reagent and boiled for 10 minutes in a water bath. Thereafter, the samples were cooled in another bath, 5 ml KIO₃ solution was added, and the samples were mixed. After 10 minutes, the samples were measured in a spectrophotometer at 570 nm.

F. convolvulus plants grown in the greenhouse (see above) were placed in a barley field (3-5 leaves, growth stage 13-15 , which was sprayed with the herbicide Glean 20 DF in recommended field rate (4 g chlorsulfuron ha^-1). Glycine (75 g l^-1) was added to the spraying mixture (200 l ha^-1) for measurements of deposition. The field was sprayed on May 18th 1999, at eastern-south-eastern wind (i.e. at right angle to the tractor track) of approximately 2.5 m s^-1. The plants were placed at different distance from the spraying track (0, 1, 1.5, 2.25, 3.38, 5.06, 7.59, 11.39 and 17.09 m from the tip of the sprayer) in 8 columns in the field. Control plants were placed approximately 40 m upwind from the spraying track. After spraying, some plants (1-3 per distance and column) were transferred to the greenhouse to allow effects of the spraying to occur, while other plants (1 per distance and column) were analysed for deposition of the herbicide by measuring the deposition of the glycine mixed into the spraying solution. Beside the plants, also glass plates and hair curlers were placed on racks in the field at different heights (10, 30, 60 and 90 cm above the ground, curlers only) and distances (0, 1, 1.5, 2.25, 3.38, 5.06, 7.59, 11.39, 17.09 and 40 m from the tip of the sprayer) to get estimates of herbicide deposition.

In the laboratory, plants, glass plates and curlers were washed with water, and the deposited glycine measured by spectrometry, as described above. Plants for estimates of herbicide effects were harvested after 7 days at min. 20° C, 16 h light and 70 % RH. Above-ground biomass was dried at 80 ° C, and dry weight was determined.

Effects of nozzle size on growth and of vertical and horizontal position on deposition were analysed by analysis of variance (ANOVA). Means were compared by Tukey t-test. Results were evaluated at the 5 % significance level.

9.2 Results

Choice of spraying nozzle had no impact on the effect of chlorsulfuron on the growth of F. convolvulus (Figure 9.1). Growth was depressed at chlorsulfuron dosages of 0.0625 times the recommended field rate and higher (Figure 9.1).

Measures of deposition on glass plates corresponded to 2.3, 1.9 and 1.7 g a.i. ha^-1 for the small, medium and large nozzle size, respectively, at 0.5 times the field rate (nominally 2 g a.i.ha^-1). Comparison is difficult, since no replication was performed. Consequently, no experimental background for the field studies was obtained from the greenhouse study concerning actual exposure.

Figure 9.1
Impact of nozzle size on chlorsulfuron effects on F. convolvulus grown under laboratory conditions.

Results from four rows were excluded due to failure of the spraying procedure (water in device, i.e. no deposition of herbicide). Correlations between the different measures of chlorsulfuron deposition (curlers, glass plates, plants) were high (correlation coefficients of 0.94-0.98). In the following, deposition on glass plates is used, since that deposition measure is the only one expressed on an area basis (g ha^-1).

Deposition decreased with increasing distance to the sprayer (Figure 9.2), but was almost unaffected by their vertical position (data not shown). The chlorsulfuron deposition on glass plates in the drift zone (1 m or more from the spraying track) was less than 5% of the nominal recommended field rate, compared to 25-40% under the spray nozzles. Actual deposition in the drift zone was thus £ 7% of the actual deposition under the sprayer (Figure 9.2), i.e. at full field rate. Effects on plant growth were small or absent: only plants placed right beneath the sprayer were significantly smaller than the control plants (and all other plants), whereas plants sprayed at 3.38 m distance to sprayer, corresponding to an estimated exposure of 1.1% of the deposition at recommended field rate (i.e. right under the nozzles), were significantly larger than the control plants (Figure 9.3).

Figure 9.2
Deposition of chlorsulfuron on glass plates, expressed as fraction of the average deposition at recommended field rate.

Figure 9.3
Growth of F. convolvulus as function of chlorsulfuron exposure, expressed as fraction of average actual exposure at recommended field rate.

9.3 Discussion

Effects on the growth of F. convolvulus occurred at comparable chlorsulfuron dosages in laboratory and field studies (at 0.0625 times the recommended field rate and higher). For plants exposed in the field, an indication of a hormetic effect was seen at a distance of 3.38 m from the sprayer, corresponding to c. 1.1% of the actual exposure at the recommended field rate. Kjær (1994) found a similar effect at 0.1 times the field rate in a laboratory experiment. No such effect was seen in the present laboratory experiment, possibly due to the choice of test dosages. For plants exposed in the field, negative effects on growth only occurred on plants placed directly under the sprayer. The drift zone where effects occurred was thus less than 5 m wide. However, the wind speed was low in the field (c. 2.5 m s^-1), and the drift zone may be wider under more windy conditions.

The measured deposition on e.g. glass plates may not exactly mimic the exposure of plants, but the relationship between distance and deposition is probably well described, due to the high correlation between the various deposition measures (see above).

The lack of effect of droplet size on toxicity in the laboratory study does not necessarily imply that no droplet size effect exists in the field. In the laboratory, there was hardly any evaporation, whereas in the field evaporation of water may increase the concentration of chlorsulfuron, especially within the smallest droplets, leading to higher concentrations in small droplets than in large ones.

The conclusions drawn from the present study are:

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