- Email: [email protected]
Uptake and persistence of imidacloprid in grapevines treated by chemigation
ARTICLE IN PRESS
Crop Protection 25 (2006) 831–834 www.elsevier.com/locate/cropro
Uptake and persistence of imidacloprid in grapevines treated by chemigation Frank J. Byrne, Nick C. Toscano Department of Entomology, University of California, Riverside, CA 92521, USA Accepted 8 November 2005
Abstract The uptake and persistence of a systemic formulation (240 g l1 SC) of imidacloprid was studied in grapevines treated by chemigation, with the objective of defining suitable application rates for control of the glassy-winged sharpshooter Homalodisca coagulata Say. The sharpshooter is an important vector of Pierce’s Disease in southern California, and insecticide treatments are necessary for effective management of insect populations and disease transmission. Uptake of imidacloprid was most rapid at the highest rates of application (281 and 562 g ha1), reaching target threshold levels within the xylem fluid of 10 mg l1 within 2 days in younger vines (4 years old). At 141 g ha1, however, uptake was slow and threshold levels were not achieved in every vine. In older vines (20 years old), 6–8 days elapsed before threshold levels were detected in vines treated with 281 and 562 g ha1. Despite the initial delay in uptake, once the target threshold was reached, it was maintained throughout the season. It is clear from available information on the population dynamics of the glassy-winged sharpshooter, particularly relating to its seasonal movement from citrus orchards to neighboring vineyards, that appropriate timing of insecticide treatments can play a crucial role in the management of this pest. r 2005 Elsevier Ltd. All rights reserved. Keywords: Imidacloprid; Glassy-winged sharpshooter; Chemigation; Grapevines
1. Introduction Imidacloprid is a neonicotinoid insecticide that is becoming increasingly important for pest management within the grape industry in southern California. The systemic properties of imidacloprid have favored its selection by pest management specialists for the control of sucking pests such as the glassy-winged sharpshooter Homalodisca coagulata Say. This insect is an exotic pest of California agriculture (Blua et al., 1999) that has become the principal vector of the plant pathogenic bacterium Xylella fastidiosa Well, the causal agent of Pierce’s Disease in grapevines (Purcell and Saunders, 1999). The disease is transmitted during insect feeding on the xylem fluid of host plants. In the Temecula Valley viticultural region, vineyards have been severely affected by a Pierce’s Disease epidemic that began in 1996. In this region, substantial reductions in sharpshooter pest densities have been Corresponding author. Tel.: +1 951 827 7078.
E-mail address: [email protected] (F.J. Byrne). 0261-2194/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2005.11.004
achieved following the implementation of area-wide treatment programs (Hix et al., 2003). These programs have relied almost exclusively on the use of imidacloprid (applied as a 240 g l1 SC) to reduce sharpshooter populations on citrus, a favored host of the sharpshooter. Citrus is known to support high densities of sharpshooters throughout the year (Castle et al., 2005), and it is especially important as an over-wintering host for the sharpshooter at a time when grapes and many of its ornamental hosts are dormant. Epidemiological studies conducted in Temecula Valley have provided strong evidence to suggest that proximity to citrus may influence the incidence and severity of Pierce’s Disease in adjacent vineyards (Perring et al., 2001). Vineyard owners in Temecula Valley are advised to maintain minimum imidacloprid titers of 10 mg l1 within grapevines in order to ensure adequate protection from Pierce’s Disease infection. This concentration was tentatively set following extensive studies in citrus orchards which showed that insect numbers declined on trees in which the imidacloprid titers within the xylem fluid had
ARTICLE IN PRESS 832
F.J. Byrne, N.C. Toscano / Crop Protection 25 (2006) 831–834
attained the 10 mg l1 level (Castle et al., 2005). The latter study was conducted on mature citrus (30-year-old Valencia oranges) using the maximum recommended label rate of 562 g imidacloprid ha1, and was novel in its approach to correlating insect mortality with direct measurements of insecticide concentrations within the xylem fluid. Threshold levels of imidacloprid were maintained for up to 3 months and suppressed GWSS populations during that period (Castle et al., 2005). Although it might be assumed that the substantially lower plant biomass in vines relative to mature citrus trees would allow for lower application rates within vineyards, there was insufficient information available to show that application rates of imidacloprid below 562 g ai ha1 would provide adequate protection to vines. In a recent study (Byrne et al., 2005), we showed that imidacloprid concentrations within the xylem fluid of vines treated with 281 g imidacloprid ha1 were maintained above the 10 mg l1 threshold for up to 14 weeks. However, these vines were only 2 years old and the impressive uptake data may not reflect the situation in older vines. The aim of this study was to monitor the uptake of imidacloprid in vines treated with different application rates of a 240 g imidacloprid l1 SC, in order to establish the application rate necessary to provide threshold levels of insecticide within the xylem fluid. 2. Materials and methods 2.1. Study sites Two commercial vineyards producing wine grapes were chosen for imidacloprid uptake studies. The first site provided us with a 0.4-ha block of 4-year old Cabernet Sauvignon wine grapes (ColWay). The second site provided us with a 1.1-ha block of 20-year old Chardonnay wine grapes (CalCon). At ColWay, the study area was divided into three treatment blocks that were comprised of 11 rows, each with 32 vines/row. The central 4 rows of each treatment block were chosen for sampling. Within each row, four adjacent vines were sampled making a total of 16 vines from the four rows. At CalCon, the study area was divided into three treatment blocks comprised of 10 rows each, with 90 vines/row. Three rows were chosen from the middle of each treatment block, and four vines/row were sampled from each, making a total of 12 vines. At both sites, vines selected for sampling were located within 20 vines from the start of each row. We were not permitted to designate any rows within our study sites for control purposes due to the continued threat of Pierce’s Disease. We have, however, established from previous work conducted on grapevines (Byrne et al., 2005), and from pre-treatment sampling at the sites used in this study, that the imidacloprid quantification method (described below) yields readings of zero for all untreated vines.
2.2. Insecticide applications Imidacloprid (Admire 2Fs—240 g l1 soluble concentrate; Bayer CropScience, 2 T.W. Alexander Drive, Research Triangle Park, NC 27709) was applied by chemigation, utilizing the drip irrigation systems established at both vineyards. Water was delivered to each vine by two pressure-compensating drippers that delivered water at a rate of 4.546 l h1. Vines were pre-irrigated for at least 1 h immediately before the application of insecticide to ensure adequate wetting of the soil. Imidacloprid was then applied at 141, 281, and 562 g ha1 to designated treatment plots at each vineyard. After chemigation was completed, the irrigation lines were flushed with water for two hours, to clean the irrigation lines and to water the imidacloprid into the soil. ColWay was treated on May 28th, 2003, while CalCon was treated on June 19th, 2003. Thereafter, the irrigation was run in accordance with the vineyard manager’s established agronomic practices in which the timing of irrigation was based upon a crop coefficient determined from measurements of soil moisture content and plant water status. 2.3. Imidacloprid quantification in xylem fluid Xylem fluid was extracted using a plant water stress console (pressure chamber) (Scholander et al., 1965) from the terminal end of actively growing canes that were cut from vines on each sampling date (Byrne et al., 2005). Concentrations of imidacloprid within the extracts were determined using a competitive ELISA technique. The ELISA kits are available commercially (QuantiPlate kit for imidacloprid, cat. # EP 006; EnviroLogix Inc., 500 Riverside Industrial Parkway, Portland, ME 04103, USA) and were calibrated before use to test for matrix effects associated with xylem fluid (Byrne et al., 2005). These experiments established a lower detection limit of 4 mg imidacloprid l1. 2.4. Statistical analyses All statistical analyses were performed using the JMPs Statistical Discovery Software program (SAS Institute Inc., 2000). A nested model analysis of variance (ANOVA) was used to determine whether there were significant differences in the distribution of imidacloprid within individual vines located in different rows. A repeated measures multivariate analysis of variance (MANOVA) model was used to assess the significance of different rates of application on the temporal distribution of imidacloprid within vines. 3. Results 3.1. Imidacloprid application rates and uptake/persistence Fig. 1a shows the temporal distribution of imidacloprid titers within the xylem fluid of grape vines at the ColWay
ARTICLE IN PRESS F.J. Byrne, N.C. Toscano / Crop Protection 25 (2006) 831–834
833
Fig. 2. Distribution of imidacloprid within vines that were sampled in adjacent rows treated with 562 g imidacloprid ha1. The vines were sampled from the ColWay vineyard experiment on July 9th, 2003, 42 days after the application. Each bar represents the mean imidacloprid concentration of 6 canes per vine, with three adjacent vines sampled per row.
Fig. 1. Temporal profiles of imidacloprid concentrations within the xylem fluid of grape vines treated with 240 g l1 imidacloprid. The ColWay vines were 4 years old at the time of treatment, while the CalCon vines were at least 20 years old. ColWay was treated on May 28th, 2003, while CalCon was treated on June 19th, 2003. Each point represents the mean (7SEM) imidacloprid concentration present in the xylem fluid extracted from 16 (ColWay) and 12 (CalCon) vines. The 0.59, 1.17, and 2.34 1/ha application rates are equivalent to 141, 281, and 562 g imidacloprid ha-1, respectively.
vineyard. At all application rates, imidacloprid was detected within the first xylem fluid samples that were extracted two days after treatment. However, differences in the temporal profiles for the three rates were highly significant (F 2;45 ¼ 37:73, Po0:0001). Pair-wise comparisons of the rates showed that the 141 and 281 g ha1 rates differed significantly (F 1;30 ¼ 10:78, P ¼ 0:0026), while the degree of separation was even greater between the 281 and 562 g ha1 rates (F 1;30 ¼ 30:28, Po0:0001). There was a similar trend in the uptake and seasonal persistence of imidacloprid in vines at the CalCon vineyard, where the 141, 281, and 562 g ha1 application rates resulted in concomitant increases in xylem fluid concentrations of the insecticide (Fig. 1b). However, despite an overall significant difference in imidacloprid titers between the three rates (F 2;33 ¼ 19:60, Po0:0001), the degree of separation between responses at the 141 and 281 g ha1 rates was not significant (F 1;22 ¼ 3:45, P ¼ 0:077), whereas responses between the 281 and
562 g ha1 rates were highly significant (F 1;22 ¼ 16:12, P ¼ 0:0006). At ColWay, there was a sharp decline in the concentrations of imidacloprid at about 100 days after treatment (mid to late August) (Fig. 1a). Imidacloprid levels also began to decline during the same period at the CalCon vineyard, despite an interval of three weeks between the application dates at both sites (Fig. 1b). 3.2. Imidacloprid distribution within vines The degree of variation in the distribution of imidacloprid within vines at the ColWay vineyard was determined on July 9th, 2003, from multiple samples extracted from adjacent vines treated with 562 g ha1 (Fig. 2). The differences in imidacloprid titers between treated rows were highly significant (F 3;60 ¼ 51:25, Po0:0001). However, the distribution of imidacloprid within vines located in the same rows was very uniform (F 8;60 ¼ 0:94, P ¼ 0:49). 4. Discussion In this study, we quantified the titers of imidacloprid within treated grapevines in order to determine the chemigation rate necessary to ensure target threshold levels of insecticide within the xylem. This type of information is useful for pest management because it will contribute to the development of more effective application strategies aimed at protecting vines from pest attack during vulnerable periods of the season. Optimization of application rates could also minimize potential deleterious effects on natural enemies, the environment and human health that might arise from the use of unnecessarily high application rates.
ARTICLE IN PRESS 834
F.J. Byrne, N.C. Toscano / Crop Protection 25 (2006) 831–834
There were differences in the behavior of imidacloprid between the two vineyard locations. The rate of uptake was generally slower in the older vines (20 years old) located at the CalCon vineyard. Six days elapsed before the threshold value of 10 mg imidacloprid l1 was reached following treatment at 562 g ha1, while at half this rate, 8 days were needed before the threshold was reached. Further reductions in the application rate resulted in unsatisfactory delays in uptake (up to 20 days), and many vines did not reach the target threshold (Fig. 1). The 141 g ha1 rate performed better at the ColWay vineyard where the vines were much younger (4 years old), achieving the threshold within 7 days. However, both the 281 and 562 g ha1 rates were far more effective and achieved the threshold level within 2 days. At both vineyards, the 562 g ha1 application rate resulted in peak titers of imidacloprid within the xylem fluid that were far in excess of recommended threshold levels. Furthermore, the 562 g ha1 rate did not extend the window of protection much beyond that afforded by the 281 g ha1 rate, particularly at the CalCon site. For these reasons, the 281 g ha1 rate has been recommended to growers, as it quickly establishes effective concentrations within the xylem fluid that are maintained for a significant part of the season. With a statutory limit of 562 g ha1 per season, the use of the lower application rate also offers growers the option of an additional application of imidacloprid later in the season, should the need arise. At ColWay, the distribution of imidacloprid levels in individual vines located within the same rows was generally quite consistent (Fig. 2). However, more dramatic differences in levels were observed between rows, suggesting inconsistent delivery of imidacloprid during chemigation. At this vineyard, the point of origin of the imidacloprid injection into the irrigation lines was located nearest the rows showing the highest levels of uptake. This highlights the need for growers to inspect the irrigation lines before insecticide applications in order to achieve a more uniform placement of insecticide throughout the treatment area. In Temecula Valley vineyards, movement of the glassywinged sharpshooter from citrus groves to adjacent vineyards begins in early June (Hix et al., 2003). Therefore, during years when there are high pest densities present on citrus, there will be a specific need to time insecticide treatments in vineyards that will guarantee protection to vines from invasive insects. We have shown that uptake of imidacloprid occurs rapidly within grapevines at the 281 g ha1 application rate, thereby offering growers the opportunity to respond quickly to incipient outbreaks.
The initial detection of imidacloprid within the xylem of grapevines contrasts dramatically with that of mature citrus trees, where between 4 and 6 weeks can elapse before levels within citrus xylem reach the target threshold (Castle et al., 2005). However, the persistence of imidacloprid at threshold levels within both citrus and grapevine systems was equally impressive, and has accounted for the success of this insecticide at suppressing glassy-winged sharpshooter populations in citrus orchards and vineyards throughout southern California. Furthermore, its excellent persistence provides growers with a very effective pest management option during the most vulnerable period of pest attack, and reduces the need for multiple applications throughout the season. Acknowledgments We gratefully acknowledge Mac Learned of Bayer CropScience for his technical assistance and expertise with imidacloprid applications. We thank Ben Drake of Drake Enterprises Inc., Temecula, California, for providing vineyard study sites. And we thank Kristen Mello and Greg Ballmer for assistance with sample collection. This work was funded by the California Department of Food and Agriculture’s Plant Health and Pest Prevention Services Award # 03-0275. References Blua, M.J., Philips, P.A., Redak, R.A., 1999. A new sharpshooter threatens both crops and ornamentals. Calif. Agric. 53, 22–25. Byrne, F.J., Castle, S.J., Bi, J.L., Toscano, N.C., 2005. Application of competitive enzyme-linked immunosorbent assay for the quantification of imidacloprid titers in xylem fluid extracted from grapevines. J. Econ. Entomol. 98, 182–187. Castle, S.J., Byrne, F.J., Bi, J.L., Toscano, N.C., 2005. Spatial and temporal distribution of imidacloprid and thiamethoxam in citrus and impact on Homalodisca coagulata (Say) populations. Pest Manag. Sci. 61, 75–84. Hix, R.L., Toscano, N.C., Gispert, C., 2003. Area-wide management of the glassy-winged sharpshooter in the Temecula and Coachella Valleys. In: Tariq, M.A., Oswalt, S., Blincoe, P., Spencer, R., Houser, L., Ba, A., Esser, T. (Eds.), Proceedings of the Pierce’s Disease Research Symposium, 8–11 December 2003. Coronado, San Diego, CA, pp. 292–294. Perring, T.M., Farrar, C.A., Blua, M.J., 2001. Proximity to citrus influences Pierce’s disease in Temecula Valley vineyards. Calif. Agric. 55, 13–18. Purcell, A.H., Saunders, S.R., 1999. Glassy-winged sharpshooters expected to increase plant disease. Calif. Agric. 53, 26–27. SAS Institute Inc., 2000. JMP User’s Guide, version 4. Cary, NC. Scholander, P.F., Hammel, H.T., Bradstreer, E.D., Hemmingsen, E.A., 1965. Sap pressure in vascular plants. Science 148, 339–346.
Crop Protection 25 (2006) 831–834 www.elsevier.com/locate/cropro
Uptake and persistence of imidacloprid in grapevines treated by chemigation Frank J. Byrne, Nick C. Toscano Department of Entomology, University of California, Riverside, CA 92521, USA Accepted 8 November 2005
Abstract The uptake and persistence of a systemic formulation (240 g l1 SC) of imidacloprid was studied in grapevines treated by chemigation, with the objective of defining suitable application rates for control of the glassy-winged sharpshooter Homalodisca coagulata Say. The sharpshooter is an important vector of Pierce’s Disease in southern California, and insecticide treatments are necessary for effective management of insect populations and disease transmission. Uptake of imidacloprid was most rapid at the highest rates of application (281 and 562 g ha1), reaching target threshold levels within the xylem fluid of 10 mg l1 within 2 days in younger vines (4 years old). At 141 g ha1, however, uptake was slow and threshold levels were not achieved in every vine. In older vines (20 years old), 6–8 days elapsed before threshold levels were detected in vines treated with 281 and 562 g ha1. Despite the initial delay in uptake, once the target threshold was reached, it was maintained throughout the season. It is clear from available information on the population dynamics of the glassy-winged sharpshooter, particularly relating to its seasonal movement from citrus orchards to neighboring vineyards, that appropriate timing of insecticide treatments can play a crucial role in the management of this pest. r 2005 Elsevier Ltd. All rights reserved. Keywords: Imidacloprid; Glassy-winged sharpshooter; Chemigation; Grapevines
1. Introduction Imidacloprid is a neonicotinoid insecticide that is becoming increasingly important for pest management within the grape industry in southern California. The systemic properties of imidacloprid have favored its selection by pest management specialists for the control of sucking pests such as the glassy-winged sharpshooter Homalodisca coagulata Say. This insect is an exotic pest of California agriculture (Blua et al., 1999) that has become the principal vector of the plant pathogenic bacterium Xylella fastidiosa Well, the causal agent of Pierce’s Disease in grapevines (Purcell and Saunders, 1999). The disease is transmitted during insect feeding on the xylem fluid of host plants. In the Temecula Valley viticultural region, vineyards have been severely affected by a Pierce’s Disease epidemic that began in 1996. In this region, substantial reductions in sharpshooter pest densities have been Corresponding author. Tel.: +1 951 827 7078.
E-mail address: [email protected] (F.J. Byrne). 0261-2194/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2005.11.004
achieved following the implementation of area-wide treatment programs (Hix et al., 2003). These programs have relied almost exclusively on the use of imidacloprid (applied as a 240 g l1 SC) to reduce sharpshooter populations on citrus, a favored host of the sharpshooter. Citrus is known to support high densities of sharpshooters throughout the year (Castle et al., 2005), and it is especially important as an over-wintering host for the sharpshooter at a time when grapes and many of its ornamental hosts are dormant. Epidemiological studies conducted in Temecula Valley have provided strong evidence to suggest that proximity to citrus may influence the incidence and severity of Pierce’s Disease in adjacent vineyards (Perring et al., 2001). Vineyard owners in Temecula Valley are advised to maintain minimum imidacloprid titers of 10 mg l1 within grapevines in order to ensure adequate protection from Pierce’s Disease infection. This concentration was tentatively set following extensive studies in citrus orchards which showed that insect numbers declined on trees in which the imidacloprid titers within the xylem fluid had
ARTICLE IN PRESS 832
F.J. Byrne, N.C. Toscano / Crop Protection 25 (2006) 831–834
attained the 10 mg l1 level (Castle et al., 2005). The latter study was conducted on mature citrus (30-year-old Valencia oranges) using the maximum recommended label rate of 562 g imidacloprid ha1, and was novel in its approach to correlating insect mortality with direct measurements of insecticide concentrations within the xylem fluid. Threshold levels of imidacloprid were maintained for up to 3 months and suppressed GWSS populations during that period (Castle et al., 2005). Although it might be assumed that the substantially lower plant biomass in vines relative to mature citrus trees would allow for lower application rates within vineyards, there was insufficient information available to show that application rates of imidacloprid below 562 g ai ha1 would provide adequate protection to vines. In a recent study (Byrne et al., 2005), we showed that imidacloprid concentrations within the xylem fluid of vines treated with 281 g imidacloprid ha1 were maintained above the 10 mg l1 threshold for up to 14 weeks. However, these vines were only 2 years old and the impressive uptake data may not reflect the situation in older vines. The aim of this study was to monitor the uptake of imidacloprid in vines treated with different application rates of a 240 g imidacloprid l1 SC, in order to establish the application rate necessary to provide threshold levels of insecticide within the xylem fluid. 2. Materials and methods 2.1. Study sites Two commercial vineyards producing wine grapes were chosen for imidacloprid uptake studies. The first site provided us with a 0.4-ha block of 4-year old Cabernet Sauvignon wine grapes (ColWay). The second site provided us with a 1.1-ha block of 20-year old Chardonnay wine grapes (CalCon). At ColWay, the study area was divided into three treatment blocks that were comprised of 11 rows, each with 32 vines/row. The central 4 rows of each treatment block were chosen for sampling. Within each row, four adjacent vines were sampled making a total of 16 vines from the four rows. At CalCon, the study area was divided into three treatment blocks comprised of 10 rows each, with 90 vines/row. Three rows were chosen from the middle of each treatment block, and four vines/row were sampled from each, making a total of 12 vines. At both sites, vines selected for sampling were located within 20 vines from the start of each row. We were not permitted to designate any rows within our study sites for control purposes due to the continued threat of Pierce’s Disease. We have, however, established from previous work conducted on grapevines (Byrne et al., 2005), and from pre-treatment sampling at the sites used in this study, that the imidacloprid quantification method (described below) yields readings of zero for all untreated vines.
2.2. Insecticide applications Imidacloprid (Admire 2Fs—240 g l1 soluble concentrate; Bayer CropScience, 2 T.W. Alexander Drive, Research Triangle Park, NC 27709) was applied by chemigation, utilizing the drip irrigation systems established at both vineyards. Water was delivered to each vine by two pressure-compensating drippers that delivered water at a rate of 4.546 l h1. Vines were pre-irrigated for at least 1 h immediately before the application of insecticide to ensure adequate wetting of the soil. Imidacloprid was then applied at 141, 281, and 562 g ha1 to designated treatment plots at each vineyard. After chemigation was completed, the irrigation lines were flushed with water for two hours, to clean the irrigation lines and to water the imidacloprid into the soil. ColWay was treated on May 28th, 2003, while CalCon was treated on June 19th, 2003. Thereafter, the irrigation was run in accordance with the vineyard manager’s established agronomic practices in which the timing of irrigation was based upon a crop coefficient determined from measurements of soil moisture content and plant water status. 2.3. Imidacloprid quantification in xylem fluid Xylem fluid was extracted using a plant water stress console (pressure chamber) (Scholander et al., 1965) from the terminal end of actively growing canes that were cut from vines on each sampling date (Byrne et al., 2005). Concentrations of imidacloprid within the extracts were determined using a competitive ELISA technique. The ELISA kits are available commercially (QuantiPlate kit for imidacloprid, cat. # EP 006; EnviroLogix Inc., 500 Riverside Industrial Parkway, Portland, ME 04103, USA) and were calibrated before use to test for matrix effects associated with xylem fluid (Byrne et al., 2005). These experiments established a lower detection limit of 4 mg imidacloprid l1. 2.4. Statistical analyses All statistical analyses were performed using the JMPs Statistical Discovery Software program (SAS Institute Inc., 2000). A nested model analysis of variance (ANOVA) was used to determine whether there were significant differences in the distribution of imidacloprid within individual vines located in different rows. A repeated measures multivariate analysis of variance (MANOVA) model was used to assess the significance of different rates of application on the temporal distribution of imidacloprid within vines. 3. Results 3.1. Imidacloprid application rates and uptake/persistence Fig. 1a shows the temporal distribution of imidacloprid titers within the xylem fluid of grape vines at the ColWay
ARTICLE IN PRESS F.J. Byrne, N.C. Toscano / Crop Protection 25 (2006) 831–834
833
Fig. 2. Distribution of imidacloprid within vines that were sampled in adjacent rows treated with 562 g imidacloprid ha1. The vines were sampled from the ColWay vineyard experiment on July 9th, 2003, 42 days after the application. Each bar represents the mean imidacloprid concentration of 6 canes per vine, with three adjacent vines sampled per row.
Fig. 1. Temporal profiles of imidacloprid concentrations within the xylem fluid of grape vines treated with 240 g l1 imidacloprid. The ColWay vines were 4 years old at the time of treatment, while the CalCon vines were at least 20 years old. ColWay was treated on May 28th, 2003, while CalCon was treated on June 19th, 2003. Each point represents the mean (7SEM) imidacloprid concentration present in the xylem fluid extracted from 16 (ColWay) and 12 (CalCon) vines. The 0.59, 1.17, and 2.34 1/ha application rates are equivalent to 141, 281, and 562 g imidacloprid ha-1, respectively.
vineyard. At all application rates, imidacloprid was detected within the first xylem fluid samples that were extracted two days after treatment. However, differences in the temporal profiles for the three rates were highly significant (F 2;45 ¼ 37:73, Po0:0001). Pair-wise comparisons of the rates showed that the 141 and 281 g ha1 rates differed significantly (F 1;30 ¼ 10:78, P ¼ 0:0026), while the degree of separation was even greater between the 281 and 562 g ha1 rates (F 1;30 ¼ 30:28, Po0:0001). There was a similar trend in the uptake and seasonal persistence of imidacloprid in vines at the CalCon vineyard, where the 141, 281, and 562 g ha1 application rates resulted in concomitant increases in xylem fluid concentrations of the insecticide (Fig. 1b). However, despite an overall significant difference in imidacloprid titers between the three rates (F 2;33 ¼ 19:60, Po0:0001), the degree of separation between responses at the 141 and 281 g ha1 rates was not significant (F 1;22 ¼ 3:45, P ¼ 0:077), whereas responses between the 281 and
562 g ha1 rates were highly significant (F 1;22 ¼ 16:12, P ¼ 0:0006). At ColWay, there was a sharp decline in the concentrations of imidacloprid at about 100 days after treatment (mid to late August) (Fig. 1a). Imidacloprid levels also began to decline during the same period at the CalCon vineyard, despite an interval of three weeks between the application dates at both sites (Fig. 1b). 3.2. Imidacloprid distribution within vines The degree of variation in the distribution of imidacloprid within vines at the ColWay vineyard was determined on July 9th, 2003, from multiple samples extracted from adjacent vines treated with 562 g ha1 (Fig. 2). The differences in imidacloprid titers between treated rows were highly significant (F 3;60 ¼ 51:25, Po0:0001). However, the distribution of imidacloprid within vines located in the same rows was very uniform (F 8;60 ¼ 0:94, P ¼ 0:49). 4. Discussion In this study, we quantified the titers of imidacloprid within treated grapevines in order to determine the chemigation rate necessary to ensure target threshold levels of insecticide within the xylem. This type of information is useful for pest management because it will contribute to the development of more effective application strategies aimed at protecting vines from pest attack during vulnerable periods of the season. Optimization of application rates could also minimize potential deleterious effects on natural enemies, the environment and human health that might arise from the use of unnecessarily high application rates.
ARTICLE IN PRESS 834
F.J. Byrne, N.C. Toscano / Crop Protection 25 (2006) 831–834
There were differences in the behavior of imidacloprid between the two vineyard locations. The rate of uptake was generally slower in the older vines (20 years old) located at the CalCon vineyard. Six days elapsed before the threshold value of 10 mg imidacloprid l1 was reached following treatment at 562 g ha1, while at half this rate, 8 days were needed before the threshold was reached. Further reductions in the application rate resulted in unsatisfactory delays in uptake (up to 20 days), and many vines did not reach the target threshold (Fig. 1). The 141 g ha1 rate performed better at the ColWay vineyard where the vines were much younger (4 years old), achieving the threshold within 7 days. However, both the 281 and 562 g ha1 rates were far more effective and achieved the threshold level within 2 days. At both vineyards, the 562 g ha1 application rate resulted in peak titers of imidacloprid within the xylem fluid that were far in excess of recommended threshold levels. Furthermore, the 562 g ha1 rate did not extend the window of protection much beyond that afforded by the 281 g ha1 rate, particularly at the CalCon site. For these reasons, the 281 g ha1 rate has been recommended to growers, as it quickly establishes effective concentrations within the xylem fluid that are maintained for a significant part of the season. With a statutory limit of 562 g ha1 per season, the use of the lower application rate also offers growers the option of an additional application of imidacloprid later in the season, should the need arise. At ColWay, the distribution of imidacloprid levels in individual vines located within the same rows was generally quite consistent (Fig. 2). However, more dramatic differences in levels were observed between rows, suggesting inconsistent delivery of imidacloprid during chemigation. At this vineyard, the point of origin of the imidacloprid injection into the irrigation lines was located nearest the rows showing the highest levels of uptake. This highlights the need for growers to inspect the irrigation lines before insecticide applications in order to achieve a more uniform placement of insecticide throughout the treatment area. In Temecula Valley vineyards, movement of the glassywinged sharpshooter from citrus groves to adjacent vineyards begins in early June (Hix et al., 2003). Therefore, during years when there are high pest densities present on citrus, there will be a specific need to time insecticide treatments in vineyards that will guarantee protection to vines from invasive insects. We have shown that uptake of imidacloprid occurs rapidly within grapevines at the 281 g ha1 application rate, thereby offering growers the opportunity to respond quickly to incipient outbreaks.
The initial detection of imidacloprid within the xylem of grapevines contrasts dramatically with that of mature citrus trees, where between 4 and 6 weeks can elapse before levels within citrus xylem reach the target threshold (Castle et al., 2005). However, the persistence of imidacloprid at threshold levels within both citrus and grapevine systems was equally impressive, and has accounted for the success of this insecticide at suppressing glassy-winged sharpshooter populations in citrus orchards and vineyards throughout southern California. Furthermore, its excellent persistence provides growers with a very effective pest management option during the most vulnerable period of pest attack, and reduces the need for multiple applications throughout the season. Acknowledgments We gratefully acknowledge Mac Learned of Bayer CropScience for his technical assistance and expertise with imidacloprid applications. We thank Ben Drake of Drake Enterprises Inc., Temecula, California, for providing vineyard study sites. And we thank Kristen Mello and Greg Ballmer for assistance with sample collection. This work was funded by the California Department of Food and Agriculture’s Plant Health and Pest Prevention Services Award # 03-0275. References Blua, M.J., Philips, P.A., Redak, R.A., 1999. A new sharpshooter threatens both crops and ornamentals. Calif. Agric. 53, 22–25. Byrne, F.J., Castle, S.J., Bi, J.L., Toscano, N.C., 2005. Application of competitive enzyme-linked immunosorbent assay for the quantification of imidacloprid titers in xylem fluid extracted from grapevines. J. Econ. Entomol. 98, 182–187. Castle, S.J., Byrne, F.J., Bi, J.L., Toscano, N.C., 2005. Spatial and temporal distribution of imidacloprid and thiamethoxam in citrus and impact on Homalodisca coagulata (Say) populations. Pest Manag. Sci. 61, 75–84. Hix, R.L., Toscano, N.C., Gispert, C., 2003. Area-wide management of the glassy-winged sharpshooter in the Temecula and Coachella Valleys. In: Tariq, M.A., Oswalt, S., Blincoe, P., Spencer, R., Houser, L., Ba, A., Esser, T. (Eds.), Proceedings of the Pierce’s Disease Research Symposium, 8–11 December 2003. Coronado, San Diego, CA, pp. 292–294. Perring, T.M., Farrar, C.A., Blua, M.J., 2001. Proximity to citrus influences Pierce’s disease in Temecula Valley vineyards. Calif. Agric. 55, 13–18. Purcell, A.H., Saunders, S.R., 1999. Glassy-winged sharpshooters expected to increase plant disease. Calif. Agric. 53, 26–27. SAS Institute Inc., 2000. JMP User’s Guide, version 4. Cary, NC. Scholander, P.F., Hammel, H.T., Bradstreer, E.D., Hemmingsen, E.A., 1965. Sap pressure in vascular plants. Science 148, 339–346.