Effectiveness of the standard evaluation method for hydraulic nozzles employed in
stored grain protection trials

Eficiencia de la evaluación estándar de boquillas hidráulicas utilizadas en experimentos de protección de granos almacenados

JAVIER A. VÁSQUEZ-CASTRO¹, GILBERTO C. DE BAPTISTA², CASIMIRO D. GADANHA Jr.³ and LUIZ R. P. TREVIZAN⁴

¹ Prof. Dr. Departamento de Entomología, Universidad Nacional Agraria La Molina, Av. La Universidad s/n, apartado 456, Lima100, Peru. jaque@lamolina.edu.pe.

² Prof. Dr. Departamento de Entomologia, Fitopatologia e Zoologia Agrícola, Escola Superior de Agricultura “Luiz de Queiroz”, Universidade de São Paulo, SP 13418-900, Brazil. gcbaptis@esalq.usp.br.

3 Prof. Dr. Departamento de Engenharia Rural, Escola Superior de Agricultura “Luiz de Queiroz”, Universidade de São Paulo, SP 13418-900, Brazil. cdgadanh@esalq.usp.br.

⁴ Postdoctorando. Escola Superior de Agricultura “Luiz de Queiroz”, Universidade de São Paulo, SP 13418-900, Brazil. lrptrevi@esalq.usp.br.

Key words: Application technology. Volumetric distribution. Insecticide deposition. Chromatography.

Resumen: En los granos almacenados, depósitos inferiores y gran variación en relación a las dosis teóricas de insecticidas aplicadas son frecuentemente encontrados. El objetivo del presente trabajo fue estudiar la eficiencia del método estándar (ISO 5682/1-1996) utilizado para la evaluación de boquillas de pulverización usadas en experimentos de protección de granos de maíz y trigo. La distribución volumétrica transversal y el espectro de gotas del modelo TJ-60 8002EVS se determinaron con la finalidad de calibrar un sistema de pulverización para aplicar un volumen de aplicación de 5 L/t y obtener una concentración teórica de 10 y 0,5 mg/kg de fenitrotion y esfenvalerato, respectivamente. Después del tratamiento, los granos de maíz y trigo fueron procesados y los depósitos de los insecticidas analizados mediante técnica de cromatografía gaseosa. El tipo de grano no tuvo influencia en el depósito de los insecticidas y dependió apenas del insecticida. Los depósitos en los granos solamente alcanzaron valores de 42,1 y 38,2% de la concentración teórica pretendida de fenitrotion y esfenvalerato, respectivamente. Estos resultados demuestran la ineficiencia del método estándar para la evaluación de boquillas hidráulicas empleadas en experimentos de protección de granos almacenados.

Palabras clave: Tecnología de aplicación. Distribución volumétrica. Depósito de insecticida. Cromatografía.

Introduction

In a storage facility, grains are usually treated on a conveyor belt, where hydraulic nozzles are mounted for this purpose. Under these conditions, smaller depositions and great variations with regard to theoretical insecticide doses are frequently found (Vardell et al. 1973; Rowlands 1975; Desmarchelier et al. 1987; Redlinger et al. 1988; White and Sinha 1990; Acda et al. 1994). In order to improve the quality of sprays generated by hydraulic nozzles, a number of systems have been developed under laboratory conditions, but with little success when it comes to solving the above-mentioned problems. Great variation in insecticide deposition on the mass of grains may favor the occurrence of two important biological phenomena. The first is associated with subtoxic amounts of the insecticide, which may stimulate population growth of the pest (hormoligosis) (Kuenen 1958; Luckey 1968; Morse 1998), while the second is associated with high amounts of the chemical compound (resistance), which may favor the survival of highly resistant individuals whose biological performance in the absence of the insecticide may be just identical to the susceptible strain, resulting in practical complications in the management of this phenomenon (Oliveira et al. 2005). In Brazil, failure in the chemical control of Sitophilus oryzae (L., 1763), Sitophilus zeamais Motsch., 1855 (Coleoptera: Curculionidae), Rhyzopertha dominica (Fabr., 1792) (Coleoptera: Bostrichidae), Tribolium castaneum (Herbst, 1797) (Coleoptera: Tenebrionidae), Cryptolestes ferrugineus (Stephens, 1831) (Coleoptera: Cucujidae), and Oryzaephilus surinamensis (L., 1758) (Coleoptera: Silvanidae) has been reported (Lorini and Beckel 2002). It is possible that an inadequate technology of application may have favored a loss of effectiveness of insecticides used for stored grain protection.

In a spraying system, the nozzle is the most important component, since it is responsible for the flow, generation, and distribution of droplets that will carry the insecticide to the target to be controlled. Knowing the transversal volumetric distribution of the nozzle is highly important in a spray analysis and has been the object of study of several researchers (Roth et al. 1985; Debouche et al. 2000; Cunha and Teixeira 2001; Womac and Bui 2002). In the evaluation of volumetric distribution of spray nozzles, the International Organization for Standardization has defined clean water as the standard spraying liquid (ISO 5682/1-1996). On the other hand; the physical characteristics of the sprayed liquid can affect the volumetric distribution pattern of the nozzle (Krueger and Reichard 1985), thus compromising field experiments. The objective of this work was to study the effectiveness of the evaluation method employed for hydraulic nozzles used in experiments on the protection of stored grains, as described in the ISO 5682/1-1996 (E) standard.

Material and Methods

Application technology. A twin jet model TJ-60 8002EVS hydraulic nozzle (Spraying Systems Co.) was used for the study. A channeled table (patternator) was used to carry out the spray nozzle transversal volumetric distribution analysis experiments, standardized according to standard ISO 5682/1-1996 (E). The testing table (3.5 m long, 3.0 m wide) has channels spaced at 0.025 m, positioned at a 5% slope. In front of the table, a set of graduated cylinders (250 mL) collects the liquid from each channel. Clean water was used to evaluate the nozzle. The following parameters were evaluated: actual flow and transversal volumetric distribution, at a pressure of 200 kPa and a nozzle height of 0.5 m. The weighting method was used to obtain actual flow, and the volume collected during one minute in a plastic container was weighted in a precision balance. In order to determine transversal volumetric distribution and effective swath width, the nozzle was mounted on the boom and positioned at a 90º angle in relation to the assay table. The collection time was set until one of the graduated cylinders reached a volume of 230 mL. This collection time was used for the three replications.

After the effective swath width was determined, we studied the droplets spectrum. A mobile application system was built containing the nozzle, a manometer, a CO2 tank, and a tank for the liquid to be applied. Three water-sensitive papers (0.076 m long, 0.026 m wide) were distributed on the extreme and central portions of the previously-defined effective swath width. The same height and working pressure adopted for the assay table were used, at a moving speed of 5 km h-¹. After spraying, the water-sensitive papers were collected and analyzed using a computerized image analysis system (Chaim et al. 2002), Gotas, version 1.0 (Embrapa Meio Ambiente, São Paulo, Brazil).

Under these conditions, the insecticidal emulsion contained 0.4% of the commercial product. Three replications were made, generating six experimental plots, and two insecticides were analyzed, totaling twelve subplots. The same procedure was adopted for the control treatment, but in this case the spray consisted of water only. The temperature and relative humidity during spray were 26ºC and 71%, respectively. The physical properties of the mix were determined after spraying. Surface tension was determined by the burette method, according to the NBR 13241 standard for surface tension determination in agrochemicals (Associação Brasileira de Normas Técnicas 1994). Viscosity was determined with a Brookfield, model LVDV-III Ultra viscometer at 26ºC.

Deposition Analysis

Grain. Half an hour after the spray, grains was collected and processed with dry ice. To achieve this, a model TRF70 forage chopper was used. The dry ice was mixed with the grain at a 1:1 ratio prior to grinding, in order to maintain a temperature value that would minimize insecticide degradation during the operation. The analytical method was adapted from Ohlin Ohlin (1998). Homogenized samples (10 g) were placed in Schott bottles (100 mL) for residue extraction. Ethyl acetate (50 mL) and sodium sulfate (10 g) were added and later homogenized in a stirring table for 1 hour at 360 cycles min-1. After this operation, the extracts were centrifuged for 5 min at 2,600 rpm for better separation of the liquid phase from suspension materials. Aliquots of the supernatant (10 mL) were transferred to 12-mL test tubes, corresponding to 2 g of the original sample, and were then added of dodecane (50 µL). The extracts were evaporated in a Turbo-Vap evaporator, in a water bath (30°C) aided by moving air previously dried through a blue silica gel desiccant filter. The insecticide residues were then resuspended in a cyclohexane+ethyl acetate mixture (1+1 by volume) (5 mL), homogenized in a vortex mixer/ultrasound and filtered through a Millipore, FG, 0.2 µm pore membrane filter mounted on a plastic hypodermic syringe (5 mL).

The extracts were cleaned by gel permeation chromatography (GPC) and eluted with a cyclohexane+ethyl acetate mixture (1+1 by volume). After this operation, the extracts were evaporated in a Turbo-Vap evaporator previously added of dodecane (50 µL) and were later resuspended in the cyclohexane+ethyl acetate mixture (1+1 by volume) (20.0 and 1.95 mL) for the fenitrothion and esfenvalerate residues, respectively.

The samples were analyzed by gas chromatography, with a Thermo Electron Corporation, model Finnigan Trace gas chromatograph, equipped with an electron capture detector (ECD, Ni⁶³) and a Restek Corp. RTX-5MS chromatography capillary column (30 m-long, 0.25 mm diameter, and 0.25 µm film thickness), with injections made in the splitless mode. The chromatograph was operated under the following conditions: column temperature = 100oC (initial); then at 280oC (25oC/ min ramp), remaining at this temperature for 10 min; injector temperature = 230oC; detector temperature = 320oC; purge time = 1 min; gas flow (mL/min): H₂ (skidding) = 1.2; N₂ (make up) = 45; and purge flow = 65. Under these conditions, retention time was 6 min and 20 sec for fenitrothion and 10 min and 25 sec for esfenvalerate, approximately. Residue amounts were calculated using the ChromQuest version 4.0 software, by comparing the chromatographic peak heights for the samples against the chromatographic peak heights for the corresponding analytical standards. The analytical method used for corn and wheat grains was validated by means of matrix fortification at the levels of 0.05; 0.5; and 10.0 mg/kg for fenitrothion, and 0.05; 0.1; and 1.0 mg/kg for esfenvalerate, with three replications for each level (nine fortified samples for each matrix). Recoveries between 70-120% were considered acceptable.

Glass slide. Three glass slides were placed into flasks (600 mL). Ethyl acetate (500 mL) was added and the insecticides were later extracted by ultrasound for 15 min. Aliquots (2 mL) were transferred to test tubes (12 mL) and were then added of dodecane (50 µL). The extracts were evaporated in a Turbo- Vap evaporator, in a water bath (30°C) aided by moving air previously dried through a blue silica gel desiccant filter. Later, the insecticide residues were resuspended with the cyclohexane+ ethyl acetate mixture (1+1 by volume) (2 mL) and homogenized in a vortex mixer/ultrasound, and then diluted at an extract (1 mL) + cyclohexane+ethyl acetate mixture rate of 1+1 by volume (9 mL). The insecticide depositions on the glass slides were analyzed by gas chromatography, in the same manner as for depositions on the grains. Statistical analysis. The data were submitted to analysis of variance, using a mathematical model for a completely randomized design in a split-plot arrangement, and the F test was used to evaluate the significance of factors grain type, insecticide, and interactions in the models (Steel and Torrie 1960, Pimentel-Gomes 1987).

Results and Discussion

Application technology. The actual flow of the nozzle with water was 0.66 L/min, 1.5% higher than the nominal flow of 0.65 L/min, according to the information provided by the manufacturer. The variation between actual and nominal flow was within the acceptable limit, since according to the World Health Organization (WHO 1976), the acceptable flow variation limit of a spraying nozzle is ± 4% in relation to the nominal flow indicated by the manufacturer. At our working conditions, a total swath width of 0.875 m was obtained, with a coefficient of variation (c.v.) of 41% (Fig. 1). The c.v. was higher than the 7% limit established by the international standard (European Committee for Standardization 1997).

In order to obtain an insecticidal mix distribution as uniform as possible, and considering that in Brazil a c.v. of up to 10% is acceptable, we calculated effective swath width and c.v. values of 0.425 m and 9%, respectively. Under these conditions, 65% of the sprayed water volume was collected within the effective swath width (Fig. 1). Therefore, the spraying equipment was calibrated to apply a total volume of 7.6 L/t, since 35% of this volume would remain outside the treatment area. Consequently, the grains would receive an effective application volume of 5 L/t as intended. The nozzle flow in the spraying equipment at a pressure of 200 kPa was 0.66 L min-1; this value was very similar to the flow obtained with water in the laboratory test. In this situation, the cart moving speeds were 2.4 and 3.1 km/h for the corn and wheat sprays, respectively. The droplets spectrum for the nozzle under evaluation, working at pressure, height, and moving speed values of 200 kPa, 0.5 m, and 5 km/h, respectively, is presented in Table 1.

Because of the difficulty in determining the droplets spectrum, researchers and nozzle manufacturers frequently refer to the mean volumetric diameter to characterize sprays (Bouse 1994). According to the brochure of the manufacturer, the TJ-60 8002EVS nozzle produces fine droplets at all recommended working pressures; however, in the present study we obtained droplets of sizes between medium and large. The droplet size categories used in this experiment were the same as in the international ASAE (X-572) and BCPC standards. The differences in droplet diameter and consequently in droplet size category were possibly caused by the measurement technique used, since the international standards specify a laser system to evaluate the droplets spectrum. In this work, we used water-sensitive paper to obtain droplet stains to make diameter measurements at a later time using specific software. Several methods can be used to measure droplets spectrum. In this respect, computing methods have been used quite often (Wolf et al. 2000; Wolf 2003, 2005). On the other hand, all computing methods used for droplet measurement take into consideration a spreading factor; in the case of the software used in our study, it was validated (Chaim et al. 2002). At the center of the effective swath width, the droplets were smaller when compared with the droplets at the extreme points of the swath. During the droplet formation process, the hydraulic energy of the liquid is transformed into droplet kinetic energy (Amberg and Buttler 1969). One explanation for the results is that larger droplets have greater mass and therefore acquire higher kinetic energy. Consequently, large droplets have a greater capacity to overcome air resistance to horizontal movement, and may travel longer distances when compared with smaller droplets. In the same way, the volume and coverage values at the center of the effective swath width were lower than at the edge. This was probably due to the vortex effect generated by the cart moving at a speed of 5 km h-1; very small droplets would then be dispersed outside the treatment area by the wind turbulence.

Deposition analysis. The insecticide recovery percentages in the fortified corn and wheat grains were acceptable (70-120%), thus validating the analytical method. None of the two insecticides were recovered from the control indicating that the grains were free from contamination by those compounds. The analysis of variance only detected a significant effect (P ≤ 0.05) for insecticide (Table 2). This result demonstrates than Effectiveness of the standard evaluation method for hydraulic neither grain type nor the interaction between grain type and insecticide has an influence on pesticide deposition Consequently, deposition only depended on insecticide. The fenitrothion deposits were significantly higher than expected than those for esfenvalerate, both on grains and on glass slides (Fig. 3). In spite of the fact that the physico-chemical properties of these insecticides would determine greater esfenvalerate stability, more fenitrothion was recovered. The greater recovery of fenitrothion was due to the higher sensitivity of the chromatograph detector to this molecule. On the other hand, the depositions of both insecticides were always higher on the glass slides when compared with depositions on the grains. Probably, some spray droplets reached the plastic tarp through the empty spaces between the grains, resulting in lower depositions than those intended. Nevertheless, the analytical procedure for grains is much more complex than for the glass slides, and some degree of insecticide loss occurred in the agronomic matrix.