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Published online 27 October 2006
Published in J Environ Qual 35:1994-1998 (2006)
DOI: 10.2134/jeq2005.0484
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Influence of Irrigation Methods and an Adjuvant on the Persistence of Carbaryl on Pakchoi

Mari Marutania,* and Veronica Edirveerasingamb

a Agricultural Experiment Station, College of Natural and Applied Sciences, University of Guam, UOG Station, Mangilao GU 96923
b Environmental Sciences and Health, University of Nevada, Reno, NV 89503

* Corresponding author (marutani{at}uog9.uog.edu)

Received for publication December 27, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Influence of irrigation methods and use of an adjuvant on the persistence of the carbaryl (1-naphthyl N-methylcarbamate) on pakchoi [Brassica rapa L. subsp. chinensis (Rupr.) Olsson] was studied using a commercial enzyme linked immunosorbent assay kit. After applying carbaryl at a.i. 10.6 g L–1 with or without an adjuvant (Latron B-1956) to leaves, plants were provided water daily by either overhead or basal application. Pesticide residue on leaf tissues was examined immediately after pesticide application and on 2, 4, 6, and 8 d after pesticide application. Use of the adjuvant did not affect the initial deposit of the pesticide, however pesticide persistence was improved with the adjuvant regardless of irrigation. Overhead irrigation contributed to rapid loss of the pesticide by washing carbaryl from the leaf surface. The longest half-life of carbaryl (6.5 d) was detected on plants receiving basal irrigation plus the adjuvant while the shortest half-life (2 d) was observed when plants were treated with overhead irrigation and no adjuvant.

Abbreviations: DAP, days after pesticide application • ELISA, enzyme linked immunosorbent assay


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
THE FATE of pesticides on field grown crops is affected by environmental factors including sunlight, air temperature, relative humidity, wind speed, and precipitation (Crosby, 1969; Demeterio, 1981, 1983; McDowell et al., 1985; Willis and McDowell, 1987; Willis et al., 1988, 1996). For example, the half-life of carbaryl (EPA Reg. No. 19713-52) was reported earlier with the range of 1 d to more than 1 mo depending on the type of crop and the cultivation environment. In the temperate area, the half-life of carbaryl on citrus was 42 d on orange [Citrus sinensis (L.) Osbeck] and 28 d on lemons (Citrus limon Burm. f.) (Gunther et al., 1962). The half-life of other fruit crops in the temperate region ranged from 7 to 10 d on peaches [Prunus persica Batsch forma (L.)], apples (Malus spp.), strawberries (Fragaria spp.) and gooseberries (Ribes grossularioides Maxim.) (Bogomolova, 1968; Polize et al., 1971). In India, the half-life of carbaryl on eggplant (Solanum melongena L.) was 6.5 d when grown in a temperate climate (Deshmukh and Lal, 1970), while in a tropical environment the half-life was 1 d (Mann and Chopra, 1969). Cultural practices may also influence the amount of pesticide residues on plants. Decreasing light intensity with shade covers slowed the degradation of carbaryl on a field-grown pakchoi, ranging the half-life of carbaryl from 2 d (0% shade) to 9 d (75% shade) in tropical condition (Marutani and Edirveerasingam, 2003). Crosby (1969) emphasized that ultraviolet (UV) of sunlight was responsible for most pesticide photolysis.

Guam's environment is characterized as a lowland tropical island climate. The island is located in the western Pacific at the 13° N, 144° E and with the total area of 549 km2, consisting of diverse soil types from deep acidic volcanic soils to shallow calcareous alkaline soils (Young, 1988). The relative humidity ranges from 72 to 80% in the afternoon and higher at night (86–94%), and the mean daily air temperature ranges from 22 and 30°C (NOAA, 1998). The average monthly rainfall ranges from 10 cm in March during the dry season to 38 cm in August during wet months (NOAA, 1998). It is a common practice to have supplemental irrigation in farming even during the rainy season since there are several days without any precipitations that may hinder plant growth significantly especially in shallow sandy soils in hot weather. Occasional tropical storms and typhoons visit the island bringing heavy rain and farm destruction.

Carbaryl is a pesticide commonly used to control insect pests of pakchoi, a popular leafy vegetable consumed in Guam (Yudin and Butz, 1998). This crop is grown in open fields and water is supplied with either overhead irrigation or by a drip irrigation system installed at the base of plants. An adjuvant is usually added to agrochemicals to improve its initial deposition by formation of a film over pesticides (Foy, 1993; Reeves, 1993) and/or to improve their retention (Grayson et al., 1996; Hall et al., 1998; Holloway and Western, 2003; Putman et al., 2003; Reddy and Locke, 1996; Reeves, 1993). To perform an effective pest control, we need to find out the persistence of carbaryl on pakchoi grown under different irrigation systems and effects of using an adjuvant under the agroecological environment in Guam. Carbaryl breakdown can be due to photodecomposition (Crosby, 1969; Crosby et al., 1965) and hydrolysis (Aly and El-Dib, 1971; Kuhr, 1970). The stability of carbaryl is also affected by pH (Fisher and Lohner, 1986). Carbaryl like other agrochemicals is influenced by rainfall on plants (Willis et al., 1988; Walgenbach et al., 1991).

This study compared the degradation rate of carbaryl on pakchoi as affected by irrigation methods, and the use of an adjuvant in pesticide applications on Guam. Understanding the effects of these cultural practices on pesticide residue would assist farmers to use an effective pest management practice, and provide safe produce to consumers.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Seeds of pakchoi were sown in 72-cell seedling trays with peat moss-based potting mix at a plant nursery. Three-week-old seedlings were transplanted to the field with soil classified as Guam cobbly clay (clayey, gibbastic, nonacid, isohyperthermic, Lithic Ustorthents) in Mangilao, Guam (13°26' N, 144°47' E). During the early stages of plant establishment in the field, before the experiment, noncarbaryl insecticides were applied to control insect pests. The transplants were fertilized weekly with a recommended amount of a complete fertilizer (16–14.1–13.3; N–P–K), and were watered twice daily with a drip irrigation system to field capacity.

The experiment was conducted in a randomized complete block design with four replications, with two treatments arranged in a split plot design. The irrigation treatment was the main plot and use of the adjuvant was the subplot. The size of each subplot was 1.8 by 0.9 m raised bed with eight plants spaced 0.45 m apart in two rows.

To protect plants from rainfall to prevent the inference of precipitation to the irrigation experiment, a clear polyethylene cover (Mitsubishi Vinyl Co. Ltd., Tokyo, Japan) was placed on top of a wire 0.75-m tall tunnel frame with 1-m width. The total rainfall was 63 mm during the experiment and the daily rainfall ranged from 0 to 25.7 mm. Relative humidity ranged from 75 to 95% at the experimental site during the study. The average light intensity under the plastic covering was recorded daily at 1200 h with a quantum meter (Spectrum Technologies, Plainsfield, IL) ranging from 1567 to 2806 mol m–2 s–1 (n = 8). The air temperature was also recoded daily at 1200 h with a hygro-thermometer (Cole Parmer, Vernon Hills, PA), ranging from 28.9 and 31.1°C.

Four-week-old transplants with 8 to 10 mature leaves were treated with a.i. 10.6 g L–1 of carbaryl, Sevin 80S (Bayer Crop Science, Research Triangle Park, NC) with or without the spreading of the adjuvant, Latron B-1956 (Rohm and Hass Co., Philadelphia, PA). The main functional ingredient of Latron B-1956 is modified phthalic glycerol alkyd resin that is a water-dispersible, nonionic surfactant. The insecticide was mixed with water and applied at an equivalent of a.i. 9.9 kg ha–1. Tap water of pH 7.6 was used for both applying the pesticide and irrigating plants. A backpack sprayer with a diaphragm pump (413.7 kPa) and a fine flat fan nozzle was used. After the pesticide was applied and air-dried, four leaves from each subplot were randomly sampled and placed in a brown paper bag and labeled as 0 days after pesticide application (DAP) with or without the adjuvant. The leaf samples were stored in a portable cooler, and carbaryl level was measured within 6 h using the ELISA kit (Strategic Diagnostic, Newton, PA) (Marutani and Edirveerasingam, 2003).

Ten grams of composite leaf samples were homogenized in 20 mL of acetone for 60 s with a homogenizer (Cole-Parmer, Vernon Hills, IL). Top 4 mL acetone layer was transferred to a tube containing 1 to 2 g of polyvinylpolypyrrolidone (PVPP) (Sigma Chemical, St. Louis, MO) to remove secondary compounds and vortexed for 15 s. Two grams of salt reagent (Strategic Diagnostics, Newton, PA) was added to each tube and allowed to stand for 5 min. The top layer (50 µL) was then transferred into a 13 by 100 mm glass tube and was evaporated to dryness using N gas. The sample was redissolved in 2.5 mL of RaRID assay, magnetic-separation immunoassay sample diluent (Strategic Diagnostic, Newton, PA) and stored at 4°C. Carbaryl antibody coupled paramagnetic particle solution (500 µL) was added to each test tube containing 200 µL of sample and 250 µL of carbaryl enzyme conjugate. After vortexing for 1 to 2 s, the content was incubated for 20 min at 25°C and was placed in a magnetic separation rack for 2 min. The solution was decanted and the sample was washed twice with 1 min of buffer solution. Color solution (500 uL) containing hydrogen peroxide and 3, 3', 5, 5'-tetramethylbenzidine was added to the sample, vortexed for 2 s, and incubated at room temperature for 20 min. Within 15 min after adding 500 µL of stopping solution, the concentration of carbaryl in each sample was examined using a spectrophotometer at 450 nm (U-2000, Hitachi, Japan).

From the second day (1 DAP), all plants were watered daily at 1800 h. With overhead irrigation, each plant received 500 mL of water applied above the canopy through a very fine wire mesh screen to ensure even distribution of sprinkle water on the entire plant. A garden watering can was used to deliver water by gravity. In the basal irrigation treatment, the same amount of water was applied to the soil surface under the plant canopy without touching any plant leaves. At 2, 4, 6, and 8 DAP, four leaves were sampled from each subplot at 0800 h, and the 10-g leaf tissues were processed to determine the concentration of carbaryl using the ELISA kit within 6 h after sampling.

Data Analysis
The half-life of carbaryl was calculated according to Aly and El-Dib (1971). After log transformation, the concentration of the pesticide was plotted against the date of sampling for each treatment. The half-life of carbaryl was then calculated using the equation, t1/2 = –ln(0.5)/K, where t1/2 = the time taken for 50% of the pesticide to degrade and K = the rate loss constant. The valueK was obtained after multiplying the slope of the regression equation by 2.303 (Aly and El-Dib, 1971).

For each sampling day, the original data without transformation were subjected to analysis of variance (ANOVA) and means were separated by least significant difference at the 0.05 probability level. All analyses were conducted using StatView v.5.01 (SAS Institute, 1998).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Before irrigation treatments applied, the mean amount of initial carbaryl residues on fresh leaf tissues was 6.16 mg kg–1 (SD = 0.37, n = 16) on 0 DAP. The mean was calculated from the composite of all data since there was no significant difference between two adjuvant treatments (P > 0.5).

The half-life of carbaryl on leaves was determined for all four treatments. With the overhead irrigation the half-life of carbaryl was 5 d with the adjuvant added to the pesticide and 2 d without the adjuvant. With the basal irrigation, the pesticide had the longer half-life of 6.5 and 4 d with and without the adjuvant, respectively. Under the same irrigation system the half-life of carbaryl on pakchoi was extended 2.5 to 3 d with the adjuvant, and that the half-life of carbaryl on plants under basal irrigation system was 1.5 to 2 d longer compared to overhead irrigation system with the same adjuvant treatment.

Mean concentrations of carbaryl recovered from sampled leaves differed for the various sampling times (Table 1). The pesticide was detected on all sampling dates for all treatments. The lowest concentration of 0.44 mg kg–1 (7% of the original amount detected on 0 DAP) was found from foliar samples receiving overhead irrigation + adjuvant on 8 DAP. On 2 DAP the mean insecticide concentration recovered from plants did not differ between the irrigation methods (P > 0.05), whereas the effect of the adjuvant was apparent (LSD0.05 = 1.11) in increasing the pesticide retention even after one irrigation application on 2 DAP. On 4, 6, and 8 DAP, both irrigation method and adjuvant treatments influenced the retention of carbaryl of pakchoi leaves (Table 1). There was much less pesticide recovery with overhead irrigation than with basal irrigation (P < 0.01). The adjuvant prolonged pesticide retention on the plant canopy (P < 0.01).


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  		Table 1. Effect of irrigation method and use of the adjuvant on carbaryl recovery (= amount of the pesticide in fresh wt. of plant tissues) from pakchoi leaves on 2, 4, 6, and 8 d after pesticide application (DAP) (n = 4). Number in a parenthesis after the value of the carbaryl concentration expresses the percent of the insecticide retained compared to the initial amount (6.16 mg kg–1) detected on 0 DAP (n = 16).

 
The graph of the log concentration of carbaryl against days after pesticide application indicated that the log concentration of carbaryl on plants receiving the four treatments decreased linearly with time (Fig. 1). The coefficient of determination (r2) of the four linear regression equations was all significant (P < 0.001), ranging from 0.846 to 0.909. The slopes of the lines indicated the rates of carbaryl removal from plant tissues. The slope (–0.047) of the linear regression equation for basal irrigation with the adjuvant indicated that plants retained the pesticide longest. In contrast, the overhead irrigation without the adjuvant had a rapid loss of pesticide and the smallest slope value (–0.147) of the regression equation. Two intermediates were the overhead irrigation with the adjuvant (slope = –0.063) and the basal irrigation without the adjuvant (slope = –0.090) (Fig. 1).


Figure 1
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  		Fig. 1. Relationship between log transformed carbaryl concentration (expressed as mg kg–1 fresh wt. of leaf tissues) and days after pesticide application (DAP) on basal and overhead irrigation methods and with or without the adjuvant addition in pesticide application.

 
The adjuvant in this study did not alter the initial deposit of a.i. carbaryl on pakchoi, but influenced significantly the dissipation rate or metabolism of the pesticide subsequently (Table 1; Fig. 1). Enhancement of the initial deposition of a pesticide depended on types of pesticides, adjuvants, and plant species. It is noted that a "sticker"-type adjuvant increased the initial deposit of esfenvalerate on corn (Zea mays L.) on 0 d after treatment (Reeves, 1993). In our study the initial reading was taken after air-drying the pesticide solution on foliages. The adjuvant, Latron B-1956, did not show its possible benefits of added adhesion and viscoelasticity to reduce run-off and shattering of pesticide droplets of the sprayed pesticide solution on leaf surface. It appeared that the formation of a film over the pesticide on drying the carbaryl solution might have protected the active ingredients of the pesticide from further degradation without affecting the initial concentration of the pesticide on the leaf surface as described by Reeves (1993). Physiochemical properties of cuticle of plant species may take an important role to influence the effectiveness of the adjuvant (Kirkwood, 1999). Further studies will be needed to elucidate a mechanism of interrelationship of carbaryl, the adjuvant and cuticle of pakchoi to improve the initial amount of the pesticide on the leaf surface.

Regardless of the irrigation treatment, adjuvant application significantly improved retention of carbaryl on pakchoi during the experiment. Several studies also indicated that adjuvants improved retention of various agrochemicals (Grayson et al., 1996; Hall et al., 1998; Holloway and Western, 2003; Putman et al., 2003; Reddy and Locke, 1996; Reeves, 1993). For example, foliar herbicide residues on two weeds were higher at 24 h after application when plants were treated with the adjuvant than without the adjuvant regardless of rainfall-simulated treatments (Reddy and Locke, 1996). Effectiveness of fungicides was also improved with adjuvants in controlling downy mildew on grapevine (Vitis vinifeta L.), and the level of effectiveness varied with the type of adjuvants (Grayson et al., 1996). The effect of adjuvants on pesticide retention depends on interrelationships among a thin cuticle membrane of leaf surface, type of pesticides, and type of adjuvants (Kirkwood, 1999). Our data indicated that initial amount of carbaryl on pakchoi foliage was not influenced by the adjuvant, however the retention of the pesticide was significantly improved with the adjuvant.

Overhead irrigation resembled rainfall since plants received water from above the plant canopy. Several studies indicated that the amount and/or the intensity of simulated rainfall affected the fate of applied agrochemicals on plants (Bondada et al., 2000; McDowell et al., 1985; Reddy and Locke, 1996; Willis and McDowell, 1987; Willis et al., 1986, 1988, 1992, 1996). Carbaryl was influenced by rainfall (Willis et al., 1988) and was more susceptible to rainfall loss than esfenvalerate, methomyl, and endosulfan (Walgenbach et al., 1991). Willis et al. (1996) showed that carbaryl level on soybean [Glycine max (L.) Merr.] plants was affected more by rainfall amount than rainfall intensity. In the present study, overhead irrigation might have washed off a significant amount of carbaryl from the plant surface as was the case in earlier rainfall-simulated studies. In our study, with basal irrigation physical washoff of carbaryl from plant surface did not occur. Under these conditions the disappearance of carbaryl was probably due to photodecomposition (Crosby, 1969; Crosby et al., 1965; Marutani and Edirveerasingam, 2003) and hydrolysis (Aly and El-Dib, 1971; Kuhr, 1970) with a film of water on the leaf surface produced by dew and moisture from leaf tissues. The fate of carbaryl was found be related to pH of water. The stability of carbaryl decreases with increasing pH values (pH 4 < pH 6 < pH 8) (Fisher and Lohner, 1986). The present study used water with pH of 7.6. The slightly alkaline water would have contributed to rapid breakdown of the pesticide. The disappearance rate of the pesticide may also be related to dilution contributed by plant growth (Johnson and Stansbury, 1965).

In this study, the commercial ELISA kit was a useful tool in detecting carbaryl on pakchoi. One disadvantage of using the kit was that it was not truly portable. Chemical analyses were still done in a laboratory. Development of a user-friendly kit for use in remote areas would further advance pesticide studies.

The present study indicated that irrigation methods and the use of an adjuvant influenced the stability of carbaryl on pakchoi. It is recommended to use an adjuvant to prolong life of carbaryl and drip irrigation to prevent rapid decomposition of the pesticide. Additional studies need be conducted to predict pesticide movement in the environment if overhead irrigation is used, and to predict the efficacy of pesticides on pest populations.


   ACKNOWLEDGMENTS
 
This research was supported by USDA-NAPIAP project (GUA00132). This paper is a portion of a thesis of V. Edirveerasingam. We thank Ruben dela Cruz and Jesse Manglona for technical assistance in the field, and Dr. R. Schlub, Dr. M. Golabi, and Dr. J. McConnell for editorial comments.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 





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