Mixture toxicity of imidacloprid and cyfluthrin to two non-target species, the fathead minnow Pimephales promelas and the amphipod Hyalella azteca.

Two species, the fathead minnow Pimephales promelas and the amphipod Hyalella azteca, were tested to examine acute toxicity to two insecticides, cyfluthrin and imidacloprid individually and as a mixture. Cyfluthrin was acutely toxic to P. promelas and H. azteca with EC50 values and 95 % confidence intervals of 0.31 µg L(-1) (0.26-0.35 µg L(-1)) and 0.0015 µg L(-1) (0.0011-0.0018 µg L(-1)), respectively. Imidacloprid was not acutely toxic to P. promelas at water concentrations ranging from 1 to 5000 µg L(-1), whereas it was toxic to H. azteca with a EC50 value of 33.5 µg L(-1) (23.3-47.4 µg L(-1)). For the P. promelas mixture test, imidacloprid was added at a single concentration to a geometric series of cyfluthrin concentrations bracketing the EC50 value. A synergistic ratio (SR) of 1.9 was found for P. promelas, which was calculated using the cyfluthrin-only exposure and mixture-exposure data. Because cyfluthrin and imidacloprid were toxic to H. azteca, the mixture test was designed based on an equipotent toxic unit method. Results from the mixture test indicated a model deviation ratio (MDR) of 1.7 or 2.7 depending on the model. Mixture test results from the simultaneous exposure to cyfluthrin and imidacloprid with both species indicated a greater than expected toxic response because the SR or MDR values were >1. Because these two insecticides are commonly used together in the same product formulations, nontarget species could be more affected due to their greater-than-additive toxicity observed in the current study.

A simultaneous extraction method for organophosphate, pyrethroid, and neonicotinoid insecticides in aqueous samples.

A method was developed for the extraction and analysis of 2 organophosphate, 8 pyrethroid, and 5 neonicotinoid insecticides from the same water sample. A salted liquid-liquid extraction (LLE) was optimized with a solid-phase extraction (SPE) step that separated the organophosphates (OPs) and pyrethroids from the neonicotinoids. Factors that were optimized included volume of solvent and amount of salt used in the LLE, homogenization time for the LLE, and type and volume of eluting solvent used for the SPE. The OPs and pyrethroids were quantified using gas chromatography-mass spectrometry, and the neonicotinoids were quantified using liquid chromatography-diode array detector. Results showed that the optimized method was accurate, precise, reproducible, and robust; recoveries in river water spiked with 100 ng L(-1) of each of the insecticides were all between 86 and 114 % with RSDs between 2 and 8 %. The method was also sensitive with method detection limits ranging from 0.1 to 27.2 ng L(-1) depending on compounds and matrices. The optimized method was thus appropriate for the simultaneous extraction of 15 widely applied insecticides from three different classes and was shown to provide valuable information on their environmental fate from field-collected aqueous samples.

Application of kinetic modeling to predict the fate of two indoxacarb metabolites and their bound residues in soil.

Insecticide indoxacarb metabolites JT333 and MP819 were used as model compounds to assess the utilization of kinetic modeling to elucidate metabolic pathways, determine degradation kinetics of non-extractable residues (NER) and predict the accumulation potential of the released NER in soil. Soil adsorption coefficients and degradation product formation were determined in different soils in laboratory. Inverse kinetic modeling was applied to explore the dynamics of dissipation of parent (P), formation of extractable metabolites (MET), NER and CO2, and to identify their routes of degradation in soil. These two compounds share similar structural characteristics, have high affinity to soil (Koc>5000L/kg), short half-life (DT50 of 4-9days), and significant CO2 formation in soil. However, kinetic modeling showed that they degraded via different pathways. The P-MET-CO2 conversion route was the major degradation pathway for JT333 in aerobic soil. Multiple pathways were involved in MP819 degradation, while the formation of NER was predominant. The time-exposure area under the curves (AUC) for the MET or NER in soils were derived from the time-%concentration plots for the evaluation of rate limiting steps in their degradation pathways. In P-MET-CO2 pathway the MET-CO2 conversion is the rate limiting step for both compounds. Higher P-MET conversion/MET-CO2 conversion rate constant ratio resulted in larger MET AUC. The rate of NER degradation appeared much slower compared to the rates of P-MET and MET-CO2 conversions, likely due to the rate-limiting step of NER release from the bound-state, indicating that in this situation the free-state NER would be unlikely to accumulate in soil. The study reported here demonstrates the utility of kinetic modeling to quantify the dynamics of NER formation/dissipation vs. P-MET-CO2 conversion, and the application of kinetic modeling to predict the possibility of free-sate NER accumulation in soil, therefore, reveals the potential for the quantitative NER environmental risk assessment.

A site-specific ecological risk assessment for corn-associated insecticides.

A site-specific ecological risk assessment (ERA) was conducted to examine the simultaneous use of genetically modified corn (Bt corn) with a neonicotinoid seed coating, clothianidin, and use of a granular insecticide, tefluthrin, to protect crops from pest damage. A field study was conducted on site, and exposure data from the literature were summarized to determine the matrices and exposure concentrations that nontarget species could typically experience within an agricultural ecosystem. To determine ecological effects on nontarget species, acute toxicity bioassays were conducted on earthworms (Eisenia fetida), amphipods (Hyalella azteca), and Elmid riffle beetle larvae (Ancyronyx spp.) in which the test species were exposed to single insecticides as well as the mixture of the 3 insecticides. In the risk characterization section of the ERA, stressor-response profiles for each species tested were compared with field distributions of the insecticides, and a margin of safety at the 10th percentile (MOS10) was calculated to estimate risk. No acute toxicity was observed in any of the 3 nontarget species after exposure to senescent Bt corn leaf tissue. Large MOS10 values were calculated for clothianidin to the nontarget species. When bioassays were compared with tefluthrin field distributions, very low MOS10 values were calculated for earthworms (0.06) and H. azteca (0.08) because the environmental concentrations often exceeded the stressor-response profile. No increased toxicity was observed when nontarget species were exposed to a mixture of the 3 insecticides. In summary, the genetically modified corn insecticidal proteins and clothianidin were not found at environmental concentrations exceeding benchmark values for ecological effects, but tefluthrin was consistently detected in the environment at levels that could be causing toxicity to nontarget species, especially if this pyrethroid is able to travel off site.

Laboratory and field validation of a Cry1Ab protein quantitation method for water.

The widespread planting of crops expressing insecticidal proteins derived from the soil bacterium Bacillus thuringiensis (Bt) has given rise to concerns regarding potential exposure to non-target species. These proteins are released from the plant throughout the growing season into soil and surface runoff and may enter adjacent waterways as runoff, erosion, aerial deposition of particulates, or plant debris. It is crucial to be able to accurately quantify Bt protein concentrations in the environment to aid in risk analyses and decision making. Enzyme-linked immunosorbent assay (ELISA) is commonly used for quantitation of Bt proteins in the environment; however, there are no published methods detailing and validating the extraction and quantitation of Bt proteins in water. The objective of the current study was to optimize the extraction of a Bt protein, Cry1Ab, from three water matrices and validate the ELISA method for specificity, precision, accuracy, stability, and sensitivity. Recovery of the Cry1Ab protein was matrix-dependent and ranged from 40 to 88% in the validated matrices, with an overall method detection limit of 2.1 ng/L. Precision among two plates and within a single plate was confirmed with a coefficient of variation less than 20%. The ELISA method was verified in field and laboratory samples, demonstrating the utility of the validated method. The implementation of a validated extraction and quantitation protocol adds consistency and reliability to field-collected data regarding transgenic products.

A multi-year field study to evaluate the environmental fate and agronomic effects of insecticide mixtures.

A mixture of insecticides used in corn production was monitored over a three-year period in a field study to determine how long each persists in the environment, where each insecticide travels within the corn field, and the efficacy of using soil-applied insecticides with genetically modified corn. The genetically modified corn contained the insecticidal Cry1Ab and Cry3Bb1 proteins (Bt corn) and the Cry1Ab protein was found to persist only during the corn growing season in soil, runoff water, and runoff sediment with highest concentrations measured during pollination. Very low concentrations of Cry1Ab proteins were measured in soil collected in the non-Bt corn field, and no Cry1Ab proteins were detected in shallow groundwater or soil pore water. Clothianidin, a neonicotinoid insecticide used as a seed coating, was detected in all matrices and remained persistent throughout the year in soil pore water. Tefluthrin, a pyrethroid insecticide applied at planting to control corn rootworm larvae (Diabrotica spp., Coleoptera: Chrysomelidae) populations, was consistently detected in soil, runoff water, and runoff sediment during the corn growing season, but was not detected in groundwater or soil pore water. Tefluthrin did not have an effect on root damage from corn rootworm larvae feeding to Bt corn, but did prevent damage to non-Bt corn. A slight reduction in grain yield was observed in the non-Bt, no tefluthrin treatment when compared to all other treatments, but no significant difference in grain yield was observed among Bt corn treatments regardless of soil insecticide application. In the current study, the use of tefluthrin on Bt corn did not significantly affect crop damage or yield, and tefluthrin may travel off-site in runoff water and sediment.