Pyrethroid insecticides in urban catch basins: A potential secondary contamination source for urban aquatic systems.

Pesticide contamination is a threat to many aquatic habitats, and runoff from residential homes is a major contributor of these chemicals in urban surface streams and estuaries. Improved understanding of their fate and transport can help identify areas of concern for monitoring and management. In many urban areas, runoff water congregates in numerous underground catch basins before draining into the open environment; however, at present essentially no information is available on pesticide presence in these systems. In this study, we collected water samples from a large number of underground urban catch basins in different regions of California during the active pest management season to determine the occurrence and profile of the widely used pyrethroid insecticides. Detectable levels of pyrethroids were found in 98% of the samples, and the detection frequency of individual pyrethroids ranged from no detection for fenpropathrin to 97% for bifenthrin. In the aqueous phase, total pyrethroid concentrations ranged from 3 to 726 ng/L, with a median value of 32 ng/L. Pyrethroids were found to be enriched on suspended solids, with total concentrations ranging from 42 to 93,600 ng/g and a median value of 2,350 ng/g. In approximately 89% of the samples, whole water concentrations of bifenthrin were predicted to have toxic units >1 for sensitive aquatic invertebrates. The high detection frequency of bifenthrin and overall pyrethroid concentrations, especially for particle-bound residues, suggest that underground urban catch basins constitute an important secondary source for extended and widespread contamination of downstream surface waters by pesticides such as pyrethroids in urban regions.

When vector control and organic farming intersect: Pesticide residues on rice plants from aerial mosquito sprays.

Conflicts often exist between the use of pesticides for public health protection and organic farming. A prominent example is the use of insecticides for mosquito control in rice fields designated for organic farming. Rice fields, with static water and other conducive conditions, are favorable mosquito habitats. Best management practices are urgently needed to ensure the integrity of organic farming while addressing the need for public health protection. In this study, we evaluated aerial ultra-low-volume (ULV) applications of two classes of mosquito adulticides, pyrethrins and organophosphates, and their deposition and residues on rice plants throughout an active growing season in the Sacramento Valley of California. Frequent applications of pyrethrin synergized with piperonyl butoxide (PBO) and rotating applications of synergized pyrethrins and naled, an organophosphate, were carried out on two large blocks of rice fields. Aerial ULV application of either synergized pyrethrins or naled was able to generate uniform droplets above the fields with high efficacy for mosquito control. Rice leaf samples were collected before and after a subset of applications, and rice grains were sampled at harvest. Frequent applications of synergized pyrethrins resulted in some accumulation of the synergist PBO on rice leaves, but pyrethrins and naled dissipated rapidly from the leaves after each application with no noticeable accumulation over repeated applications. At harvest, no detectable residues of the pesticides or PBO were found in the rice grains. The absence of pesticide residues in the rice grains at harvest suggested that the ULV aerial application led to deposition of only very low levels of residues on rice plants during the growing season. When coupled with the short persistence and/or poor mobility of the insecticides, such applications resulted in negligible pesticide residues in rice grains.

Evaluation of efficacy and human health risk of aerial ultra-low volume applications of pyrethrins and piperonyl butoxide for adult mosquito management in response to West Nile virus activity in Sacramento County, California.

The Sacramento and Yolo Mosquito and Vector Control District (SYMVCD, also referred to as "the District") conducts surveillance and management of mosquitoes in Sacramento and Yolo counties in California. Following an increase in numbers and West Nile virus (WNV) infection rates of Culex tarsalis and Culex pipiens, the District decided on July 26, 2007, to conduct aerial applications of Evergreen EC 60-6 (60% pyrethrins: 6% piperonyl butoxide) over approximately 215 km2 in the north area of Sacramento County on the nights of July 30, July 31, and August 1, 2007. At the same time, the District received notification of the first human WNV case in the area. To evaluate the efficacy of the applications in decreasing mosquito abundance and infection rates, we conducted pre- and post-trapping inside and outside the spray zone and assessed human health risks from exposure to the insecticide applications. Results showed a significant decrease in abundance of both Cx. tarsalis and Cx. pipiens, and in the minimum infection rate of Cx. tarsalis. Human-health risks from exposure to the insecticide were below thresholds set by the US Environmental Protection Agency.

Impact of aerial spraying of pyrethrin insecticide on Culex pipiens and Culex tarsalis (Diptera: Culicidae) abundance and West Nile virus infection rates in an urban/suburban area of Sacramento County, California.

In response to an epidemic amplification of West Nile virus (family Flaviviridae, genus Flavivirus, WNV), the Sacramento and Yolo Mosquito and Vector Control District (SYMVCD) sprayed ultralow-volume (ULV) formulations of pyrethrin insecticide (Evergreen EC 60-6: 6% pyrethrin insecticide, 60% piperonylbutoxide; MGK, Minneapolis, MN, applied as 0.003 kg/ha [0.0025 lb/acre] ) over 218 km2 in north Sacramento and 243.5 km2 in south Sacramento on three consecutive evenings in August 2005. We evaluated the impact of this intervention in north Sacramento on the abundance and WNV infection rates of Culex pipiens L. and Culex tarsalis Coquillett. Mortality rates of caged Cx. tarsalis sentinels ranged from 0% under dense canopy to 100% in open fields. A comparison of weekly geometric mean mosquito abundance in CO2-baited traps in sprayed and unsprayed areas before and after treatment indicated a 75.0 and 48.7% reduction in the abundance of Cx. pipiens and Cx. tarsalis, respectively. This reduction was statistically significant for Cx. pipiens, the primary vector of WNV, with highest abundance in this urban area, but not for Cx. tarsalis, which is more associated with rural areas. The infection rates of WNV in Cx. pipiens and Cx. tarsalis collected from the spray zone were 8.2 and 4.3 per 1,000 female mosquitoes in the 2 wk before and the 2 wk after applications of insecticide, respectively. In comparison, WNV infection rates in Cx. pipiens and Cx. tarsalis collected at same time interval in the unsprayed zone were 2.0 and 8.7 per 1,000, respectively. Based on the reduction in vector abundance and its effects on number of infective bites received by human population, we concluded that the aerial application ofpyrethrin insecticide reduced the transmission intensity of WNV and decreased the risk of human infection.