Spring rainfall is a mixed blessing for many. While plants require the sunshine and warm spring temperatures for growth, water is also necessary. Following droughty growing seasons, spring rains are needed to replenish moisture in the soil profile. Granular fertilizer is also applied to the soil to replace nutrients lost during the previous growing season from leaf removal, pruning, or harvesting of a crop. Rainfall soon after nitrogen fertilization helps dissolve the granules and moves the nutrient into the soil where plant roots absorb and utilize it for growth.
However, rain also promotes disease infection and interferes with spring chores, such as pruning and mowing, and the application of pesticides. Rainfall soon after a spraying can also affect the performance of the pesticide. Some of the factors that influence the "rainfastness" of an insecticide are the capacity of the chemical to penetrate the plant tissue, the inherent toxicity of the product, the target plant tissue, and the amount of rainfall.
Researchers from Michigan State University found that the total amount of rainfall is more important the duration. Also, insecticides tend to be more rainfast on apple foliage than on fruit when there is less a half inch of precipitation. However, as the amount of precipitation increases to two inches, both types of apple tissues tend to become more susceptible to pesticide wash-off so reapplication is necessary sooner to protect the crop. Organophosphate-type insecticides, such as Guthion and Malathion, are very susceptible to wash-off from rain because they do not readily penetrate cuticle layers on plant tissues. However, because Guthion is highly toxic, it does not require reapplication after precipitation as soon as some of the other insecticides. Pyrethroid, carbamate (such as Sevin), and insect growth regulator insecticides are generally moderately susceptible to wash-off. Once a neonicotinoid product is absorbed by the plant, it is systemic, and therefore very rainfast, even though the surface residues can wash off. Diamide and spinosyn-type insecticides (e.g., Spinosad) are also very rainfast.
For apple trees, early sprays are generally applied every 7 to 10 days and summer cover sprays are applied every 10 to 14 days. However, when one or two inches of rainfall occur, reapplication of Guthion is necessary to control codling moth larvae on apples seven days after the first application. In contrast, Assail should be reapplied after 7days when only one-half inch of precipitation occurs. For grapes and blueberries, most of the insecticides used on these crops must be reapplied after 7 days when a half-inch of rainfall occurs. For more specific examples see: http://news.msue.msu.edu/news/article/rainfast_characteristics_of_insecticides. Also, product labels usually contain information regarding the minimum time required for pesticide absorption before rainfall.
Thirty year monthly averages for central Missouri are 4.5 inches of rainfall in April and June and 5 inches of precipitation for May. Therefore, spring showers bring not only flowers, but also headaches when it comes to protecting fruit trees from pests!
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REVISED: April 30, 2012