Tracer techniques for the comparison of sprayer performance

CROP PROTECTION (1987) 6 (2), 123-129

Tracer techniques for the comparison of sprayer performance G. R.'CAYLEY, D. C. GRIFFITHS, P. J. HULME, R. J. LEWTHWAITEAND B. J. PYE

Rothamsted Experimental Station, Harpenden, Herts, A L 5 2JQ~ U K

ABSTRACT. The synthesis and use of two series of tracers is described. One, for field use, comprises a series of chlorinated esters and the other, for glasshouse use, is a series of substituted benzoic acids. Compounds belonging to the same series were identically formulated for spraying, and were extracted and separated from each other using identical orocedures. With a different tracer in each sprayer, all applications were made to the same plots, thus removing the error due to variations in plant number, height, leaf position and angle, in addition to errors due to sampling procedures. As opposed to a randomized block trial in which individual treatments are applied to separate plots, the tracer techniques described allow more information to be obtained from a set number of samples which, coupled with a different statistical treatment of the data, improves the precision of the comparisons.

Introduction Recently, much attention has been paid to novel (particularly electrostatic) methods of pesticide application, with the aim of increasing the amount of applied chemical reaching the target while at the same time decreasing application volumes. This has stimulated research in the evaluation of pesticide application and has led to attempts to define the criteria for optimum pesticide performance in terms of the magnitude and location of spray deposits. It is difficult to measure both the size and the distribution of deposits of active ingredient: quantitative measurements can be made by washing deposits from leaf surfaces and then using chromatographic techniques for analysis (Arnold et al., 1984a); a fluorescent dye can be added to the spray liquid and an optical method used to obtain a qualitative distribution of spray deposits (Herrington et al., 1985), or the dye may even be washed off and used to estimate the amounts and location of deposits of the active ingredient (Hislop, Cooke and Harman, 1983). The use of this latter technique is questionable because formulations of fluorescent dyes can affect the biological performance of the active ingredient (in field trials, 'Uvitex' formulations decreased aphid numbers, Arnold et al., 1984a; D. C. Griffiths, unpublished work) and with some sprayers, particularly electrostatic, changes in formulation alter the location of spray deposits by modifying the conductivity of the spray solution (Griffiths et al., 1984). To measure the distribution of behaviour-control-

ling chemicals such as pheromones, where application rates are low (a few grams per hectare) and the compounds can be detected at these low concentrations only by single-ion monitoring GC/MS (gas chromatography coupled mass spectrometry), the use of model tracer compounds that can be detected by less sophisticated analytical techniques is more appropriate. Thus, with both the aphid alarm pheromone (E)-/J-farnesene, and the mosquito oviposition pheromone, (-)-(5R,6S)-6-acetoxy-5-hexadecanolide, fluorinated derivatives can be prepared (Briggs et al., 1986) which have physicochemical properties similar to the parent compounds, are easily analysed by electron capture GC and still retain high biological activity, thus making them ideal tracers for determining the fate of the pheromone after application. When it is necessary to determine only the size of the initial spray deposit, a wider selection of tracer chemicals can be used, although the tracer must still satisfy several criteria. Tracers should be reliably and reproducibly extracted, avoiding interfering materials, and should then be capable of detection at low concentrations. For field use, to give adequate time for application and sampling, the tracers should be stable to sunlight and plant degradation and be of low volatility. Suitable compounds, therefore, are readily extracted with solvents such as hexane and are lipophilic with high molecular weights so that they are not adsorbed and translocated by plants (Briggs, Bromilow and Evans, 1982). Maximum sensitivity for analysis by GC with electron capture detectors is obtained with

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Tracer techniques for sprays

halogenated compounds. For work in the laboratory or glasshouse, smaller numbers of plants are treated and samples can be taken much more rapidly; thus the requirements for the tracers are much less stringent. Tracer chemicals are ideally suited for the comparison of sprayers and the measurement of spray drift. In such studies it is conventional to apply treatments to different plots in a replicated block design. However, chemical deposits achieved in this way vary, partly because of non-uniformity of the plots, especially if measurements are made on individual plants when variations in plant density, plant height or leaf angle may produce larger differences than those resulting from the the sprayers. Such variation is partly overcome by taking a large number of samples from many replicated plots but this is very time consuming and labour intensive. A more efficient procedure is to use a family of tracers which can be formulated identically and applied close in time to precisely the same targets. After application, samples are taken, extracted and analysed for all the tracers in one operation, thus further reducing errors due to sampling and sample preparation. Sharp (1974) used fluorescent compounds as tracers but it was recommended that sampling should take place in the weak morning or evening sun and within 15 minutes of application because the fluorescence fades in sunlight. This would limit sequential application in the field. Johnstone (1977) and Parkin et al. (1985) used coloured dyes to monitor the simultaneous application of two treatments, but sensitivity of detection of dyes is limited because it is rare to find compounds with molar extinction coefficients greater than 105. Absorbance envelopes of the dyes that have been used overlap and, although it is possible in theory to resolve the contribution due to a number of dyes, this necessitates measuring the absorbance at a number of well-separated wavelengths: thus, in practice a combination of three dyes would appear to be the most that could be separated without resorting to chromatography. An alternative would be to use neutron activation analysis of metal ions as described by Dobson, Minski and Matthews (1983), but this requires access to a reactor. In this paper we present results on the characterization and use of families of tracers with characteristics suitable for measuring spray deposits in the glasshouse and field, and their use in a simplified procedure of field experimentation, giving more accurate results than conventional methods. Materials and methods

alumina, products were checked for purity by gas chromatography with electron capture detection (see below). The even-numbered n-alkyl alcohols are cheaper and more readily available than the odd-numbered compounds. Initially, the trichloroacetates of the C~4, C~6, C~s and C22 as well as C17 alcohols were synthesized and then, to give a larger range of tracers, the dichloroacetates of the C~6, C~s and C22 alcohols. Estimates of volatility based on the work of Briggs (1981) and Dobbs and Grant (1980) suggested that esters based on tetradecanol were likely to be the most volatile that could be reliably used in field trials and this was therefore chosen as the starting point for chemical synthesis. Pyrethroid tracers Permethrin and cypermethrin, both commercially available ester pyrethroid insecticides, were included as tracers in some experiments. They are easily detected by GC and have retention times which complement those of the other esters. Benzoic acid tracers A series of commercially available benzoic acids were selected as tracers, because they are involatile and easily analysed by HPLC. Formulation of tracers Permethrin and ester tracers were dissolved in xylene containing 2% Ethylan BV (Lankro Chemicals) to make stock solutions of 25% w/v which were diluted with water for spraying. Cypermethrin was used as the experimental formulation Cymbush 10EC (ICI). 20% w/v solutions of benzoic acids in water were prepared by neutralizing the acids with a 3% excess of diethanolamine and the addition of 1°7o Ethylan BV. Solutions were further diluted with water for spraying. Volatility studies (esters only) The rates of loss of three esters (C~4TCA, C16DCA, C18DCA) from glass were determined. Samples of individual esters (1/ag) dissolved in hexane (5 gl) were applied to 13mm diameter microscope cover slips (equivalent to 75 g a.i./ha) and placed in shallow trays in a 20°C constant temperature room with air moving at 0.5m/s. At intervals the esters were washed from four replicate cover slips with hexane (1 ml) and the amounts remaining determined by GC.

Ester tracers Dichloroacetate (DCA) and trichloroacetate (TCA) esters of a range of n-alkyl alcohols were synthesized from the appropriate alcohol and the haloacetyl chloride using the general procedure described by Elliott et al. (1980). After column chromatography on CROP PROTECTION Vol. 6 April 1987

Recovery tests and analysis Esters and pyrethroids. Formulated material was diluted with ethanol to enable esters (25/ag in 0.5 ml) to be applied to four replicate samples of 25 barley leaves, mean leaf area 330cm 2, giving a deposit of

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G. R. CAYLEYet al. 76ng/cm 2 (equivalent to 36/ag/g dry weight of plant material). After 2 hours, samples were extracted with 50 ml of 5% acetone in hexane by shaking samples on an orbital shaker for 30 min. The solvent was decanted and dried over anhydrous sodium sulphate and then analysed by electron capture GC. For individual esters, a 60 cm glass column packed with 2% OV-101 on Chromosorb W was operated isothermally, at oven temperatures between 180°C and 220°C depending on the ester, in a Hewlett Packard 5880A GC with 63 Ni electron capture detector at 300°C and the injection port 20°C above the oven temperature. For mixtures, the same packed column was used with an initial oven temperature of 180 ° C for 2min followed by temperature programming at 3°C per minute to 240°C. Alternatively, a 10 m wide-bore capillary (i.d. 0-53mm), coated with methyl silicone (Hewlett Packard), was temperature programmed from 190°C to 250°C at 5°C per minute after 1 min at the initial temperature using a carrier gas (nitrogen) flow of between 6 and 15 ml/min.

manifold system and a series of ducts to produce a curtain of air moving at a velocity of 15 or 30m/sec (Pye and Cayley, 1986). Tracer chemicals were applied at 100 g/ha with the 'Jumbo' sprayer in 101 water and for comparison cypermethrin was applied at 100g a.i./ha using a hydraulic sprayer fitted with Lurmark F110-200 jets delivering 200 l/ha. All treatments were applied under steady wind conditions (< 1 m/s) to the same four replicate plots of spring barley, at growth stage 37 (Tottman and Makepeace, 1979) starting with the hydraulic treatment. After all treatments, and within 90 min of the start of spraying, four replicate samples of each of the upper three fully expanded leaves from 25 plants were taken from each plot for chemical analysis. Results

Although volatilization of C14-C22 esters would be expected to be zero order (Hartley, 1969) as reported elsewhere (Dobbs and Grant, 1980; Dobbs, Hart and

Ten micrograms of formulated acids were applied in 200/ag water to broad bean or barley leaves, mean leaf area 110 and 180cm 2 respectively. Samples were placed in a screw-top jar and, after 5 min, extracted by shaking for l m i n with 10ml of a 50/50 mixture of acetone/50mM chloroacetic acid. The solvent was decanted and analysed by H P L C using a 15 cm 3~m Spherisorb ODS-2 column protected by a 5cm 5/am Spherisorb ODS-2 guard column. The mobile phase, 65% methanol, 35°7o 50 mM chloroacetic acid, was pumped at 1 ml/min and the eluant monitored at 240nm with a variable wavelength ultraviolet detector. Retention times of the four acids used (ptoluic acid, p-chlorobenzoic acid, m-iodobenzoic acid and 3,4-bichlorobenzoic acid) were 4.78, 5.99, 7.39 and 10.87 min respectively. Benzoic acids.

Glasshouse trial

Broad beans were sown in boxes 3cm apart in rows with 7cm between rows. When the beans were 15cm tall they were sprayed with an 'Ape 80' sprayer (Arnold and Pye, 1980) mounted on a spray track. Four consecutive applications were made with each of the tracers at a fate of 500 g a.i./ha in 7.8 1of spray with the sprayer operating alternately in the uncharged or charged mode. Four replicate samples of seven pairs of leaves were taken from the upper and lower parts of the plants, extracted and analysed as above. Field trial

The ester tracers were used to assess the effect of air assistance on the deposition, in a spring barley crop, of charged drops produced by the 'Jumbo' electrostatically charged rotary atomizer (Arnold, 1983). Air assistance was provided using a Briggs and Stratton 5hp engine driving a centrifugal fan linked to a

0

I

I

I

I

2

4

6

8

FIGURE ]. Separation of 2ng amounts of (|) C]6DCA, (2) CI7TCA, (3) permethfin and (4) cypermethrin on the packed GC column. See text for conditions. CROP P R O T E C T I O N Vol. 6 Apri! 1987

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Tracer techniquesfor sprays and cypermethrin, were therefore used to extend the range o f tracers to longer retention times. Recoveries of close to 90% were found for all the ester tracers applied to barley leaves at a rate of 76ng/cm 2 (Table 1) and using the wide-bore capillary column the sensitivity o f detection was less than

4

6

0

3

6

9

12

Time (min)

FIGURE2. Separationof 2rig amountsof (1) CI4TCA, (2) CI6DCA, (3) C16TCA, (4) C17TCA, (5) CIaTCA, (6) permethrin and (7) cypermethrin on the wide-borecapillaryGC column.See text for conditions. 4

Parsons (1984), the rates o f loss fit m u c h closer to a firstorder process. T h e half-lives for C~4, T C A , Cl6 D C A and Cla D C A were 16, 72 and 374 hours respectively). The packed G C columns were found to be adequate to separate the trichloroacetates o f the even-numbered alcohols but, to obtain complete separation of even and odd trichloroacetates, the additional resolution o f the wide-bore capillary column was necessary; typical chromatograms are shown in Figures 1 and 2. T h e dichloroacetates chromatograph similarly to the trichloroacetates with one fewer methylene group and so can provide a cheaper method o f producing a wide range o f tracers. For longer retention times the C22 alcohol was used but this always had small impurities of shorter-chain alcohols which could confuse the results: two commercially available pyrethroids, permethrin CROP PROTECTION Vol. 6 April 1987

0

I

I

I

I

3

6

9

12

Time (rain) FXGURE3. Separation of (1) 4-methyl-, (2) 4-chloro-, (3) 3-iodo- and (4) 3,4-dichlorobenzoic acids on a 3/~m spherisorb ODS-2 column. See text for conditions.

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G. R. CAYLEYet al. TABLE 1. Recovery of ester tracers from barley leaves; application rate 76 ng/cm2 Tracer Cl4 T C A Cl6 DCA C16 T C A C~7 TCA C18 T C A Permethrin

Recovery (%)

SE

92" 8 90.0 87- 2 90" 1 86" 4 86" 1

1.26 6-70 2.25 1.11 3- 61 2.46

TABLE 4. Effect of air assistance on the deposition of electrostatically charged drops in spring barley (GS 37) Deposit on leaves (tag/g) Sprayer

Upper

Middle

Lower

Hydraulic 'Jumbo' 'Jumbo' + small air 'Jumbo' + large air

26" 5 119.8 94" 0 99.9

31.7 49 '9 45.4 54.0

34.7 17.9 22" 6 27.1

SED

TABLE 2. Recovery of substituted benzoic acid tracers from bean and barley plants Beans

Tracer 4-Methylbenzoic acid 4-Chlorobenzoic acid 3-Iodobenzoic acid 3,4-Dichlorobenzoic acid

Barley

Recovery (%)

SE

Recovery (%)

SE

92.2 91.0 92.4 95.3

1.05 1 • 20 1 •63 1.76

88- 7 87.2 89.8 92" 3

1" 13 1- 23 1.41 1" 51

5 ng/cm 2. Addition of more acetone to the extracting solvent did improve the extraction efficiency but also extracted plant material which interfered with the analysis of low concentrations of C14 TCA. However, at higher application rates or with other plant species, 10% acetone in hexane may be a more suitable solvent. The benzoic acid tracers were well separated on the 3/~m Spherisorb ODS-2 column, as shown in Figure 3. Recoveries from beans and barley were close to 90% at an application rate of 55ng/cm 2 and there was no difference between tracers (Table 2). The sensitivity of detection is less than 5 ng/cm 2 for all of the tracers. In the glasshouse trial the charged sprayer deposited 2.4 times more chemical on the upper parts of the beans than the uncharged sprayer but on the lower leaves larger deposits were achieved with the uncharged sprayer (Table 3). On the upper leaves, in particular, the repeat sprays gave similar mean deposits, showing that the deposition of low-volume sprays was not affected by spray deposits already on the leaf surface. However, this is not necessarily true of larger-volume sprays, where run-off may occur. In the field trial, three ester tracers were used in the electrostatic sprayers and cypermethrin in the hydraulic sprayer. Analysis of the tracers showed that

4.27

2- 32

2.19

all electrostatic treatments increased the spray deposits on the upper and middle leaves of the plants compared with the hydraulic sprayer (Table 4). On the lowest leaves the largest deposits were achieved with the hydraulic sprayer. Air assistance applied to the 'Jumbo' electrostatic sprayer significantly changed the distribution of spray deposits. Air at both 15 and 30 m/s decreased the deposits on the upper leaves and increased deposits on the lower leaves. Discussion

For a tracer technique to give reliable information on the performance of sprayers it is essential that the tracers are stable, readily and efficiently extracted, and analysed at realistic levels. The required sensitivity of detection of tracers in a cereal crop can be estimated. Assuming an application rate of 100 g a.i./ha (typical for insecticides and fungicides in cereals) and a maximum leaf area index of spring barley of about 5 (Gallagher, 1979) or winter wheat of approximately 10 (Prew et al., 1983), if spray deposits are uniform throughout the crop canopy then a theoretical deposit of about 200ng/cm 2 would be obtained for spring barley or 100ng/cm 2 for winter wheat. Thus the tracers and extraction procedures described in this paper giving 90°70 recoveries at 76 ng/cm 2 and limits of detection of less than 5 ng/cm 2 should be adequate for most purposes and, where greater sensitivity is required, higher application rates could be used. The tracer studies showed that applying electrostatic charge to a rotary atomizer markedly changed the distribution of spray deposits on bean plants, increasing deposits on the upper leaves of the plants while decreasing them on the lower ones, confirming

TABLE 3. Effect of repeat spraying on deposits on broad beans Deposits (tag/cm2) Leaves

Spray sequence

Sprayer

R1 *

R2

R3

R4

Mean (SED)t

Upper

Uncharged Charged Uncharged Charged

2.13 7.31 1.88 6.91

1.27 5.01 1.25 4.65

2.71 6.39 2-41 6-19

1.27 6-00 1.77 5-62

1"85 6.18 1.83 5.84

Lower

Uncharged Charged Uncharged Charged

1.74 1.09 2.00 1.04

1.03 0.64 1-18 0.61

2.29

1.24 0.99 1.49 0.87

1'57 1-06 1.77 0.99

1.55 2.43 1.43

*R = replicate no.; "I'SED= standard error of mean difference. CROP PROTECTION

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results obtained in field trials (Arnold a al., 1984a). In previous trials (Arnold et al., 1984b) we have shown that electrostatic charging can give increased deposits of fungicide in spring barley but that the deposit distribution is not adequate to give improved disease control. In this trial we have shown that the addition of air assistance to an electrostatic sprayer can increase deposits on the lower leaves, making them similar to those obtained with the hydraulic sprayer, while still giving increased deposits on the upper leaves and offering the promise of improving mildew control. Subsequently we have shown that the air-assisted sprayer does improve the performance of electrostatically applied fungicides in a spring barley crop (Cayley et al., 1985). Despite the considerable effort devoted to spray application techniques in recent years, pesticide application techniques are still very inefficient. Nevertheless, in principle, doses could be substantially reduced if pesticides could be more effectively transported to the target site (see for example GrahamBryce, 1977). The optimum pesticide distribution for maximum biological effect is rarely known and it is not uncommon to obtain the same biological performance from spray deposits differing widely in distribution and amount, although in some instances (control of pea moth; control of aphids on cereal ears) there is a good correlation between spray deposits on specific plant parts and the biological effect (Cayley et al., 1985). During the development of spraying systems, and to obtain maximum benefit from pesticide application, it is essential to determine the size and distribution of spray deposits. Experiments on sprayer performance are usually carried out on a randomized block design which, although best for determining biological performance of sprays applied in different ways, is not ideal for the comparison of deposit distributions. Comparisons of several sprayers require large numbers of plots and hence a large site. This introduces errors due to plotto-plot variation caused by different plant densities and crop heights. There are also individual plant variations in growth stage and leaf angle. Errors arising from these variables can be reduced by taking large numbers of samples but this then requires the use of many staff and increases not only the possibility of observer error but also the likelihood of errors in sample preparation and analysis. Increasing the size and number of plots to increase precision also increases the application time, thus giving more opportunity for changes in meteorological conditions during the course of the trial. The effect of all these variables can be reduced by applying all treatments to the same plots, using a different tracer in each sprayer. Although not done in this study, the technique could also allow the sprayer boom to be modified to enable at least two simultaneous applications. Sequential use of tracers on the same targets has the further advantage of making the whole trial procedure much more rapid. As each sample provides information on all sprayers, fewer CROP P R O T E C T I O N Vol. 6 April 1987

TABLE5. Influence of statistical treatment on precision Analysis Unstructured 16 samples Randomized blocks 4 x 4 samples All treatments to each sample

Mean square for error 237 "4 209.5 176.9

samples need to be taken to provide the same amount of data as from single treatments applied to individual plots, and time on sample preparation and analysis is reduced. Where necessary, this could enable more samples to be taken to improve the precision of measurements. A further benefit of this approach is that the statistical treatment of the data can be improved by treating every plant, or sample, as a 'block' which contains all the treatments (Table 5). In this way the residual mean square for error is decreased by 18°7o, compared with a trial set out as four blocks of four plots, or by 34% when compared with 16 randomized plots. This calculated improvement is achieved purely by the improved statistical treatment. In practice, our suggested procedure would improve the precision by a much greater amount, because all the treatment comparisons are obtained from the identical plants. The techniques employed in this paper are also ideal for determining the effects of spray formulation and sprayer-operating conditions on chemical deposition in crops, and the influence of spray characteristics and meteorological conditions on the drift of spray drops.

Acknowledgements We thank A. Murray for help and advice on statistical analysis.

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