Weather affects nearly all aspects of agriculture but we received several questions regarding insecticide treatment. Decreased residual effectiveness can have a direct cost for agricultural operations. Ineffective treatments may result in additional costly treatments and the potential for crop damage. An accurate forecast can help make the educated decisions required on a daily basis. For example, an application of insecticide will have reduced residual effectiveness depending on several weather factors;
Amount of Sun
Wind Speed and Direction
The problem with answering this question is the multitude of factors involved. How much rain is expected? What will be the intensity of the rainfall? Specific crop? Spray technique and wind direction at the time of application? Targeted pest? Amount of organic matter in the soil? Soil temperature and moisture? Is the insecticide absorbed by the plant (systemic) or coat the exterior (contact). Most of the dedicated studies I found cover a specific set of factors and will not necessarily assist in making an educated decision for your specific situation.
Here are some helpful points that can help you make a decision. Of course specific limitations can be taken directly from the product label.
If the insecticide is absorbed by the plant, the absorption may take a few hours depending on the plant and the insecticide. If the insecticide coats the surface you have an obvious reason to be concerned. However, the ability of the contact insecticide to remain affective is dependent on the many weather factors listed above.
A more soluble insecticide also means that it will be absorbed quickly by the plants.
Insecticide will attach to organic matter in the soil which can help maintain the insecticide where you need it. In fact some insecticides do better when a light rain helps move the insecticide into the soil.
Where does the targeted insect feed on the crop in question? If the insect is going to come in immediate contact with the insecticide, the upcoming rain may be of little concern.
At a minimum try to spray 4 hours prior to the rain and not directly following a rainfall where the leaves are still wet. For Acephate you will want to add another 8 hours.
Spray with lower temperatures and higher humidity (reduces evaporation of the insecticide). Try to spray when the relative humidity is above 40% and the temperature is below 77ºF (25ºC).
Ideal wind speeds for spraying are between 2-6 mph (3.2-9.6 kmh)