NOTE Carryover of Imidacloprid and Disulfoton in Subsurface Drip-Irrigated Hop 1 Lawrence C. Wright and Wyatt W. Cone Department of Entomology, Washington State University Irrigated Agriculture Research and Extension Center 24106 N. Bunn Road Prosser, Washington 99350-9687 USA J. Agric. Urban Entomol. 16(1): 59 64 (January 1999) KEY WORDS Homoptera, Aphididae, Phorodon humuli, Humulus lupulus, hops, imidacloprid, disulfoton, drip irrigation Subsurface drip irrigation not only increases water and soil nitrogen use efficiencies but also crop yield and quality compared with other irrigation systems (Phene 1995). Other advantages of applying systemic insecticides through subsurface drip systems over spray applications are that drip applications can be made in virtually any weather or field condition; applications can be made quickly and inexpensively; airborne drift is eliminated; exposure of applicators and field workers to the chemical is reduced; more of the insecticide may reach the plant, allowing for lower dosages; coverage of the insecticide may be more uniform; and it may be safer for natural enemies and other nontarget organisms. The main disadvantages of subsurface drip systems are the relatively high cost of installation and maintenance of the system (Phene et al. 1987); possibility of poor uptake of insecticides by the plants, which depends upon the insecticide, water ph, and soil type; the danger of chemicals entering the surface or groundwater (However, nitrate and salt contamination of groundwater has been prevented in subsurface drip irrigation systems [Phene 1995].); and the potential accumulation of insecticide residues in the soil or plant. Pierzgalski (1995) reported on subsurface drip irrigation of hops (Humulus lupulus L.) in Poland. Hop is a dioecious, perennial, climbing, herbaceous plant that dies down to the roots in the winter (Neve 1991). In the spring, shoots emerge from the soil and grow up strings tied to a trellis that is about5minheight. In the late summer the hops are harvested by removing the plants from the fields and taking them to a stationary picking machine where the cones are separated from the foliage. The hop aphid, Phorodon humuli (Schrank), is a serious pest of hop in most of the areas of the Northern Hemisphere (Neve 1991). In the south central area of Washington state, where 76.9% of the U.S. hops were grown in 1996 (Wash. Agric. 1 Accepted for publication 10 October 1998. 59
60 J. Agric. Urban Entomol. Vol. 16, No. 1 (1999) Stat. Service 1997), almost every hop yard is treated at least once per year to control hop aphids. The climate in this area is warm and dry during the summer and hops must be irrigated. In recent years, many hop growers have installed drip irrigation systems to increase the efficiency of water use. We tested the efficacy of insecticides applied thorough a subsurface drip irrigation system in an experimental hop yard for 6 yr. Some of the untreated plots had large numbers of aphids and other untreated plots in the same hop yard had low numbers. Typically, untreated hops have large infestations of hop aphids. We suspected that insecticides applied in the previous year may have been responsible for the variation in the number of aphids in untreated plots. The objective of this study was to determine if insecticides applied through the drip system reduced the number of hop aphids for one or more years following application. Materials and Methods Drip-irrigated hop yard. The drip system was installed in a mature hop yard in the spring of 1992. The 1.32-cm ID in-pipe drip line (Rootguard, Geoflow, Inc., San Francisco, California) was buried 40 cm below the surface. The emitters were spaced 91.4 cm apart resulting in 5,064 emitters per hectare. Each emitter delivered 1.89 liter/h, which is equivalent to 0.96 mm/h. Four varieties, Mt. Hood, Liberty, Willamette, and Chinook, were planted in the hop yard with plant spacing at 2.13 2.13 m. Each variety block (0.19 ha) was divided into 12 plots that were each five rows wide and seven hills long (10.67 14.94 m). Plots were irrigated independently of each other, so each plot was treated separately. Insecticide treatments. The test chemicals were mixed in a volume of water and injected into the system with an injection pump (Model 400, Ozawa R&D, Inc., Ontario, Oregon). The chemicals applied and the range of their rates in kg (AI)/ha were as follows: imidacloprid (BAY NTN 33893, Bayer Corp., Kansas City, Missouri), 0.056 to 0.28; disulfoton (Di-Syston 8, Bayer Corp., Kansas City, Missouri), 0.56 to 2.24; dimethoate (Dimethoate 400, United Agricultural Products, Greeley, Colorado), 0.56 to 2.24; CGA-215944 (Ciba-Geigy Corp., Greensboro, North Carolina), 0.11 to 0.22; oxamyl (Vydate, Du Pont, Wilmington, Delaware), 0.56; oxydemeton-methyl (Metasystox-R, Gowan Co., Yuma, Arizona), 0.56 to 1.12; phorate (Phorate technical, United Agricultural Products, Greeley, Colorado), 0.56 to 2.24; and triazamate (RH-7988, Rohm & Haas Co., Philadelphia, Pennsylvania), 0.067 to 0.336. Treatments were applied from 1992 through 1997 and were randomly assigned to the plots every year, but insecticides and rates varied from year to year. Aphid sampling. Hop aphids were sampled from 1993 through 1997. We sampled once or twice per week with starting dates ranging from early June to early July and ending from mid-august to early September. Ten leaves were collected per plot at a height of 2m(Wright et al. 1990) and taken to the laboratory, where the aphids were brushed off the leaves with a mite brushing machine (Leedom Engineering, San Jose, California) onto a sticky glass plate. Aphids were counted under a binocular microscope, and numbers were expressed as aphids per leaf. Statistical analysis. Two separate analyses of variance (ANOVA) were used to determine if insecticides reduced the number of aphids in the year following the
WRIGHT & CONE: Insecticide Carryover in Drip Irrigated Hops 61 applications. In the first analysis, we used aphid counts from plots that were treated with insecticides in the initial year (year 0) and untreated the following year (year 1). The treatments were dimethoate, disulfoton, imidacloprid, untreated (no insecticide), and a pooled treatment consisting of CGA-215944, oxamyl, oxydemeton-methyl, phorate, and triazamate. Insecticide, year, variety, and the insecticide by year interaction were the sources of variation. Mean aphids per leaf per untreated plot in the year following the insecticide applications was used as the measure of insecticide activity. A one-tailed Dunnett s test was used to determine if the treatment means were significantly smaller than the untreated mean (PROC GLM, SAS Institute 1988). The second ANOVA was done to determine if imidacloprid or disulfoton applied in year 0 reduced aphid numbers in year 1 even when other treatments had been applied to those plots in year 1. This analysis compared disulfoton, imidacloprid, and a pooled treatment, which combined all other treatments including the untreated control. The seasonal means of aphids per leaf per plot in the pooled treatments in year 1 were the observations. Most of the insecticides were not effective in controlling aphids in the year of application, so pooling the treatments was similar to increasing the number of untreated observations. The ANOVA and Dunnett s test were conducted the same as in the first analysis. The last analysis tested the 2-yr effect of imidacloprid and disulfoton on aphid abundance in plots that had been untreated for 2 yr following the initial application. The imidacloprid- and disulfoton-treated plots were compared separately with a pooled treatment using t-tests, which were computed with PROC TTEST (SAS Institute 1988). The imidacloprid test compared imidacloprid with a pooled treatment that consisted of all treatments including disulfoton. Imidacloprid was omitted from the pooled treatment in the disulfoton test. Results and Discussion Treated year 0, untreated year 1. Insecticides significantly reduced aphid abundance (F 4.15; df 4, 37; P 0.0071) in the first ANOVA that used only the untreated plots in year 1. Year was also a significant source of variation (F 4.64; df 4, 37; P 0.004) but variety (F 1.55; df 3, 37; P 0.219) and the insecticide by year interaction (F 1.62; df 10, 37; P 0.138) were not significant. Although aphid abundance varied significantly among years, the nonsignificant year by insecticide interaction indicates that the two factors were independent of each other. Imidacloprid- and disulfoton-treated plots had significantly fewer aphids than the untreated plots but dimethoate and the pooled treatment plots did not (Table 1). Treated year 0, pooled treatments year 1. In the second ANOVA, in which all treatments except imidacloprid and disulfoton in year 1 were pooled, only insecticide (F 5.23; df 2, 113; P 0.007) significantly reduced aphid abundance. Year (F 1.72; df 4, 113; P 0.151), variety (F 1.52; df 3, 113; P 0.213), and the insecticide by variety interaction (F 0.790; df 7, 113; P 0.593) were not significant. Imidacloprid-treated plots had significantly fewer aphids than plots receiving the pooled treatments, but the disulfoton plots did not (Table 2). Both analyses indicate that imidacloprid residues remained in the soil or in the plants and reduced the number of hop aphids for 1 yr following
62 J. Agric. Urban Entomol. Vol. 16, No. 1 (1999) Table 1. Mean numbers of hop aphids in untreated plots that were treated in the previous year with insecticides applied through a subsurface drip irrigation system. Treatment, yr 0 Treatment, yr 1 Mean aphids, yr 1 a n b Untreated Untreated 37.11 ± 11.08 14 Pooled insecticides c Untreated 25.89 ± 10.67 10 Dimethoate Untreated 15.10 ± 11.34 10 Disulfoton Untreated 8.49 ± 3.46*** 12 Imidacloprid Untreated 3.92 ± 2.65*** 13 ***indicates that the mean was significantly smaller (one-tailed Dunnett s test, P 0.05) than the untreated mean. a Mean numbers of hop aphids per leaf per season ± the standard error of the means. b Number of plots. c Pooled insecticides were CGA-215944, oxamyl, oxydemeton-methyl, phorate, and triazamate. application. The carryover of disulfoton is more doubtful because it significantly reduced aphids in only the first analysis. Two-year carryover. The last test was to determine possible 2-yr effects of imidacloprid or disulfoton. Plots treated with imidacloprid in 1 yr and untreated for the next 2 yr (n 4) averaged 0.799 aphids per leaf per plot per year, whereas plots receiving other treatments followed by two untreated years (n 10) averaged 51.64 (t 3.944, df 9, P 0.0034). Disulfoton-treated plots that were followed by 2 yr of no chemical treatments averaged 54.9 aphids per leaf (n 4), whereas the other plots (imidacloprid excluded) that had been untreated for 2 yr (n 6) averaged 49.4 per leaf (t 0.197, df 8, P 0.849). Therefore, it appears that the effect of imidacloprid, but not disulfoton, lasted for 2 yr. The results do not necessarily indicate the comparative magnitude of residues because differential aphid susceptibility could bias the results. Soil type may influence the persistence of chemicals in the soil. Soils in the hop-growing region of Washington are primarily silt loam with smaller areas of brown loam and loamy fine sand (Rasmussen 1971, 1976; Lenfesty & Reedy 1985). Soil in the drip-irrigated hop yard was typical of the region: an analysis of the soil in the drip yard revealed that the soil texture was silt loam with 56.0% silt, 31.0% sand, 13.0% clay, and 0.8% organic matter. These data indicate that disulfoton reduced aphid numbers for 1 yr and imidacloprid for up to 2 yr following application. The long-lasting activity of imidacloprid may be due to its effectiveness in extremely low concentrations and its stability in the soil (Elbert et al. 1991) or possibly the plant. Szeto et al. (1983) found that disulfoton residues declined relatively rapidly in the soil, but more slowly in asparagus (Asparagus officinalis L.) plants. Wildman & Cone (1986) applied disulfoton to asparagus through a drip irrigation system and found disulfoton residues in spears the following growing season. Probably the biggest danger of applying imidacloprid or disulfoton through a drip system is exceeding
WRIGHT & CONE: Insecticide Carryover in Drip Irrigated Hops 63 Table 2. Mean numbers of hop aphids in plots receiving pooled treatments that were treated in the previous year with pooled treatments, disulfoton, or imidacloprid. Treatment, yr 0 Treatment, yr 1 Mean aphids, yr 1 a n b Pooled treatments c Pooled treatments 19.62 ± 4.09 72 Disulfoton Pooled treatments 13.67 ± 3.96 27 Imidacloprid Pooled treatments 2.08 ± 1.13*** 31 ***indicates that the mean was significantly smaller (one-tailed Dunnett s test, P 0.05) than the pooled treatment mean. a Mean number of aphids per leaf per season ± standard error of the means. b Number of plots. c Only imidacloprid and disulfoton were significantly different from the untreated plots (see Table 1). Therefore, CGA-215944, oxamyl, oxydemeton-methyl, phorate, triazamate, dimethoate, and the untreated control were combined into the pooled category. the tolerance level for hops, but contamination of the soil or water also may be a potential problem. If growers apply imidacloprid, disulfoton, or other insecticides only when hop aphid numbers reach or exceed the economic threshold level, however, the danger of accumulation should be minimized. Acknowledgment We thank Joe Perez and Lisa Pike for technical assistance; Robert Evans, Michael Mahan, and Martin Kroeger for drip yard design, operation, and maintenance; and Guy Reisenhauer for statistical advice. We gratefully acknowledge the Washington Hop Commission for financial support. References Cited Elbert, A., B. Becker, J. Hartwig & C. Erdelen. 1991. Imidacloprid-a new systemic insecticide. Pflanzenschutz-Nachrichten Bayer 44: 113 136. Lenfesty, C. D. & T. E. Reedy. 1985. Soil survey of Yakima county area, Washington. USDA Soil Conservation Service. U.S. Government Printing Office, Washington, D.C. Neve, R. A. 1991. Hops. Chapman & Hall, London. Phene, C. J., K. R. Davis, R. B. Hutmacher & R. L. McCormick. 1987. Advantages of subsurface irrigation for processing tomatoes. Acta Horticulturae 200: 101 113. Phene, C. J. 1995. The sustainability and potential of subsurface drip irrigation, pp. 359 367. In F. R. Lamm [Ed.], Microirrigation for a changing world: conserving resources/ preserving the environment. American Society of Agricultural Engineers, St. Joseph, Michigan. Pierzgalski, E. 1995. Application of subsurface irrigation on a hop plantation, pp. 729 734. In F. R. Lamm [Ed.], Microirrigation for a changing world: conserving resources/ preserving the environment. American Society of Agricultural Engineers, St. Joseph, Michigan. Rasmussen, J. L. 1971. Soil survey of Benton county area, Washington. USDA Soil Conservation Service. U.S.G.P.O., Washington, D.C.
64 J. Agric. Urban Entomol. Vol. 16, No. 1 (1999). 1976. Soil survey of Yakima Indian Reservation irrigated area, Washington, part of Yakima county. USDA Soil Conservation Service, U.S.G.P.O., Washington, D.C. SAS Institute. 1988. SAS/STAT user s guide, release 6.03 ed. SAS Institute, Cary, North Carolina. Szeto, S. Y., R. S. Vernon & M. J. Brown. 1983. Degradation of disulfoton in soil and its translocation into asparagus. J. Agric. Food Chem. 31: 217 220. Washington Agricultural Statistical Service. 1997. Washington agricultural statistics 1996 1997. Washington Agricultural Statistics Service, Olympia, Washington. Wildman, T. E. & W. W. Cone. 1986. Drip chemigation of asparagus with disulfoton: Brachycorynella asparagi (Homoptera: Aphididae) control and disulfoton degradation. J. Econ. Entomol. 79: 1617 1620. Wright, L. C., W. W. Cone, G. W. Menxies & T. E. Wildman. 1990. Numerical and binomial sequential sampling plans for the hop aphid (Homoptera: Aphididae) on hop leaves. J. Econ. Entomol. 83: 1388 1394.