MONITORING SUREDYE® DEPOSITION DURING

AN AERIAL APPLICATION

Introduction

In the past, emergency eradication programs for the Mediterranean fruit fly (Medfly) have involved the aerial application of a malathion-bait mixture over large regions of farming and residential properties. Recently, promising results have been found with applications of a food dye (SureDye®) and bait mixture as a replacement for malathion in trials performed by the United States Department of Agriculture (USDA) in Hawaii. On April 17, 1996, the California Department of Food and Agriculture's (CDFA) Pest Detection and Emergency Projects branch began a trial aerial application program of SureDye® and bait to assess its pesticide efficacy. Prior to application, sterile Medflies were released in a selected orchard in Orange County and the population was to be monitored through trapping. The SureDye® mixture was applied weekly by helicopter as an ultra-low volume spray for a total of 9 applications. The dye concentrations in the preliminary test are much less than the malathion concentrations currently used in aerial bait sprays.

SureDye® is a mixture of Red Dye #28 and Yellow Dye #8, both of which are xanthene dyes registered for use as a color additive in drugs and cosmetics by the Food and Drug Administration. Scientists speculate that the red dye reacts with light and is transformed into a substance that destroys the insects' digestive tracts. The yellow dye is believed to increase the activity of the red dye (NIEHS 1996). The SureDye® compounds degrade rapidly under sunlight and applications of SureDye® are not likely to cause significant impacts on the environment. The reported half life of the dyes are approximately one hour under sunlight. Neither dye persists for more than a few hours in air or water due to rapid photodegradation (Liquido 1996).

This monitoring study was conducted as a two-phase project. The first phase involved the initial development of the appropriate sampling and analytical methodologies. The second phase consisted of a comprehensive sampling during application and analysis of samples.

Sampling and Analytical Methods Development

Analytical Methods Development

The primary analytical laboratory for this study was the CDFA Center for Analytical Chemistry. Available methods for residue analysis of SureDye® were expanded to include absorbent deposition sheets (Kimbies®). Method development included the determination of reporting limits for both dyes, which were 10 ug per sample for the yellow dye and 12 ug per sample for the red dye. Because the dye is applied in a yeast and fructose bait, the dense mixture made it difficult to generate reproducible and low level quantitative spikes. With extended investigation into various mixtures and methods for the formation of laboratory spikes, the final recommendation was to use the dye and bait mixture as formulated, with a final filtration of the extract through 0.45 micrometer pore size filters to remove particles. The reported average recoveries from the method validation study for the red dye and yellow dye were 93 and 88 percent, respectively.

Sampling

During the sixth application, on May 29th, 30 mass deposition sites within the treatment area were sampled to determine the amount of pesticide reaching the ground. Sites were selected in the middle "access rows", which were located every five rows within the orchard to allow vehicles through the orchard. A 0.093 square meter of absorbent Kimbie® material was placed 20 centimeters above ground level on cardboard platforms. Application took approximately one hour. Following application, the samples were collected when spray droplets were sufficiently dry. The samples were then folded in aluminum foil, put into envelopes and placed on dry ice for transport. The samples remained frozen until analysis in the laboratory. Because of the rapid photodegradation of SureDye®, all samples were covered and protected from exposure to sunlight.

The application was made to approximately 150 hectares of oranges in Orange County. The SureDye® mixture was applied by helicopter approximately seven meters above ground level, just above tree height. The application swath width was approximately 12 meters, covering two rows of trees.

Results

The results are presented in Figure 1. The application rate of 39.2 grams of ai per hectare of total red and yellow dye is equivalent to a theoretical deposition rate of 3958 micrograms per cubic meter (ug/m2). The average of all the measured concentrations was 1827 ug/m2. Measured concentrations of SureDye® ranged from 585.6 to 5916 ug/m2 (Table 1).

Table 1. Results of deposition sampling for SureDye(R) application.

Figure 1. Results of deposition sampling for SureDye® application.

Overall, the samples were much lower in concentration than the theoretical concentration of the application rate. As a measurement of precision, the coefficient of variation (CV) is calculated as the standard deviation divided by the average of all the calculations and multiplied by 100%. The CV's for the red and yellow dye concentrations were 61.5% and 61.7%, respectively. The results indicate a high variation between samples, or low precision.

The relative deviation of the concentrations indicated poor accuracy between the samples and the theoretical application rate for the red and yellow dye. The overall relative percent deviation [((observed - theoretical)/theoretical) x 100] is 53.8%. Twenty-eight of the thirty samples fell well below the targeted value for red dye. Twenty-nine of the thirty samples fell well below the target application rate for yellow dye. The sample collected from location 10 was atypical in comparison to other samples, with a measured concentration nearly 4 standard deviations away from the average concentration measured.

Table 1. Summary of SureDye® deposition concentrations.

Red dye

(ug/m2)

Yellow dye

(ug/m2)

Total Dye mixture

(ug/m2)

Average 1326 46.5 169.7
Minimum 422 15.2 54.4
Maximum 4260 153.8 549.6
Theoretical 2721 114.9 367.7
CV 61.5% 61.7% 59.5%


The low precision and accuracy of the measured concentrations could be due to the nature of the application, and location of the samples. A visual inspection of the dye coverage following the application indicated a good distribution of material. The sampling media were located between the widest possible tree rows, but were still probably influenced by the orchard canopy. The usual procedure for deposition sampling requires placement of the sample from the closest obstacle to be at least twice the distance of the height of the closest obstacle. The only other possible location for the sampling media would have been on platforms placed at tree height. The low clearance of the helicopter and high wind turbulence made the option undesirable.

Another possible explanation could involve the short half-life of the compound. The rapid degradation rate of the compound under sunlight may have affected some of the samples before they were collected. A non-analytical method, such as droplet distribution on drop cards, may give a better measure of coverage.

Consistent and accurate application may also be difficult to achieve because of the physical nature of the mixture, which was a thick and concentrated mixture of dye, yeast, fructose and water. Distribution of the material may be variable due to clogging of equipment, difference in droplet size, or pattern of dispersion. The thickness of the mixture made it difficult for the chemist to reproduce accurate and consistent droplets for spiking purposes. Measured droplets of the mixture varied widely in weight. The comparison of concentrations within discrete areas from the actual field application, such as measured with the kimbies, would also result in variable concentrations.



References

Liquido, Nicanor. 1996. USDA Agricultural Research Service. Tropical Fruit and Vegetable Research Laboratory. Hilo, HI. Personal Communication. March 28, 1996.

National Institute of Environmental Health Services. 1996. Environmental Health Perspectives Innovations Web magazine. Vol 104, Number 2, February. http://ehpnet1.niehs.nih.gov/docs/1996/104(2)/innovations.html