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Wednesday, November 30, 2011

Understanding the Impact of Pesticides and Choosing Those with the Least Impact

Karl Foord, UMN Extension Educator

I often see recommendations to use the pesticide with the least impact when controlling pests. However, prior to spraying every effort should be made to avoid pest outbreaks by using the best management practices for a particular crop. For example, most fungi need a period of wetness for their spores to germinate. Managing systems to permit maximum airflow reduces drying time on leaves and reduces the opportunities for fungal spores to germinate.

For the purposes of this article let's assume that all best efforts were made and a spray as the last resort was required. How would you go about choosing the one with the least impact? The first question might be impact on whom, with the second being how one would measure such impact. At a University of California Davis website a series of pesticides is listed. Each pesticide is rated according to its impact on aquatic live, beneficial insects, honeybees, and humans. The human impact is separated into acute and long term impacts. Acute being what can happen to you today, and long term being what can happen over a number of years due to continued exposure at lower dosage rates.

Each chemical is given a potential hazard rating based on a series of other documents and warnings on the chemical's label. These are complicated but can be accessed at the website previously mentioned. The ratings range from no risk, no known risk, and very low risk to very high risk or no data available. For those pesticides labeled for strawberry, the impact information has been consolidated into a table where the materials have been ranked from lowest risk to those of highest risk (table 1). For example if you encountered slugs (mollusks) in your strawberries, the less impactful of the two active ingredients would be iron phosphate and not metaldehyde. So looking for a product with this as the active ingredient would be the first choice.

If you encountered tarnished plant bug in your strawberries, you would want to choose an insecticidal soap as a first choice over malathion. If you were forced to go to malathion you would realize that you would want to avoid any situation where the spray could get into surface water. You would also want to be particularly sensitive beneficial insects and honeybee pollinators and not spray when they are active, most likely after dark.

This table should permit you to select the least impactful chemical, and to apply it in a manner producing the least impact through an understanding what organisms were at risk from the application.

Karl Foord

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