pesticide spray from plane
Aerial crop spraying

The Issue: Pesticide Drift

Pesticides are designed to be applied to crops to kill pests or (in the case of herbicides) weeds. When the pesticide is applied to the wrong area, or drifts due to wind conditions, pesticide drift can occur, killing or ruining organic plants, killing bees, and poisoning animals and humans.  

As the Iowa Department of Agriculture and Land Stewardship (IDALS) explains, “Pesticide drift can be recognized as a cloud of pesticide spray or dust, or an unpleasant odor. Pesticide application can be done by plane, helicopter or tractor. Other times you may not see or smell the pesticides when spray drift occurs. If the drifting pesticide is an herbicide, you may notice plant damage. If the drifting pesticide is an insecticide or fungicide, you will not see any plant damage but the plants could be contaminated.” 

The Pesticide Bureau of IDALS controls pesticide registration and applicator licensing, and investigates pesticide misuse complaints, including pesticide drift. The Bureau has records of 413 drift cases from 2010 to 2015. The number is likely to be conservative, since the records reflect only reported cases.  Most of the pesticide drift cases —  357 — are agriculture-related, while another 56 involve urban use. Few of the cases are reported in the news media. 

For humans, pesticide drift exposure could happen to workers in fields, or children playing on a farm. The IDALS Pesticide Bureau notes that symptoms to exposure can include eye, nose or throat irritation; difficulty breathing; skin irritation; rash; headache; stomachache; diarrhea; nausea; vomiting; dizziness; tremors; muscle weakness; blurred vision; eye irritation; excessive sweating; or fever.

For organic farmers, spray drift has lasting effects, as it can ruin plants in the field, and affect future production for years. The Practical Farmers of Iowa developed a 2013 report that summarized Iowa spray drift cases from 2008 to 2012. The study,  IDALS Pesticide Bureau Case Files for Alleged Spray Drift to Organic, Fruits and Vegetables, and Horticulture 2008-2012, summarizes the reports from 58 files of the Pesticide Bureau. In sum, “47 showed chemical residues at off-target areas, indicating drift had occurred. Fines were issued to applicators in only 11 cases, at an average of $716 per fined case.”

Pesticide drift cases in Iowa should be reported immediately to:

IDALS Pesticide Bureau
(515) 281-8591


Current Research on Pesticide Spray Drift

Brown, R. B., Carter, M. H., & Stephenson, G. R. (2004). Buffer zone and windbreak effects on spray drift deposition in a simulated wetland. Pest Management Science, 60(11), 1085-1090. doi: 10.1002/ps.926

This study measured spray drift into wetlands from surrounding agricultural fields. Spray drift deposits were measured in a simulated wetland under different wind conditions and wetland zone area. The study also tested margins between crop land and wetland that would allow for adequate protection from herbicide spray drift. Ultimately it concluded that a 10m wide area between wetland and areas being sprayed is adequate for stopping drift from traditional application methods in normal wind conditions (4.0 m/s-1). Adding an additional 10m to the buffer zone increased protection of sensitive organisms, as well as full wetland protection in higher than average wind conditions.

de Snoo, G. R., & de Wit, P. J. (1998). Buffer zones for reducing pesticide drift to ditches and risks to aquatic organisms. Ecotoxicology and Environmental Safety, 41(1), 112-118. doi:10.1006/eesa.1998.1678

Spray nozzles used or pesticide application have the potential to affect the amount of pesticide that drifts during field application. This study investigated droplet drift into surrounding fields, in ditch banks, and in ditches surrounding sprayed fields.  Results showed that there are major differences between the types of spray nozzles used (type LFR6-80 flat tip had least drift in all conditions) and that drift increases as wind speed increases. Creation of a 3m buffer zone decreased drift into ditches by 95%. With a 6m buffer zone, the drift into ditches was almost zero in normal to slightly above normal wind conditions.  The study also completed a risk assessment of 17 commonly used pesticides, testing whether they are toxic to algae, crustaceans, and fish. The toxicology portion of the study concluded that only 8 of the 17 tested pesticides pose a significant risk to aquatic ecosystems.

Felsot, A. S., Unsworth, J. B., Linders, J. B., Roberts, G., Rautman, G., Harris, C., & Carazo, E. (2010). Agrochemical spray drift; Assessment and mitigation – a review. Journal of Environmental Science and Health, 46, 1-23. doi: 10.1080/03601234.2010.515161

“During application of agrochemicals spray droplets can drift beyond the intended target to non-target receptors, including water, plants and animals. Factors affecting this spray drift include mode of application, droplet size, which can be modified by the nozzle types, formulation adjuvants, wind direction, wind speed, air stability, relative humidity, temperature and height of released spray relative to the crop canopy. The rate of fall of spray droplets depends upon the size of the droplets but is modified by entrainment in a mobile air mass and is also influenced by the rate of evaporation of the liquid constituting the aerosol. The longer the aerosol remains in the air before falling to the ground (or alternatively striking an object above ground) the greater the opportunity for it to be carried away from its intended target. In general, all size classes of droplets are capable of movement off target, but the smallest are likely to move the farthest before depositing on the ground or a non-target receptor. It is not possible to avoid spray drift completely but it can be minimized by using best-management practices. These include using appropriate nozzle types, shields, spray pressure, volumes per area sprayed, tractor speed and only spraying when climatic conditions are suitable. Field layout can also influence spray drift, whilst crop-free and spray-free buffer zones and windbreak crops can also have a mitigating effect. Various models are available to estimate the environmental exposure from spray drift at the time of application.”

Gibbs, J. L., Yost, M. G., Negrete, M., & Fenske, R. A. (2016). Passive sampling for indoor and outdoor exposures to chlorpyrifos, azinphos-methyl, and oxygen analogs in a rural agricultural community. Environmental Health Perspectives. doi: 10.1289/EHP425

Two pesticides (chlorpyrifos and azinphos-methyl) and their oxygen analogs were sampled at 14 farm and 9 non-farm residences following aerial spray application. Samples were measured using air samples both outside and inside of houses, and surface testing inside houses. In general, indoor levels of pesticide were lower than outdoor levels in all cases. Farm households had higher indoor and outdoor air concentrations of both pesticides.  Surface levels of spray residue were also higher in farm and proximal houses than they were for other residences. Many of the indoor air samples for both pesticides were below the limits of detection, meaning that they were in low enough concentrations that they couldn’t be detected at all by researchers. Mean air sample indoor/outdoor ratios were 0.17 and 0.4 for each of the pesticides, meaning that indoor air was universally cleaner than outdoor air.  

Lazzaro, L., Otto, S., & Zanin, G. (2008). Role of hedgerows in intercepting spray drift: Evaluation and modelling of the effects. Agriculture, Ecosystems & Environment, 123(4), 317-327. doi: 10.1016/j.agee.2007.07.009

“This study focuses on droplet drift, with the aim of evaluating the hedgerow efficacy in reducing drift from broadcast air-assisted sprayers and then to construct a simple model for estimating the spray drift level in surrounding fields. Three experiments were conducted in North-East Italy in 2004 and 2005, in winter, summer and autumn to obtain suitable optical porosity values in order to evaluate their effects. Three study situations (no hedgerow, single, double hedgerow) and two sprayer–hedgerow interaction scenarios (sprayer working perpendicular to or parallel with the hedgerow) were considered. Hedgerows were 7–8 m in height, while spray release height ranged from 1 to 2 m. The sampling method proved to be effective, with more than 73% of total amount sprayed being intercepted. Where there was at least one hedgerow, off-site spray reductions ranged from 82.6 (with optical porosity of 74.7%) to 97% (with optical porosity of 10.8%). The presence of a double hedgerow did not produce a higher interception rate. Analysis of the spatial pattern of drift showed that where there is a hedgerow with an optical porosity of 74–75%, the aerial drift caused by common broadcast air-assisted sprayers becomes negligible at a distance of 6–7 m. Hedgerows thus proved to be effective in intercepting spray drift leaving cultivated fields.”

Marrs, R. H., Frost, A. J., Plant, R. A., & Lunis, P. (1993). Determination of buffer zones to protect seedlings of non-target plants from the effects of glyphosate spray drift. Agriculture, Ecosystems & Environment, 45(3-4), 283-293. doi:10.1016/0167-8809(93)90077-3

“In this paper four bioassay experiments are reported, where seedlings grown in trays were exposed downwind of glyphosate applications and taken to a glasshouse for assessment. Three experiments were done with Lychnis flos-cuculi (perennial plant commonly known as Ragged-Robin) seedlings including one with different surrounding grass structures, and Experiment 4 tested the response of 15 species typical of semi-natural vegetation. The mortality of Lychnis flos-cuculi varied between experiments and appeared more or less unaffected by grassland structure except immediately downwind of the sprayer. The multi-species experiment indicated a wide sensitivity to spray drift, and one species was affected between 15 and 20 m downwind. Thus, seedlings of some species were affected at greater distances than established plants, indicating either greater capture of drift or a greater sensitivity. On sites where seedling establishment is an important mechanism for community regeneration, buffer zones may need to be 20 m wide.”

Marshall, E. J. P., & Moonen, A. C. (2002). Field margins in northern Europe: Their functions and interactions with agriculture. Agriculture, Ecosystems & Environment, 89(1-2), 5-21. doi: 10.1016/S0167-8809(01)00315-2

Studies demonstrate a variety of interactions between fields and their margins. Agricultural operations, such as fertiliser and pesticide application, have effects on the flora. Some margin flora may spread into crops, becoming field weeds. Margins also have a range of associated fauna, some of which may be pest species, while many are beneficial, either as crop pollinators or as pest predators. The biodiversity of the margin may be of particular importance for the maintenance of species at higher trophic levels, notably farmland birds, at the landscape scale. Margins contribute to the sustainability of production, by enhancing beneficial species within crops and reducing pesticide use. In northwestern Europe, a variety of methods to enhance diversity at field edges have been introduced, including sown grass and flower strips. The impact of these on weed flora and arthropods indicate mostly beneficial effects though conflicts exist, notably for the conservation of rare arable weed species.”

Ucar, T., & Hall, F. R. (2001). Windbreaks as a pesticide drift mitigation strategy: a review. Pest Management Science, 57(8), 663-675. doi: 10.1002/ps.341

“The use of natural and artificial barriers to mitigate pesticide drift from agricultural and forest applications is discussed. This technique has been considered as an alternative to current methods at a time when environmental concerns are under great public scrutiny. There has been a variety of research experiments on this subject from New Zealand to The Netherlands which have documented reductions in spray drift of up to 80–90%. However, there are still enormous data gaps to utilize this method accurately. The aerodynamic factors of wind barriers and shelter effects on crop growth and yield have been well investigated. In contrast, some of the important aspects of drift mitigation, eg porosity and turbulence, have been difficult to obtain and no standard methodologies are currently available to evaluate and classify windbreaks and shelterbelts or to determine their efficiency in reducing drift. Thus there is a significant opportunity to incorporate windbreaks into the tool set of drift mitigation tactics. Government policies, initiatives, legislation, etc, which currently address water quality, BMP, stewardship, buffers, etc, are issues which so far have not included windbreaks as a valuable drift mitigation strategy.”

Ward, M. H., Lubin, J., Giglierano, J., Colt, J.S., Wolter, C., Bekiroglu, N., Camann, D., Hartge, P. & Nuckols, J. R. (2006). Proximity to Crops and Residential Exposure to Agricultural Herbicides in Iowa. Environmental Health Perspectives, 114(6), 893-897. doi:  10.1289/ehp.8770

“Rural residents can be exposed to agricultural pesticides through the proximity of their homes to crop fields. Previously, we developed a method to create historical crop maps using a geographic information system. The aim of the present study was to determine whether crop maps are useful for predicting levels of crop herbicides in carpet dust samples from residences. From homes of participants in a case–control study of non-Hodgkin lymphoma in Iowa (1998–2000), we collected vacuum cleaner dust and measured 14 herbicides with high use on corn and soybeans in Iowa. Of 112 homes, 58% of residences had crops within 500 m of their home, an intermediate distance for primary drift from aerial and ground applications. Detection rates for herbicides ranged from 0% for metribuzin and cyanazine to 95% for 2,4-dichlorophenoxyacetic acid. Six herbicides used almost exclusively in agriculture were detected in 28% of homes. Detections and concentrations were highest in homes with an active farmer. Increasing acreage of corn and soybean fields within 750 m of homes was associated with significantly elevated odds of detecting agricultural herbicides compared with homes with no crops within 750 m . . . Our results indicate that crop maps may be a useful method for estimating levels of herbicides in homes from nearby crop fields.”

Wang, M., & Raumann, D. (2008). A simple probabilistic estimation of spray drift – factors determining spray drift and development of a model. Environmental Toxicology and Chemistry, 27(12), 2617-2626. doi:10.1897/08-109.1

“Spray drift represents a major mode of exposure in off-crop habitats or surface waters after pesticide spray application. Currently, the estimation of exposure by spray drift is based on a deterministic estimation of the amount of drifting residues, either with the use of default drift values or deterministic models, which, however, do not reproduce the entire range of spray drift observed in reality… For the development of a probabilistic spray drift model, previously published data from a series of field trials was analyzed to reveal how these data could be used for the parameterization of a probabilistic model. This analysis showed that wind speed, agricultural equipment (nozzle type, spray pressure), and relative humidity showed the strongest effect on spray drift. But remarkably, the effect differed for different distances from sprayed fields. For example, higher wind speed increased spray drift only at larger distances while it even reduced spray drift very close to field borders. Also spray pressure influenced spray drift predominantly close to fields. After identifying the parameters with the strongest effects, a probabilistic model for the estimation of the exposure by spray drift in off-crop habitats was developed. Spray drift can be simulated for any given distance from fields. It is demonstrated how the exposure and the amount of effects can be estimated when applying this model in real landscapes.”

Weppner, S., Elgethun, K., Chensheng, L., Hebert, V., Yost, M. G., & Fenske, R. A. (2006). The Washington aerial spray drift study: Children’s exposure to methamidophos in an agricultural community following fixed-wing aircraft applications. Journal of Exposure Science and Environmental Epidemiology, 16, 387-396. doi: 10.1038/sj.jea.7500461

In this study, eight children living near agricultural fields treated with methamidophos (an organophosphorus insecticide) by aerial application were tested for exposure on the day of insecticide application and one day following application. Samples were taken from children’s homes and nearby playgrounds. Children’s hands were swabbed and urine samples were taken. Air concentrations were shown to increase from 0.05 ug/m3 on the day before application to a high concentration of 0.48 ug/m3 on the day of application. Urine concentrations of the insecticide also increased on the day of application and decreased slightly on the day following application. Hand swabs showed highest levels of residue shortly after sprays occurred. Playgrounds showed increased residues, but little to no change was found inside homes. No conclusions were made regarding the overall health or safety of the children that participated.