Effect of insecticide formulation and adjuvant combination on agricultural spray drift.

Loss of crop protection products when agricultural spray applications drift has economic and ecological consequences. Modification of the spray solution through tank additives and product formulation is an important drift reduction strategy that could mitigate these effects, but has been studied less than most other strategies. Therefore, an experimental field study was conducted to evaluate spray drift resulting from agricultural ground applications of an insecticide formulated as a suspension concentrate (SC) and as a wettable powder (WP), with and without two adjuvants. Droplet sizes were also measured in a wind tunnel to determine if indirect methods could be substituted for field experimentation to quantify spray drift from these technologies. Results suggest that spray drift was reduced by 37% when comparing the SC to the WP formulation. As much as 63% drift reduction was achieved by incorporating certain spray adjuvants, but this depended on the formulation/adjuvant combination. The wind tunnel data for droplet spectra showed strong agreement with field deposition trends, suggesting that droplet statistics could be used to estimate drift reduction of spray solutions. These findings can be used to develop a classification scheme for formulated products and tank additives based on their potential for reducing spray drift.

Effects of nozzle types and 2,4-D formulations on spray deposition.

The objective of this study was to evaluate the effects of nozzle types and 2,4-D formulations on spray deposition on different targets. Two field experiments were carried out in a completely randomized design, and treatments were arranged in a factorial scheme. Species in experiment 1 were Sumatran fleabane (Conyza sumatrensis) and Brazil pusley (Richardia brasiliensis) and in experiment 2 were soybeans (Glycine max) and Benghal dayflower (Commelina benghalensis). For both experiments, the first factor corresponded to spray nozzles with different settings (AD 110.015 - 61 and 105 L ha-1; AD 015-D - 75 and 146 L ha-1; XR 110.0202 - 200 L ha-1; and ADIA-D 110.02 - 208 L ha-1) and the second factor consisted of two formulations of 2,4-D (amine and choline). The formulation of 2,4-D choline has contained Colex-D™ Technology. Similar or higher spray deposition was observed on the leaves and artificial targets when using 2,4-D choline as compared to the 2,4-D amine formulation, and these differences in deposition were more evident for nozzles applying lower spray volumes. Deposition was more affected by nozzle type when amine formulation was used, compared to choline formulation.

Determinants of acute mortality of Hippodamia convergens (Coleoptera: Coccinellidae) to ultra-low volume permethrin used for mosquito management.

There are relatively few experimental studies and risk assessments of the effects on non-target insects from ultra-low volume (ULV) insecticides used for management of adult mosquitoes. Therefore, we evaluated factors that may influence the ability of an insect to intercept the insecticide at the time of application by using Hippodamia convergens (Coleoptera: Coccinellidae) in field bioassay experiments in 2011 and 2015. Treatment factors included different distances, two cage heights (ground-level and 1.5 m above ground) to the point of the application, and covered vs. uncovered cage faces (2015 only). Insecticides used included a water-based formulation (Aqua-Reslin®) and an oil-based formulation (Permanone® 30-30) of permethrin. Cage height was highly significant both years, with much less acute (i.e., short-term exposure) mortality at ground-level compared with 1.5 m. In 2011, acute mortality was less at ground-level (mean = 3.2%, median = 0%) compared to 1.5 m (mean = 85.2%, median = 100%). Cage type also was highly significant, with less mortality in covered cages compared to uncovered cages. Mortality by cage height and cage type was as follows: ground level, covered cage (mean = 2.8%, median = 0.1%); ground level, uncovered cage (mean = 41.9%, median = 9.6%); 1.5 m, covered cage (mean = 6.8%, median = 0%); 1.5 m, uncovered cage (mean = 83.7%, median = 100%). Results suggest that acute mortality to non-target insects may vary considerably based on their height and their ability to directly intercept the insecticide as the aerosol passes through the area being sprayed.

A probabilistic analysis reveals fundamental limitations with the environmental impact quotient and similar systems for rating pesticide risks.

Comparing risks among pesticides has substantial utility for decision makers. However, if rating schemes to compare risks are to be used, they must be conceptually and mathematically sound. We address limitations with pesticide risk rating schemes by examining in particular the Environmental Impact Quotient (EIQ) using, for the first time, a probabilistic analytic technique. To demonstrate the consequences of mapping discrete risk ratings to probabilities, adjusted EIQs were calculated for a group of 20 insecticides in four chemical classes. Using Monte Carlo simulation, adjusted EIQs were determined under different hypothetical scenarios by incorporating probability ranges. The analysis revealed that pesticides that have different EIQs, and therefore different putative environmental effects, actually may be no different when incorporating uncertainty. The EIQ equation cannot take into account uncertainty the way that it is structured and provide reliable quotients of pesticide impact. The EIQ also is inconsistent with the accepted notion of risk as a joint probability of toxicity and exposure. Therefore, our results suggest that the EIQ and other similar schemes be discontinued in favor of conceptually sound schemes to estimate risk that rely on proper integration of toxicity and exposure information.

The mosquito ultra-low volume dispersion model for estimating environmental concentrations of insecticides used for adult mosquito management.

Computer models for pesticide drift are widely used tools by regulatory agencies to estimate the deposition of pesticides beyond the intended target area. Currently, there is no model in use that has been validated or verified as an accurate means of estimating concentrations of insecticides after ground-based ultra-low volume (ULV) applications used for adult mosquito management. To address the need for a validated model we created a spreadsheet-based model called Mosquito Ultra-Low Volume Dispersion (MULV-Disp) to aid in the adoption and to provide easier use of a validated model. We explain the origin, use, and utility of MULV-Disp, which can be used by regulatory agencies and other interested parties to estimate deposition of ULV insecticides.

A refined aquatic ecological risk assessment for a pyrethroid insecticide used for adult mosquito management.

The use of pyrethroid insecticides has increased substantially throughout the world over the past few decades as the use of organophorous, carbamate, and organochlorine insecticides is being phased out. Pyrethroids are the most common class of insecticides for ultralow-volume (ULV) aerosol applications used to manage high densities of adult mosquitoes. Pyrethroids are highly toxic to nontarget organisms such as certain aquatic organisms, and there have been concerns about the effect of applications of ULV insecticides on these organisms. To address the uncertainties associated with the risks of ULV applications and the contradictory findings of other ecological risk assessments, the authors performed a probabilistic aquatic ecological risk assessment for permethrin using actual environmental deposition on surfaces to estimate permethrin concentrations in water. The present study is the first ecological risk assessment for pyrethroids to quantitatively integrate the reduction in bioavailability resulting from the presence of dissolved organic matter. As part of the risk assessment, the authors incorporated a species sensitivity distribution to take into account the differences in toxicity for different species. The 95th percentile estimated concentration would result in less than 0.0001% of the potentially affected fraction of species reaching the lethal concentration that kills 50% of a population. The results of the present study are supported by the weight of evidence that pyrethroids applied by ground-based ULV equipment will not result in deleterious effects on aquatic organisms.

Environmental fate model for ultra-low-volume insecticide applications used for adult mosquito management.

One of the more effective ways of managing high densities of adult mosquitoes that vector human and animal pathogens is ultra-low-volume (ULV) aerosol applications of insecticides. The U.S. Environmental Protection Agency uses models that are not validated for ULV insecticide applications and exposure assumptions to perform their human and ecological risk assessments. Currently, there is no validated model that can accurately predict deposition of insecticides applied using ULV technology for adult mosquito management. In addition, little is known about the deposition and drift of small droplets like those used under conditions encountered during ULV applications. The objective of this study was to perform field studies to measure environmental concentrations of insecticides and to develop a validated model to predict the deposition of ULV insecticides. The final regression model was selected by minimizing the Bayesian Information Criterion and its prediction performance was evaluated using k-fold cross validation. Density of the formulation and the density and CMD interaction coefficients were the largest in the model. The results showed that as density of the formulation decreases, deposition increases. The interaction of density and CMD showed that higher density formulations and larger droplets resulted in greater deposition. These results are supported by the aerosol physics literature. A k-fold cross validation demonstrated that the mean square error of the selected regression model is not biased, and the mean square error and mean square prediction error indicated good predictive ability.

The joint toxicity of type I, II, and nonester pyrethroid insecticides.

Evidence suggests that there are separate binding domains for type I and II pyrethroid insecticides on the voltage gated sodium channel of the nerve cell axon, but there are no studies that have examined the mixture toxicity of nonester pyrethroids and type I and II pyrethroids. Therefore, we examined the effect of nonester pyrethroid (etofenprox), type I (permethrin), and type II (cypermethrin) pyrethroid insecticides alone and in all combinations to Drosophila melanogaster Meigen. The combination of permethrin + etofenprox and permethrin + cypermethrin demonstrated antagonistic toxicity, while the combination of cypermethrin + etofenprox demonstrated synergistic toxicity. The mixture ofpermethrin + cypermethrin + etofenprox demonstrated additive toxicity. The toxicity of permethrin + cypermethrin was significantly lower than the toxicity of cypermethrin alone, but the combination was not significantly different from permethrin alone. The toxicity of permethrin + cypermethrin + etofenprox was significantly greater than the toxicity of both permethrin and etofenprox alone, but it was significantly lower than cypermethrin alone. The mixture of permethrin and etofenprox was significantly less toxic than permethrin. The explanation for the decreased toxicity observed is most likely because of the competitive binding at the voltage-gated sodium channel, which is supported by physiological and biochemical studies of pyrethroids. Our results demonstrate that the assumption that the mixture toxicity of pyrethroids would be additive is not adequate for modeling the mixture toxicity of pyrethroids to insects.

Bystander exposure to ultra-low-volume insecticide applications used for adult mosquito management.

A popular and effective management option for adult mosquitoes is the use of insecticides applied by ultra-low-volume (ULV) equipment. However, there is a paucity of data on human dermal exposure to insecticides applied by this method. The objective of the current study was to estimate dermal exposures to the insecticide active ingredient permethrin using water- (Aqua-Reslin®) and oil-based (Permanone® 30-30) formulations with passive dosimetry. No significant differences in deposition of permethrin were observed between years, distance from the spray source, front or back of the body, or the placement of the patches on the body. However, exposure to Aqua-Reslin was significantly greater than Permanone 30-30 and average concentrations deposited on the body were 4.2 and 2.1 ng/cm2, respectively. The greater deposition of Aqua-Reslin is most likely due to the higher density of the water-based formulation which causes it to settle out faster than the lighter oil-based formulation of Permanone 30-30. The estimated average absorbed dermal exposure for permethrin from Aqua-Reslin and Permanone 30-30 was 0.00009 and 0.00005 mg/kg body weight, respectively. We also found that ground deposition of ULV insecticides can be used as a surrogate for estimating dermal exposure. The estimated exposures support the findings of previous risk assessments that exposure to ULV applications used for mosquito management are below regulatory levels of concern.

Net risk: a risk assessment of long-lasting insecticide bed nets used for malaria management.

Despite the demonstrated ability of bed nets that have been factory-impregnated with long-lasting insecticides (LLINs) to protect people from malaria and despite the ambitious plans for their widespread use, the health risks from the LLINs themselves have not been adequately investigated and reported in the peer-reviewed science literature. Here, we use a probabilistic risk assessment approach to estimate the risks to Africans from inhalation, dermal, and oral exposures to the newer LLINs with permethrin, α-cypermethrin, or deltamethrin as the insecticide active ingredient. We estimated exposures to LLINs using 17 age groups to incorporate different body weights and sleeping behaviors. Risk quotients (exposure divided by toxic threshold) at the 50th and 90th percentiles for non-cancer risks were < 1.0 for lifetime adjusted risk and all youth and adult age groups. Risk quotients for infants and toddlers (0-3 years) and child groups from 3 to 10 years were ≥ 1.0 for specific bed nets.

The effect of fluorescent tracers on droplet spectrum, viscosity, and density of pesticide formulations.

The most important factor affecting efficacy and drift of pesticide applications is the droplet spectrum. To measure pesticide drift, researchers utilize fluorescent tracers to rapidly quantify spray deposition. Although fluorescent tracers have been used for more than 50 years, no experiments have been performed on the effect they have on the properties of pesticide formulations (density and viscosity) or droplet spectrum, which affect the drift of pesticides. Therefore, we examined the effect of an oil- and water-based tracer on the volume median diameter (VMD), viscosity, and density of oil- and water-based pesticide formulations. In addition, we experimentally fit and demonstrate the utility of using distributions to characterize pesticide droplet spectra. The addition of tracers to both water- and oil-based formulations did not significantly alter the VMD, viscosity, and density. Lognormal distributions provided the best fit for the water- and oil-based formulations with and without tracer. Our results demonstrated that the addition of oil- and water-based tracers do not significantly alter pesticide formulations properties and droplet spectrum, and most likely do not alter the movement of pesticide droplets in the environment.

Limitations of the entomological operational risk assessment using probabilistic and deterministic analyses.

The Entomological Operational Risk Assessment (EORA) is used by the U.S. military to estimate risks posed by arthropod-vectored pathogens that produce human diseases. Our analysis demonstrated that the EORA matrix is formatted so that a small change in probability results in a discontinuous jump in risk. In addition, we show the overlap of different risk categories with respect to their probability of occurrence. Our results reveal that the fundamental mathematical problems associated with the EORA process may not provide estimates that are better than random chance. To ameliorate many of the problems associated with the EORA, we suggest more robust methods for performing qualitative and semiquantitative risk assessments when it is difficult to obtain the probability that an adverse event will occur and when the knowledge of experts can aid the process.

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.

Toxicity and risk of permethrin and naled to non-target insects after adult mosquito management.

We derived laboratory LC50 values, assessed non-target insect risks, and conducted a field bioassay for ultra-low-volume (ULV) aerosol applications of insecticides used to manage adult mosquitoes. The house cricket, Acheta domesticus (L.), was used as an indicator species for medium- to large-bodied ground dwelling insects. The 24-h LC(50) values for Permanone (formulated product of permethrin), Permanone + piperonyl butoxide (PBO), technical grade permethrin, and technical grade permethrin + PBO ranged from 0.052 to 0.9 microg/cm(2). The 24 h LC(50) for technical grade naled and Trumpet((R)) (formulated product of naled) were 0.038 and 0.44 microg/cm(2), respectively. The synergist ratio was 2.65 for Permanone + PBO and 1.57 for technical grade permethrin + PBO. The toxicity of technical grade permethrin was about 10-fold greater than Permanone. A risk assessment using modeled estimated environmental concentrations resulted in risk quotients (RQ) that exceeded regulatory levels of concern, but when compared to field-derived actual environmental concentrations RQs did not exceed a regulatory level of concern, except in the case of technical grade naled. These results were expected because higher tiered risk assessments using field-verified data generally lead to lower risk estimates. Field bioassays using caged crickets showed no significant mortality for permethrin or naled after a single truck-mounted ULV application. The results of the risk assessment using actual environmental concentrations are supported by the field bioassays and suggest that a single ULV application of synergized or unsynergized permethrin and naled most likely will not result in population impacts on medium- to large-bodied insects.

Economic cost analysis of West Nile virus outbreak, Sacramento County, California, USA, 2005.

In 2005, an outbreak of West Nile virus (WNV) disease occurred in Sacramento County, California; 163 human cases were reported. In response to WNV surveillance indicating increased WNV activity, the Sacramento-Yolo Mosquito and Vector Control District conducted an emergency aerial spray. We determined the economic impact of the outbreak, including the vector control event and the medical cost to treat WNV disease. WNV disease in Sacramento County cost approximately $2.28 million for medical treatment and patients' productivity loss for both West Nile fever and West Nile neuroinvasive disease. Vector control cost approximately $701,790, including spray procedures and overtime hours. The total economic impact of WNV was $2.98 million. A cost-benefit analysis indicated that only 15 cases of West Nile neuroinvasive disease would need to be prevented to make the emergency spray cost-effective.

Deposition and air concentrations of permethrin and naled used for adult mosquito management.

One of the most effective ways of managing adult mosquitoes that vector human and animal pathogens is the use of ultra-low-volume (ULV) insecticides. Because of the lack of environmental fate studies and concerns about the safety of the insecticides used for the management of adult mosquitoes, we conducted an environmental fate study after truck-mounted applications of permethrin and naled. One hour after application, concentrations of permethrin on cotton dosimeters placed at ground level 25, 50, and 75 m from the spray source were 2, 4, and 1 ng/cm2 in 2007 and 5, 2, and 0.9 ng/cm2 in 2008, respectively. One hour after application, concentrations of naled 25, 50, and 75 m were 47, 66, and 67 ng/cm2 in 2007 and 15, 6.1, and 0 (nondetectable) ng/cm2 in 2008, respectively. Deposition concentrations 12 h after application were not significantly different than 1 h after application for permethrin and naled either year. During 2007 and 2008 permethrin applications, two quantifiable air concentrations of 375 and 397 ng/m3 were observed 1 h after application. In 2007 and 2008, naled air concentrations ranged from 2300 to 4000 ng/m3 1 h after application. There were no quantifiable air concentrations between 1 and 12 h after application in either 2007 or 2008 for both naled and permethrin. Environmental concentrations observed in this study demonstrate that models used in previous risk assessments were sufficiently conservative (i.e., the models overestimated environmental concentrations). However, we also demonstrate inadequacies of models such as AgDrift and AGDISP, which currently are used by the US Environmental Protection Agency to estimate environmental concentrations of ULV insecticides.

A probabilistic risk assessment for deployed military personnel after the implementation of the "Leishmaniasis Control Program" at Tallil Air Base, Iraq.

Leishmaniasis has been of concern to the U.S. military and has re-emerged in importance because of recent deployments to the Middle East. We conducted a retrospective probabilistic risk assessment for military personnel potentially exposed to insecticides during the "Leishmaniasis Control Plan" (LCP) undertaken in 2003 at Tallil Air Base, Iraq. We estimated acute and subchronic risks from resmethrin, malathion, piperonyl butoxide (PBO), and pyrethrins applied using a truck-mounted ultra-low-volume (ULV) sprayer and lambda-cyhalothrin, cyfluthrin, bifenthrin, chlorpyrifos, and cypermethrin used for residual sprays. We used the risk quotient (RQ) method for our risk assessment (estimated environmental exposure/toxic endpoint) and set the RQ level of concern (LOC) at 1.0. Acute RQs for truck-mounted ULV and residual sprays ranged from 0.00007 to 33.3 at the 95th percentile. Acute exposure to lambda-cyhalothrin, bifenthrin, and chlorpyrifos exceeded the RQ LOC. Subchronic RQs for truck-mounted ULV and residual sprays ranged from 0.00008 to 32.8 at the 95th percentile. Subchronic exposures to lambda-cyhalothrin and chlorpyrifos exceeded the LOC. However, estimated exposures to lambda-cyhalothrin, bifenthrin, and chlorpyrifos did not exceed their respective no observed adverse effect levels.

Environmental concentrations, fate, and risk assessment of pyrethrins and piperonyl butoxide after aerial ultralow-volume applications for adult mosquito management.

One of the most effective ways of managing adult mosquitoes that vector human and animals diseases is the use of ultralow-volume insecticides. Because of concerns about the safety of the insecticides used for the management of adult mosquitoes, we conducted an environmental fate and efficacy study in Princeton and Colusa (both CA, USA) after aerial applications of pyrethrins and piperonyl butoxide (PBO). One hour before application, PBO concentrations in water were 0.008 and 0.2175 microg/L for Princeton and Colusa, respectively. One hour after the spray event in Princeton, the average PBO concentrations were 0.0125 microg/cm2 on ground-deposition pads and 0.1723 microg/L in water samples, with concentrations decreasing significantly over time. One hour after the spray event in Colusa, the average PBO concentrations were 0.0199 microg/cm2 on deposition pads and 1.274 microg/L in water samples, with concentrations decreasing significantly over time. A significant time and location effect for both deposition pads and water samples in Princeton and Colusa was observed (p<0.001 and p=0.014, respectively). Pyrethrins were not detected in nearly all ground and water samples. One hour after application, mortality of Culex tarsalis and Culex pipiens in sentinel cages was significantly higher than at the control site for both locations (p<0.001). Risk quotients for aquatic surrogate species in Princeton and Colusa were 0.002 or less at 1 h after application, which did not exceed the U.S. Environmental Protection Agency risk quotient level of concern for endangered aquatic organisms of 0.05. Our results suggest that the amounts of pyrethrins and PBO deposited on the ground and in water after aerial ULV insecticide applications are lower than those estimated by previous exposure and risk assessments.