Unpredictable weather often makes pest control difficult in Missouri during April, May, and June. Even with good planning, unexpected showers may occur after a pesticide application. Rainfall or irrigation soon after application can dilute or remove the pesticide from the targeted plant surface. The decision to re-spray a pesticide depends on its rainfastness, which is the ability of the product to remain effective after rainfall or irrigation.
Product labels generally list the amount of time needed before rainfall or irrigation, but rainfastness can vary, depending on the time, amount, and duration of the precipitation, as well as formulation and chemistry of the product. Generally, rain immediately after application removes much of the pesticide. The longer the time before precipitation, it is more likely that the pesticide will remain on the plant surface or will be absorbed into the tissue. After a product dries on the plant tissue or is absorbed, it will perform as expected in spite of rainfall or irrigation.
Many fungicides and insecticides are effective when one inch of rain occurs at 24 hours after application. However, heavy rainfall (two inches or more) within 24 hours of application generally requires reapplication of fungicides and insecticides. Also, one inch of rainfall during a one hour period results in greater loss of pesticide efficacy than a slow drizzle lasting several hours.
Pesticides are formulated as dusts, wettable or soluble powders, granules, emulsifiable concentrates, or flowable liquids. Some products include adjuvants, which are ingredients added to the formulation to improve their efficacy and rainfastness. Dusts are the most prone to wash-off during precipitation. Wettable powders or granules are generally more rainfast than dusts, but pesticides formulated as emulsifiable concentrates are usually more rainfast. When a pesticide such as Sevin is available in multiple formulations, choose the more rainfast formulation when rainfall is unpredictable
Systemic fungicides, which are absorbed by the foliage, perform better than contact products during rainy weather. Common systemic fungicide products for fruit crops include Abound, Propimax, Quadris, Topsin M). Contact fungicides, such as Captan and Daconil protect against infection on leaf surfaces.
Organophosphate insecticides, such as Diazinon, Imidan, Supracide, and Lorsban, have contact activity and are susceptible to wash-off. However, immediate re-application is rarely needed after light rainfall due to the high toxicity of organophosphates. Products such as Delegate, Entrust, Agri-Mek, Confirm, and Esteem penetrate the leaf cuticle and are less susceptible to wash-off after drying. Systemic neonicotinoid products (Actara, Admire Pro, and Assail) are highly rainfast after plant absorption, but are moderately susceptible to wash-off when rainfall occurs in less than 24 hours after application. Rainfastness ratings of several insecticides can be found at http://msue.anr.msu.edu/news/rainfast_characteristics_of_insecticides_on_fruit.
The rainfastness of herbicides varies considerably among products. For example, older formulations of glyphosate (Roundup) require application 6 to 12 hours before rainfall or irrigation. However, new formulations of glyphosate that include an adjuvant require application 30 to 60 minutes before precipitation. Other herbicide products applied to actively-growing weeds may be rainfast with an hour, while others require up to 6 to 8 hours after application. In contrast, pre-emergent herbicides applied to bare soil require rainfall within a few days after application to become active against germinating weeds.
REVISED: February 21, 2017