Rainfastness of Fruit Insecticides
In the last couple of years, fruit producers have had access to several new insecticides for managing crop pests, and a common question is "how long do they provide control". After significant precipitation, the next question is usually "how do rain events influence the performance of insecticides?" In other words "Do I need to re-apply?".
Residual activity describes the persistence of an insecticide as a crop protectant against a given pest; knowing the residual activity is important in terms of knowing when re-application is required. In very general terms, pyrethroids and carbamates have short residual activity, OPs and neonicotinoids have medium residual activity, and insect growth regulators (Rimon, Intrepid), the new spinosyn Delegate and the new anthranilic diamide Altacor have relatively long residual activity.
Product labels often provide limited - or zero - information on rainfast periods for insecticides. The rainfast period is the point at which precipitation will not alter the performance of a product. Unless specific information is provided, we assume the product is rainfast when dry. But for how long?
Dr. John Wise at Michigan State University has conducted controlled trials to determine the rainfast characteristics of insecticides used on fruit crops, as they relate to performance of products. This information has been presented at several workshops and conferences, and was recently published in the MSU Fruit Crop Advisory Team Alert (May 11, 2010)*. The research is ongoing, but here are the results thus far.
There are a number of factors that influence the impact of precipitation on insecticides.
The first is the wash-off potential of the product, which often differs between fruit and leaves. OPs are highly susceptible to wash-off. Pyrethroids, carbamates and IGRs are moderately susceptible. Surface residues of neonicotinoids are moderately susceptible to wash-off, but because of systemic / translaminar properties, any residues that have moved into plant tissue are highly rainfast. Altacor and Delegate exhibit rapid translaminar movement into plant tissues and have strong rainfastness.
The second factor is the toxicity of the insecticide to the target life stage. Even if a product is susceptible to wash-off, enough residues can remain on the plant to kill the pest if the target is really sensitive to it.
The third is the amount of rain - not the duration - from a precipitation event, and the timing of the rain event relative to the time of application. Most products stand up well to 1 cm of rain if the product was applied recently. With 2-3 cm of precipitation a week after application, plants treated with products like Delegate and Altacor will still have sufficient residues, but most others will require re-application, if warranted. If your insecticide went on a week ago and there has been 5 cm of rain, no product is going to provide reliable control and re-application would be recommended if the pest is still present.
* Here is the link to Dr. John Wise's article: http://www.ipm.msu.edu/cat10fruit/F05-11-10sm.pdf
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