Effects of formulation on transport of pyrethroids in residential settings.

Washoff of 17 pyrethroid products resulting from a 1-h, 25.4-mm rainfall occurring 24 h after application was measured in indoor studies with concrete slabs. These products included different pyrethroid active ingredients and a range of formulation types. Based on this replicated study, 5 product pairs with contrasting washoff behaviors were chosen for an outdoor study using 6 full-scale house fronts in central California. Products in 4 of these pairs were applied once to different rectangular areas on the driveway (1 product in each pair to 3 house lots and the other to the remaining 3 house lots). The products in the fifth pair were applied 3 times at 2-mo intervals to vertical stucco walls above the driveway. All house lots received natural and simulated rainfall over 7 mo. Indoor studies showed differences up to 170-fold between paired products, whereas the maximum difference between paired products in the field was only 5-fold. In the pair applied to the wall, 1 product had 91 times the washoff of the other in the indoor study, whereas in the field the same product had 15% lower washoff. These results show that, although the formulation may influence washoff under actual use conditions, its influence is complex and not always as predicted by indoor experiments. Because the formulation also affects insect control, washoff research needs to be conducted together with efficacy testing.

Quantifying soil surface photolysis under conditions simulating water movement in the field: a new laboratory test design.

Soil surface photolysis can be a significant dissipation pathway for agrochemicals under field conditions, although it is assumed that such degradation ceases once the agrochemical is transported away from the surface following rainfall or irrigation and subsequent drainage of soil porewater. However, as both downward and upward water movements occur under field conditions, relatively mobile compounds may return to the surface, prolonging exposure to ultraviolet light and increasing the potential for degradation by photolysis. To test this hypothesis, a novel experimental system was used to quantify the contribution of photolysis to the overall dissipation of a new herbicide, bicyclopyrone, under conditions that mimicked field studies more closely than the standard laboratory test guidance. Soil cores were taken from 3 US field study sites, and the surfaces were treated with [(14) C]-bicyclopyrone. The radioactivity was redistributed throughout the cores using a simulated rainfall event, following which the cores were incubated under a xenon-arc lamp with continuous provision of moisture from below and a wind simulator to induce evaporation. After only 2 d, most of the test compound had returned to the soil surface. Significantly more degradation was observed in the irradiated samples than in a parallel dark control sample. Degradation rates were very similar to those observed in both the thin layer photolysis study and the field dissipation studies and significantly faster than in the soil metabolism studies conducted in the dark. Thus, for highly soluble, mobile agrochemicals, such as bicyclopyrone, photolysis is not terminated permanently by rainfall or irrigation but can resume following transport to the surface in evaporating water.

Washoff of cypermethrin residues from slabs of external building material surfaces using simulated rainfall.

The use of pesticides by homeowners or pest-control operators in urban settings is common, yet contributions of washoff from these materials are not easily understood. In the present study, cypermethrin, formulated as Cynoff EC (emulsifiable concentrate) and Cynoff WP (wettable powder) insecticides, was applied at typical rates to 10 different building material surfaces to examine its washoff potential from each surface. Using an indoor rainfall simulator, a 1-h rainfall event was generated and washoff samples were collected from 3 replicates of each surface type. Washoff was analyzed for cypermethrin using gas chromatography-negative chemical ionization mass spectrometry. An analysis of variance for a split-plot design was performed. Many building materials had similar water runoff masses, but asphalt resulted in significantly reduced average water runoff masses (73% less). The Cynoff WP formulation generally produced greater cypermethrin washoff than the Cynoff EC formulation. In addition, results for both the WP and EC formulations indicated that smoother surfaces such as vinyl and aluminum siding had higher washoff (1.0-14.1% mean percentage of applied mass). Cypermethrin washoff from rough absorptive surfaces like concrete and stucco was lower and ranged from 0.1 to 1.3% and from 0 to 0.2%, respectively, mean percentage of applied mass. Both building material surface and formulation play a significant role in cypermethrin washoff.

Major transport mechanisms of pyrethroids in residential settings and effects of mitigation measures.

The major pathways for transport of pyrethroids were determined in runoff studies conducted at a full-scale test facility in central California, USA. The 6 replicate house lots were typical of front lawns and house fronts of California residential developments and consisted of stucco walls, garage doors, driveways, and residential lawn irrigation sprinkler systems. Each of the 6 lots also included a rainfall simulator to generate artificial rainfall events. Different pyrethroids were applied to 5 surfaces­driveway, garage door and adjacent walls, lawn, lawn perimeter (grass near the house walls), and house walls above grass. The volume of runoff water from each house lot was measured, sampled, and analyzed to determine the amount of pyrethroid mass lost from each surface. Applications to 3 of the house lots were made using the application practices typically used prior to recent label changes, and applications were made to the other 3 house lots according to the revised application procedures. Results from the house lots using the historic application procedures showed that losses of the compounds applied to the driveway and garage door (including the adjacent walls) were 99.75% of total measured runoff losses. The greatest losses were associated with significant rainfall events rather than lawn irrigation events. However, runoff losses were 40 times less using the revised application procedures recently specified on pyrethroid labels.