Reducing Impact of Dormant Spray

Project Leader: Barry Wilson, University of California, Davis, Central Valley

Cooperating Personnel: F. Zalom, I. Werner, W. Wallender, K. Giles, H. Scher, J. D. Henderson

This past year’s field study focused on evaluating the efficacy of different widths of vegetated buffer strips in reducing the concentration of diazinon in surface runoff from dormant-sprayed orchards; and determining if a relatively small amount of sprinkler irrigation following pesticide application helps reduce the diazinon concentration in surface runoff. A lack of rainfall in the previous year resulted in no runoff being generated. This year a sprinkler irrigation system was utilized to simulate rainfall.

The field study took place in a mature prune orchard northeast of Gridley. A permanent cover was established at the site. Treatments were randomly assigned within complete blocks of the orchard, where trees were planted on berms approximately 20 feet apart. All of the plots contained relatively equal resident vegetative cover (classified by species composition and relative density). Diazinon was applied to the plots at a common rate and dilution: 15.1 L active ingredient plus 363.4 L of water per acre (Diazinon AG500 Insecticide, Loveland Products Inc., Loveland, CO). The simulated dormant spray was applied directly to the ground within each plot with a CO₂-charged backpack sprayer. It was not applied to the trees and, by default to the ground, with a conventional air-blast sprayer to 1) reduce the variability of volume and total active ingredient applied in the plots, 2) reduce the potential for drift from one plot to the next, and 3) ensure that equal areas of ground were treated in all plots. The simulated spray represented a worst case scenario where all of the pesticide applied was deposited under the trees on the ground. For each treatment, diazinon was sprayed on the orchard floor between two berms (during the dormant season). Plots were designed so runoff would drain into autosampling units.

1. Control. A 50-meter long section of orchard floor was sprayed with the autosampler set up at the edge of the treated area.

2. Sprinkler Irrigation. Treatment was identical to the control. Following diazinon application, the area received 0.42 inch of sprinkler irrigation without causing runoff. This was to test the hypothesis that residual diazinon on the ground is transported into the soil by rains occurring earlier in the dormant season when soils are not yet saturated and runoff is less likely to occur.

3. 10-Meter Buffer Strip. Treatment was identical to the control. The autosampler was placed so rainfall runoff would flow across an additional 10-meter length of unsprayed vegetated orchard floor (equivalent to a 10- meter wide buffer strip) before reaching it.

4. 20-Meter Buffer Strip. Similar to treatment 3, but with a longer buffer strip.

5. 30-Meter Buffer Strip. Similar to treatment 3, but with a longer buffer strip.

6. 100-Meter Section Plus a 20-Meter Buffer Strip. A 100-meter long section of orchard floor was sprayed with diazinon. Subsequent runoff flowed across an additional 20-meter length of unsprayed vegetated orchard floor and then drained into an autosampling unit

The composite water samples collected from the autosamplers were transported on ice to UC Davis and frozen for later analysis. Diazinon concentrations were determined by liquid-liquid extraction followed by GC analysis with a nitrogen-phosphorus detector.

Diazinon residue concentrations determined from the runoff samples from each of the six treatments were normalized within each of the three replicates to the proportion of the diazinon concentration (ppm) in the control (which was set to 1.00). Data were analyzed by one way ANOVA following arcsine transformation for the proportional data, and by the Tukey-Kramer multiple range test.

The effect of buffer strips on the concentration of diazinon in runoff is shown in Table 1. ANOVA results indicate that the vegetated buffer strips provided a measurable reduction of diazinon concentration in orchard runoff. The 10 m, 20 m and 30m buffer strip widths were not significantly different from one another. There was no difference in diazinon concentration between the 50 m row length and 100 m row length areas drained over a 20 m wide buffer strip. All of these buffer treatments were significantly different from the control treatment.

Treatment	[Diazinon] ppb ¹	Normalized to Control ^{2, 3}
1. No buffer	332.1 + 99.6	1.00 + 0.00 a
3. 50 m + 10 m buffer	178.1 + 101.3	0.47 + 0.14 b
4. 50 m + 20 m buffer	229.5 + 129.9	0.56 + 0.23 b
5. 50 m + 30 m buffer	67.93 + 13.8	0.27 + 0.12 b
6. 100 m + 20 m buffer	143.6 + 99.1	0.37 + 0.17 b

³Means followed by different letters differ significantly at p<0.05 by Tukey-Kramer multiple range test.

Treatment 2 received a light sprinkler irrigation (0.42 inch of rain equivalent) without creating runoff. Later that same night, one inch of natural rainfall fell on the study site. The following day, rainfall was simulated (an average 1.75 inches of rain equivalent across all plots), and runoff from the test sections drained into the autosampler unit of each plot. Post application sprinkler irrigation reduced diazinon concentration in orchard runoff by 45% (Table 2). This difference was not statistically significant.

Table 2. Effect of Post-Spray Irrigation on the Diazinon Levels in Surface Runoff

Treatment	[Diazinon] ppb ¹	Normalized to Control ²
1. No buffer	332.1 + 99.6	1.00 + 0.00
2. Sprinkle Irrigated	250.5 + 171.2	0.59 + 0.27

The suggestion that vegetated buffer strips might not work as well in the California orchard system for storm water runoff, as in studies of tail water runoff from irrigated systems, was not supported. We conclude that vegetated buffer strips may be a promising mitigation measure for the dormant season diazinon spray. Our data suggest that there is no significant difference in efficacy between 10 m, 20 m and 30 m buffer strip widths in terms of diazinon concentration in storm water runoff. This is encouraging as it suggests that growers could devote a relatively small area of vegetated buffer and still have an impact on diazinon runoff. No significant difference was observed in relation to orchard area drained (between the 50 m and 100 m plots), suggesting that a similar width of vegetated buffer would be effective independent of orchard area being drained.

Previous work from microplot studies suggests that post dormant spray application sprinkler irrigation could reduce diazinon concentration in orchard runoff (Joyce et al., 2004). Infiltration carries diazinon residue into the soil where it can be reduced by the action of soil microbes. Our study provided a 45% reduction in diazinon concentration attributable to post application sprinkling. However, this difference was not statistically significant - possibly due to the occurrence of a natural rainfall event after the sprinkling, which made the sprinkled and non-sprinkled plots more similar than intended. Validation of this approach seems warranted given the promising results obtained in spite of the unforeseen problem.

One of the most promising aspects of this study was the use for the first time of simulated rainfall on our large-scale field plots. This gives us more control over the timing of rainfall events relative to pesticide application, soil moisture and other variables than is possible with unpredictable natural rainfall.

Reducing Impact of Dormant Sprays