Selection of pesticides with small ecological footprints is a key factor in developing sustainable agricultural systems. Policy guiding the selection of pesticides often emphasizes natural products and organic-certified pesticides to increase sustainability, because of the prevailing public opinion that natural products are uniformly safer, and thus more environmentally friendly, than synthetic chemicals.
We report the results of a study examining the environmental impact of several new synthetic and certified organic insecticides under consideration as reduced-risk insecticides for soybean aphid (Aphis glycines) control, using established and novel methodologies to directly quantify pesticide impact in terms of biocontrol services. We found that in addition to reduced efficacy against aphids compared to novel synthetic insecticides, organic approved insecticides had a similar or even greater negative impact on several natural enemy species in lab studies, were more detrimental to biological control organisms in field experiments, and had higher Environmental Impact Quotients at field use rates.
These data bring into caution the widely held assumption that organic pesticides are more environmentally benign than synthetic ones. All pesticides must be evaluated using an empirically-based risk assessment, because generalizations based on chemical origin do not hold true in all cases.
Citation: Bahlai CA, Xue Y, McCreary CM, Schaafsma AW, Hallett RH (2010) Choosing Organic Pesticides over Synthetic Pesticides May Not Effectively Mitigate Environmental Risk in Soybeans. PLoS ONE 5(6): e11250. doi:10.1371/journal.pone.0011250
Editor: Stephen J. Johnson, University of Kansas, United States of America
Received: March 30, 2010; Accepted: June 1, 2010; Published: June 22, 2010
Copyright: © 2010 Bahlai et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: The authors acknowledge funding from the Natural Sciences and Engineering Council of Canada (http://www.nserc-crsng.gc.ca/), Agriculture and Agri-Food Canada (AAFC) (http://www.agr.gc.ca/index_e.php) and the Ontario Ministry of Agriculture, Food and Rural Affairs - University of Guelph partnership (http://www.uoguelph.ca/research/omafra/). The funders had no role in data collection and analysis, decision to publish, or preparation of the manuscript. AAFC suggested the insecticide list for testing, and this is the only role any funders played in study design.
Competing interests: In support of other projects unrelated to this study, the authors' research group has received competitive research grants from grower organizations and government bodies and contracts and/or in-kind contributions from manufacturers of both organic and synthetic pesticides. Grant sources for other research projects within the last five years include: Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, Canada Food Inspection Agency, Agriculture and Agri-Food Canada, Ontario Ministry of Food, Agriculture, and Rural Affairs, United States Department of Agriculture, Instituto Nacional de Investigación Agropecuaria (Uruguay), Grain Farmers of Ontario (formerly Ontario Soybean Growers), Ontario Grape and Wine Research Inc., Ontario Wheat Producers Marketing Board, Ontario Corn Producers Association, Agricultural Adaptation Council of Canada, Romer Labs, Bayer CropScience Canada, Bayer CropScience France, Monsanto Canada, Pioneer Hi-Bred Ltd., Dow AgroSciences, BASF Canada, Syngenta Crop Protection Canada, Syngenta Seeds Canada, DuPont Canada Crop Protection, Natural Insect Control, Woodrill Seeds Ltd.
A public call for sustainability in agriculture has resulted in numerous government initiatives to develop environmentally friendly agricultural practices , , , , , . In 2003, the Canadian government initiated the Pesticide Risk Reduction Program to provide infrastructure for the development and implementation of reduced-risk approaches for managing pests in crops . This program, similar to ones in the UK  and USA , sought to reduce environmental risk associated with older chemical insecticides by replacing them with low risk alternatives. Though generalizations about the relative safety of natural and synthetic chemicals have been questioned in the past , these sustainability programs often continue to emphasize the development of organic and natural insecticides for pest control. These programs make the assumption that natural insecticides present less risk to the environment than synthetic insecticides, aligning with public opinion  and influential scientific papers purporting greater sustainability of organic practice .
The sustainability of agricultural practices is a subject of ongoing debate in the literature , , , . Many studies have compared organic, conventional and integrated pest management (IPM) production systems as a whole, but even within a commodity system, the conclusions reached in these studies are widely divergent. A 1999 study  of New Zealand apple production suggested an integrated approach was more sustainable, but a 2001 study  of the same system in Washington favoured an organic management approach. Differing outcomes may be attributed partially to differing geography, climate and pest complexes at the two locations, but it is likely that differences in assessment methodology and the inconsistencies between specific practices classed as organic or conventional at each location were also influential in obtaining the observed results. Comparing organic, conventional and integrated agriculture is not as simple as it may initially appear : each system is characterized by a suite of practices which are ideologically, rather than empirically defined , these systems are not mutually exclusive from each other , , and vary from region to region depending on regulations . Because of these variations, generalizations about the overall sustainability of one system over another are never universal . Pest management practices are often specifically highlighted in the sustainability of organic versus conventional agriculture debate, but much of the debate is fuelled by a fundamental misconception that organic farms do not use pesticides . In fact, organic farms, like conventional farms, have access to a suite of pesticides , ; the primary difference is that organic regulations prohibit all synthetic (i.e.: human-made) chemicals but allow a vast array of mineral and botanical pesticides , whereas conventional pesticides can be both naturally and synthetically derived and are regulated individually, on a per active ingredient, per formulation basis .
Generalizations about the relative sustainability of one suite of practices over another are dangerous when integrated into policy: government regulations based on faulty assumptions about agricultural systems are expensive and do not effectively reduce the environmental risks they are designed to mitigate . It is thus more productive, and more broadly applicable, to evaluate a given tactic for environmental sustainability on its individual properties and build policy based on results of these individual evaluations .
Many national and international initiatives exist to develop environmentally sustainable strategies for managing outbreaks of soybean aphid, including Agriculture and Agri-Food Canada's (AAFC) Pesticide Risk Reduction Program . Soybean aphid is a severe pest of cultivated soybean in North America , and approximately 1.2 million hectares of soybean are cultivated each year in Canada alone . Since its introduction to North America 10 years ago , numerous studies have examined the role of biological control agents in managing populations of aphids , , , , , but foliar insecticides remain necessary when populations of aphids exceed economic thresholds. The need for reduced risk pesticides in this system is profound: only two foliar insecticides are currently registered for soybean aphid control in Canada , one of which is currently under review for re-registration . A broader suite of insecticides with varied mechanisms of action are needed to ensure effective insecticide resistance management can occur .
Working with AAFC, we identified four novel products to evaluate as potential reduced risk insecticides to include in integrated pest management programs for soybean aphid (Table 1). Two of these insecticides contained synthetic active ingredients, the other two are natural insecticides permitted for use in certified organic crops in Canada . We included formulations of the two currently registered insecticides in the experiments as conventional controls.
We completed laboratory assays to estimate the direct contact toxicity of these insecticides to several natural enemy species when applied at field rates (Table 2). We used two of the soybean aphid's primary predator species in this study, multicoloured Asian ladybeetle Harmonia axyridis and insidious flower bug Orius insidiosus , . There were significant differences in mortality by treatment applied for all insect groups F6,657 = 325.25, P<.0001 for ladybeetle adults; F6,993 = 1069.34, P<.0001 for ladybeetle larvae; F6,277 = 228.11, P<.0001 for flower bug adults), but generally, the two currently registered insecticides were most toxic to natural enemies under laboratory conditions. The other four insecticides were much less toxic to the ladybeetle, though it was found that one of the organic insecticides, Beauveria bassiana, was slightly more toxic to adults, and one novel synthetic, flonicamid, was slightly more toxic to larvae than the remaining novel insecticides. The four novel pesticides all caused some mortality to the insidious flower bug, but the two organic insecticides had significantly higher toxicity than the two novel synthetic insecticides.
We conducted a two year, five site study to examine the performance of these insecticides against aphids, and selectivity with respect to natural enemies under field conditions (Fig. 1). In addition to efficacy, it is desirable for an insecticide to have a high selectivity for its target pests in order to minimize environmental impact, and to conserve biological control services provided by other organisms residing in the treated area. All synthetic insecticides had similar efficacy one week after treatment (F6,148 = 7.48, P<0.0001), though dimethoate efficacy was reduced in the second assessment week (Fig. 1a), and yield in plots treated with synthetic insecticides did not differ significantly (F6,90 = 3.51, P = 0.0036) (Fig. 2). The two organic insecticides had lower efficacy than the synthetic insecticides (Fig. 1a) at one week (F1,148 = 25.16, P<0.0001) and two weeks (F1,121 = 17.48, P<0.0001) post-treatment and did not offer significant yield protection over the untreated control (Fig. 2). Field selectivity was highest amongst synthetic insecticides, and lowest amongst organic insecticides included in this experiment (F1,119 = 9.00, P = 0.0033), and though dimethoate had the numerically lowest selectivity amongst the synthetic insecticides, it was still numerically more selective than the organic insecticides (Fig. 1b).
A) Observed efficacy. Aphid count data were Henderson-Tilton adjusted  and subjected to a mixed model ANOVA by post-treatment sampling period with year of experiment, block, pass of tractor, site, and interaction terms between block and pass, block and site, and pass and site incorporated into the model. b) Observed selectivity. Field selectivity was determined using the natural enemy-to-aphid ratio in treatment plots, for exact calculation see Materials and Methods. Observed efficacy and selectivity within sampling period marked by the same letter are not significantly different at α = 0.05 (LSD).
Data were subjected to a mixed model ANOVA with block, site, treatment incorporated into the model. Observed yields marked by the same letter are not significantly different at α = 0.05 (LSD). Data from 2008 were excluded from analysis because of low overall aphid populations.
Net environmental impact of applying each insecticide at given rates was estimated using an Environmental Impact Quotient analysis . The per-unit-EIQ was highest for cyhalothrin-λ, a conventional synthetic insecticide (Table 1), but the EIQ-field use ratings were highest amongst the older synthetic, dimethoate, and the two organic insecticides. The high EIQ-field use rating of dimethoate was due to both a high application rate and a relatively high per-unit EIQ. The EIQ-field use rating for the mineral oil insecticide, though, was more than an order of magnitude higher than that of dimethoate, due to its relatively high per-unit-EIQ and its extremely high application rate. The remaining four insecticides had relatively low EIQ-field use ratings compared with mineral oil and dimethoate.
EIQ allows relative impact of various control strategies within a crop to be ranked; it is a standard method for indexing the total environmental impact of an application of a given pesticide. EIQ relies on data which is commonly available on MSDS sheets, incorporates the application rate of a pesticide, and is not site or pest-specific, so it provides a less biased estimation than other pesticide ranking systems used to quantify environmental impact , . Because EIQ is based on a rating system and does not rely on field obtained data, some authors have criticized its use . However, we found a clear inverse relationship between field selectivity and EIQ for insecticides tested in this study when applied at field rates (Fig. 3), suggesting that EIQ rankings are relevant predictors of at least some in-field parameters for environmental impact, and our results strongly support the continued use of EIQ for ranking pesticide impact. Responses of natural enemy communities are strong indicators of ecological impact of an insecticide, because they are arthropods, like the targets, and are thus likely to be biologically similar to the target of the insecticide, and because they are often found alongside the pest at the time of an insecticide application, heightening their exposure compared to other non-target organisms.
Field selectivities presented as least square means (± SE) of field selectivities observed at four sites in 2009. Equation of regression line is Field selectivity = (3.3±1.7)/EIQ+(0.3±3.1)+site effect, with F93 = 4.23, p = 0.0035.
Looking at the issue empirically, our results show that with regards to environmental impact, target selectivity and efficacy, the novel synthetic insecticides we tested have better performance than organic insecticides; suggesting that certain organic management practices are not more environmentally sustainable than conventional ones. It has been purported that organic systems are not just better for the environment, but are more economically sustainable because of the price premiums associated with organic food . Consumers are often willing to pay more for products they believe are produced in the most sustainable way possible, but we have shown that the organic methods available are not always the most sustainable choice. Carefully designed integrated pest management systems are likely the best strategy for minimizing environmental impact of agriculture: where certified organic systems may reject the technology with the smallest environmental impact based on ideology , IPM maintains the flexibility to incorporate any strategy empirically determined to have the smallest impact. In fact, it has been argued that studies which have concluded that IPM has a greater impact than organic management  have simply tested a poorly designed IPM strategy in which the efficacy and impact of individual tactics included in the program were not effectively examined , did not accurately reflect IPM practice, or employed biased methods of evaluation . Though IPM practice does not typically come with price premiums associated with the production of organic food, IPM strategies are still commonly used by many conventional farmers , and given increased consumer awareness of the benefits of IPM practice, adoption rates are likely to rise.
It is for these reasons that we reject the organic-conventional dichotomy and emphasize that, in order to optimize environmental sustainability, individual tactics must be evaluated for their environmental impact in the context of an integrated approach, and that policy decisions must be based on empirical data and objective risk-benefit analysis, not arbitrary classifications.
Materials and Methods
Selection of insecticides for inclusion in experiments
In May 2008, the Pest Management Centre at AAFC provided us with a list of 14 potential insecticides for inclusion in our experiments. We reviewed each insecticide and eliminated those which had the same mode of action as any other insecticide registered for use against soybean aphid in Canada, and then contacted the suppliers to assess the economic feasibility of using these insecticides in field crops. Two novel synthetic and two organic insecticides were identified to be tested for management of soybean aphid, and the two registered insecticides were included in the experiment as conventional controls. Experimental application rates for novel insecticides were developed in consensus with supplier companies (Table 1). Table S1 provides a complete list of insecticides considered for inclusion in this experiment, and the rationale for products selected.
Determination of direct contact toxicity to natural enemies
Adults and larvae of multicoloured Asian ladybeetle Harmonia axyridis and adults of insidious flower bug Orius insidiosus were treated with formulated insecticides at the equivalent of 0.5, 1 and 2× field rate using an airbrush spray tower. The untreated control consisted of 1 mL of distilled water. Groups of insects (8–10) were anesthetized using CO2 then placed in a 50 mm glass Petri plate lined with a piece of 47 mm qualitative filter paper, treated using the spray tower, and then placed in post-treatment containers. Each insecticide-concentration combination was repeated four times. The spray tower was rinsed with acetone, then distilled water, between each application.
Orius insidiosus assays.
Orius insidiosus adults were obtained from commercial suppliers (BioBest Biological Systems Canada and MGS Horticultural Inc.). Repetitions of 10 adult O. insidiosus were treated, and then placed, post-treatment, in 10 cm plastic Petri plates lined with filter paper moistened with distilled water, and containing 1–2 washed baby spinach leaves, and an excess of frozen Ephistia eggs (BioBest Biological Systems Canada) for food. Mortality was recorded at 18, 24 and 48h post treatment.
Harmonia axyridis adult assays.
Harmonia axyridis were obtained from aggregations on buildings in Guelph, Ontario, Canada, and were reared in laboratory cultures using procedures described by Xue et al . Repetitions of 10 adult H. axyridis were treated, and then placed in 10 cm plastic Petri plates lined with filter paper moistened with distilled water, and containing several barley leaves infested with bird-cherry oat aphid (Aphid Banker System; Plant Products, Brampton, Ontario, Canada), and an excess of frozen Ephistia eggs (BioBest Biological Systems Canada) for food. Mortality was recorded every 24h for 168 h (7 d).
Harmonia axyridis larvae assays.
Second and third instar H. axyridis were obtained from the laboratory culture described above. Assays were performed as adult assays above, except repetitions consisted of 8 individuals and instead of being placed together in a Petri plate, were placed individually into cells of a rearing tray (BIO-RT-32, C-D International, Inc.) with Ephistia eggs and aphid-infested barley to avoid cannibalism.
Statistical analysis of bioassay data.
Mortality data was normalized using the Henderson-Tilton adjustment , and subjected to a mixed model ANOVA accounting for concentration (relative to field rate), treatment, and assessment time. Assessment time was treated as a repeated measure in the analysis.
Determination of field efficacy and selectivity
In 2009, four soybean fields in southwestern Ontario with aphid populations approaching the action threshold of 250 aphids per plant were identified in collaboration with government extension personnel in July and August, 2009. After obtaining permission from landowners, sites were assessed once weekly until aphid populations exceeded 250 aphids per plant. Upon reaching this threshold, field experiments were initiated. In our initial screening trial in 2008, treatments were applied to a single site with a moderate density of aphids (~120 aphids per plant), due to low aphid populations across our region during that year.
Field experiments employed a RCBD consisting of four blocks of 15 3.7×15.2m beds, with 3 untreated controls per block (one for each tractor pass required), our six insecticides and six other products or formulations not reported in this study. Insecticides were applied using a Teejet Duo nozzle configuration with spray tips #TT11002 at a height of 50cm above the canopy. Spray pressure at the nozzle was 276 kPa and the tractor travelled at a ground speed of 9.7km/h. Fluid delivery rate was maintained at 187 L/ha for all treatments. 2–3 soybean plants were destructively sampled from each bed at each assessment, and assessments were completed 1) immediately before treatment, 2) one week after treatment and 3) two weeks after treatment. Total numbers of aphids, ladybeetles, lacewings, parasitized aphid mummies, syrphid larvae, and flower bugs were assessed on each plant.
Aphid counts were transformed using Henderson-Tilton adjustments to account for population changes in the control between time of treatment and time of assessment, then subjected to a mixed model ANOVA accounting for site, year, tractor pass, replicate, and treatment.
Field Selectivity Calculation
Field selectivity of each insecticide was estimated by calculating the change in the ratio of natural enemies to aphids in each plot, and subjecting these data to a mixed model ANOVA as above. We defined field selectivity as the relative change in the natural-enemy-to-pest population ratio observed after treatment. We standardized the counts of natural enemies of different species by defining a Natural Enemy Unit (NEU), where 1 NEU is the number of predators or parasitoids required to kill 100 pest insects in 24h. Thus,where N is the total number of natural enemy species, ni is the total number of individuals of natural enemy species i observed on 10 plants, and Vi is the average voracity of natural enemy species i, that is, the number of pest insects it can kill in 24 h divided by 100. Using functional response data obtained by Xue et al. , we defined our soybean aphid ecosystem specific calculation as:where nladybeetles is the total number of adult and larvae of ladybeetles of Harmonia axyridis or Coccinella septempunctata, nmummies is the total number of parasitized aphids, nsyrphids is the total number of Syrphidae larvae, nOrius is the total number of Orius spp., and nlacewings is the total number of Chrysopidae observed on 10 soybean plants.
Field selectivity was defined as the ratio of NEU/Aphids (NEU/A) after treatment to NEU/A before treatment, normalized by the control, as in the Henderson-Tilton adjustment , and took the form:
This selectivity index results in values <1 if a treatment kills more natural enemies than target pests, and values >1 if a treatment kills more target pests than natural enemies. Larger numbers will indicate a more target-selective pesticide. The selectivity index assumes the applied treatment has at least some efficacy against the target pest.
Environmental Impact Assessment
EIQs were estimated using established methodology ,  incorporating data from MSDS sheets provided by the supplier of the insecticides, an EIQ-field use rating was calculated for each insecticide, using the assumption that one application at field rate per season would provide equivalent aphid control. See Table 3 for values used in the calculation of EIQ for Beauveria bassiana, which does not have an existing published EIQ value.
Complete list of insecticides under consideration provided by Agriculture and Agri-Food Canada (AAFC).
(0.04 MB DOC)
We sincerely thank L. Des Marteaux, D. Makynen, J. Smith,T. Phibbs, D. Hooker, A. Gradish and T. Baute for technical assistance on this project, R. Norris and B. Stirling for the use of their respective farms, C. Scott-Dupree for use of the airbrush spray tower, M.K. Sears and J.A. Newman for providing comments on this manuscript, and C. Petzoldt and D. Marvin for providing support with the EIQ method. We would also like to thank Syngenta, Bayer, FMC, UAP, and Laverlam for providing insecticides for our experiments, and BioBest Canada and MGS Horticultural for providing insects.
Conceived and designed the experiments: CB AWS RHH. Performed the experiments: CB YX CMM. Analyzed the data: CB. Contributed reagents/materials/analysis tools: CB YX. Wrote the paper: CB RHH.
- 1. Agriculture and Agri-Food Canada (2003) Pesticide risk reduction and minor use programs: improving ways to manage pests with new technology. Government of Canada.
- 2. Lynch S, Greene C, Kramer-LeBlanc C (1996) Proceedings of the third national IPM symposium/workshop: Broadening support for 21st century IPM. U.S. Department of Agriculture, Economic Research Service, Natural Resources and Environment Division.
- 3. Jones E (2004) Grants awarded to develop pesticide risk reduction programs. United States Environmental Protection Agency, Press release 10/14/04.
- 4. U.K. Department for Environment Food and Rural Affairs and the Forestry Commission (2005) Departmental report 2005. United Kingdom Department for Environment, Food and Rural Affairs.
- 5. European Commission (1991) Council directive of 15 July 1991 concerning the placing of plant protection products on the market. European Union.
- 6. Ministry of Science and Technology of the People's Republic of China (2001) National High-tech R&D Program (863 Program).
- 7. Ames BN, Profet M, Gold LS (1990) Nature's chemicals and synthetic chemicals: comparative toxicology. Proceedings of the National Academy of Sciences, USA 87: 7782–7786.
- 8. James KH (1990) Risk perceptions and food choice: An exploratory analysis of organic- versus conventional-produce buyers. Risk Analysis 10: 367–374.
- 9. Reganold JP, Glover JD, Andrews PK, Hinman HR (2001) Sustainability of three apple production systems. Nature 410: 926–930.
- 10. Lynch D (2009) Environmental impacts of organic agriculture: A Canadian perspective. Canadian Journal of Plant Science 89: 621–628.
- 11. Trewavas A (2001) Urban myths of organic farming. Nature 410: 409–410.
- 12. Elliot SL, Mumford JD (2002) Organic, integrated and conventional apple production: why not consider the middle ground? Crop Protection 21: 427–429.
- 13. Shepherd M, Pearce B, Cormack B, Philipps L, Cuttle S, et al. (2003) An assessment of the environmental impacts of organic farming. United Kingdom Department for Environment, Food and Rural Affairs.
- 14. Suckling DM, Walker JTS, Wearing CH (1999) Ecological impact of three pest management systems in New Zealand apple orchards. Agriculture, Ecosystems & Environment 73: 129–140.
- 15. Avery AA (2006) Organic pesticide use: What we know and don't know about use, toxicity, and environmental impacts. Crop Protection Products for Organic Agriculture. Washington, DC: American Chemical Society. pp. 58–77.
- 16. Thompson DG, Kreutzweiser DP (2006) A review of the environmental fate and effects of natural “reduced-risk” pesticides in Canada. Crop Protection Products for Organic Agriculture. Washington, DC: American Chemical Society. pp. 245–274.
- 17. Canadian General Standards Board (2008) Organic productions systems permitted substances list. Government of Canada.
- 18. Ontario Ministry of Agriculture Food and Rural Affairs (OMAFRA) (2005) Field crop protection guide 2005–2006. Ontario Ministry of Agriculture, Food and Rural Affairs.
- 19. Kleijn D, Berendse F, Smit R, Gilissen N (2001) Agri-environment schemes do not effectively protect biodiversity in Dutch agricultural landscapes. Nature 413: 723–725.
- 20. Ragsdale DW, Voegtlin DJ, O'Neil RJ (2004) Soybean aphid biology in North America. Annals of the Entomological Society of America 97: 204–208.
- 21. Statistics Canada (2009) Cereals and oilseeds review. Ministry of Industry, Government of Canada.
- 22. Heimpel GE, Ragsdale DW, Venette R, Hopper KR, Neil RJ, et al. (2004) Prospects for importation biological control of the soybean aphid: anticipating potential costs and benefits. Annals of the Entomological Society of America 97: 249–258.
- 23. Rutledge CE, Neil RJ, Fox TB, Landis DA (2004) Soybean aphid predators and their use in integrated pest management. Annals of the Entomological Society of America 97: 240–248.
- 24. Costamagna AC, Landis DA, Difonzo CD (2007) Suppression of soybean aphid by generalist predators results in a trophic cascade in soybeans. Ecological Applications 17: 441–451.
- 25. Desneux N, O'Neil RJ, Yoo HJS (2006) Suppression of population growth of the soybean aphid, Aphis glycines Matsumura, by predators: the identification of a key predator and the effects of prey dispersion, predator abundance, and temperature. Environmental Entomology 35: 1342–1349.
- 26. Xue Y, Bahlai CA, Frewin A, Sears MK, Schaafsma AW, et al. (2009) Predation by Coccinella septempunctata and Harmonia axyridis (Coleoptera: Coccinellidae) on Aphis glycines (Homoptera: Aphididae). Environmental Entomology 38: 708–714.
- 27. Health Canada Pest Management Agency (2009) PMRA Re-evaluation Workplan (April 2009 to March 2010). Government of Canada.
- 28. Brattsten LB, Holyoke CWJ, Leeper JR, Raffa KF (1986) Insecticide resistance: Challenge to pest management and basic research. Science 231: 1255–1260.
- 29. Kovach J, Petzolt C, Degnil J, Tette J (1992) A method to measure the environmental impact of pesticides. New York's Food and Life Sciences Bulletin 139: 1–8.
- 30. Levitan L, Merwin I, Kovach J (1995) Assessing the relative environmental impacts of agricultural pesticides: the quest for a holistic method. Agriculture, Ecosystems & Environment 55: 153–168.
- 31. Olson KD, Badibanga T, DiFonzo C (2008) Farmers' awareness and use of IPM for soybean aphid control: report of survey results for the 2004, 2005, 2006, and 2007 crop years. University of Minnesota, Department of Applied Economics.
- 32. Henderson CF, Tilton EW (1955) Tests with Acaricides against the brown wheat mite. Journal of Economic Entomology 48: 157–161.
- 33. Kovach J, Petzolt C, Degnil J, Tette J (2009) A method to measure the environmental impact of pesticides: Table 2, List of pesticides.