Formicpro mite treatment should be used when the outdoor temperature range is between 10-29.5 °C and the weather conditions are fair. The bees play an important role in circulating the formic acid vapours throughout the hive so fair weather is the best time to do this. If there is heavy rain forecast in the 7-day period that you choose to carry out the treatment, it would be best to select another time to carry out the treatment.