Environmental Monitoring Results of the

Mediterranean Fruit Fly Eradication Program,

Ventura County, 1994-95


The Department of Pesticide Regulation's (DPR) Environmental Hazards Assessment Program (EHAP) conducted environmental monitoring of aerial applications of a mixture of malathion (Malathion ULV®, Clean Crop) and protein bait (added to attract the flies to the pesticide; Nu-lure®, Miller Chemical & Fertilizer Corporation) in Ventura County from October 1994 through May 1995. These applications were part of the Mediterranean fruit fly (Medfly) eradication program conducted by the California Department of Food and Agriculture (CDFA). DPR conducted the monitoring to provide information about the amount of malathion and malaoxon (a breakdown product of malathion) reaching the ground, and the concentrations of these chemicals in air, surface water, and rain runoff.


The Medfly is a plant pest that causes widespread damage to agricultural crops in many countries. There are concerns that establishment and spread of Medfly in California would lead to increases in crop production costs, crop losses, pesticide usage, and consumer prices along with decreases in product quality, shelf life, and volume and suitability of products for export. Quarantines would be imposed on California-grown produce exports by Medfly-free countries to avoid infestation.

CDFA maintains a system of border stations, conducts inspections at international ports and airline terminals, and has developed a public information program to prevent Medfly infestations. If Medflies are detected, CDFA initiates an emergency eradication program that includes additional trapping, ground spraying of Medfly host plants with malathion in bait, and release of sterile Medflies. Under certain conditions, pesticides may also be applied to soil and a malathion/bait mixture may be applied by air. EHAP collects environmental monitoring data to ensure that CDFA's treatment protocols are being met. This environmental monitoring is required, as part of CDFA's activities when they aerially apply the malathion/bait mixture for exotic fruit fly eradication, to protect public health and the environment.

When an infestation was discovered, CDFA initiated an emergency eradication program. As part of this program, CDFA launched an aerial program. The aerial eradication program consisted of 14 applications of malathion/bait spray applied at 14- to 21-day intervals from October 12, 1994, through May 23, 1995, to a 16 square mile application area that encompassed the cities of Somis and eastern Camarillo. The malathion in bait was applied at the rate of 102 grams of Malathion ULV® in 789 milliliters of Nu-lure® protein bait per hectare.


EHAP collected samples of pesticide deposition on the ground (mass deposition) and measured the amounts of malathion and malaoxon in these samples for the first three aerial applications. One-square-foot sheets of absorbent material were placed at 34 sites within the 41 square kilometers treated with malathion and were collected 30 minutes after a helicopter had flown over the site.

Air samplers were placed at five sites near the center of the spray area for each of three applications. Air samples for malathion and malaoxon were collected before the application (24-hour duration), during the application (4- to 5-hour duration), immediately after the application (24-hour duration), and for the following 24-hour period.

Surface water samples were collected for three applications to measure concentrations in the water before and after each of these applications. Before each application, water samples were collected from each of two sites on Conejo Creek: (1) an inflow site that represented water flowing into the treated area from untreated area, and (2) an outflow site that represented water that had passed through the treated area. Thirty minutes after the helicopters making the application passed over the creek, EHAP collected a water sample from the outflow site to estimate the concentration of malathion in the surface water leaving the application area.

Rain runoff water samples were collected after heavy rainfall that occurred on November 7, January 20, and March 21 (3 to 12 days after an application) and analyzed for malathion and malaoxon concentrations.(1) For the first two rain events, water samples were collected at outflow sites immediately below the application boundary on Conejo Creek, Calleguas Creek, and Lewis Drain. For the third rain event, three additional sites (one on Revlon Slough and two on Calleguas Creek) were sampled to determine if malathion or malaoxon entered Mugu Lagoon.


Mass Deposition: The malathion deposition rate for two aerial applications averaged 8.21 milligrams malathion per square meter (mg/m2). This average deposition rate was 80 percent of the calculated application rate of 10.19 mg malathion/m2. Malaoxon was not detected in 75 percent of the samples. When malaoxon was found in samples, the highest malaoxon concentration was 0.07 mg/ m2.

Air: Malathion air concentrations averaged 0.004 micrograms per cubic meter (ug/m3) during the prespray interval, 0.067 ug/m3 during application, 0.049 ug/m3 during the 24 hours immediately following application, and 0.042 ug/m3 for the interval 24-48 hours after application. No malaoxon was detected during the prespray or application sampling intervals; concentrations averaged 0.010 ug malaoxon/m3 during both the first postspray and second postspray sampling intervals. The maximum concentration of malathion in air was 0.176 ug/m3, measured during an application.


Surface Water Samples. Before the three applications, malathion concentrations averaged 0.09 parts per billion (ppb), and malaoxon concentrations averaged 0.04 ppb in water collected at the inflow site; at the outflow site, no malathion or malaoxon was detected. After the applications, samples were collected from the outflow site only; malathion concentrations averaged 44.2 ppb (ranging from 39.1 to 50.3 ppb); malaoxon concentrations averaged 0.05 ppb.

Rain Runoff Water Samples. The highest malathion concentration measured occurred during a storm three days after an application. This concentration, 787.1 ppb malathion, was measured in Calleguas Creek immediately below the application boundary. The highest malaoxon concentration, 160.2 ppb, occurred at the same location during a rain storm 12 days after an application. Farther downstream where Calleguas Creek enters Mugu Lagoon, water samples were collected during a storm six days after an application. Initially, malathion and malaoxon were not detected; after four-and-one-half hours, malathion and malaoxon concentrations had increased to a maximum of 11.1 ppb and 2.62 ppb, respectively; 15 hours later, the malathion concentration had decreased to 0.16 ppb while malaoxon was not detected.


The levels of malathion and malaoxon measured as mass deposited and in water and air in Ventura County were similar to or less than those detected in the Los Angeles Basin during the 1989-90 eradication program. A risk assessment conducted by the California Environmental Protection Agency's Office of Environmental Health Hazard Assessment concluded that the aerial application of malathion in 1989-90 posed no significant risk to public health.(2)

After application of malathion/bait, levels of malathion that were potentially toxic to aquatic life(3) were measured in surface and rain runoff water samples collected in and downstream of the spray area from tributaries to Mugu Lagoon and from Mugu Lagoon; however, no fish kills were reported. DPR has forwarded this information to CDFA and the State Department of Fish and Game for their review.

John S. Sanders, Ph.D.
Branch Chief
(916) 324-4100

December 1997

1. In 1980, higher malathion concentrations in inland streams from rainwater runoff caused fish kills (B.J. Finlayson, G. Fagella, H. Jong, E. Littrell, and T. Lew. 1982. Impact on Fish and Wildlife from Broadscale Aerial Malathion Applications in South San Francisco Bay Region, 1981. State Resources Agency, Department of Fish and Game, Environmental Services Branch, Pesticide Investigations Unit. Rancho Cordova, CA. Administrative Report 82-2). To prevent such fish kills in future aerial application programs, CDFA amended its action plan to not aerially treat with malathion when there is a 50 percent or more chance of receiving significant rainfall within the next 24 hours of scheduled application [CDFA. 1989. Action Plan for Mediterranean Fruit Fly Ceratitis capitata (Weidemann). Division of Plant Industry, Pest Detection/Emergency Projects Branch. Sacramento, CA.

2. Please note that the dosage of malathion used in 1989-90 was twice that used in this aerial application program (Segawa, et al., 1991).

3. State Department of Fish and Game, Pesticides Investigations Unit, Aquatic Toxicology Laboratory Report from Robert Fujimura to Brian Finlayson, dated April 24, 1995.