The goal of a pesticide is to interrupt the life cycle of a pest at a vulnerable point. For example, a leaf miner application on a birch tree will be effective if applied at the end of April when the larvae are active. However, if the wrong materials are tank mixed, problems will occur. To avoid these problems, the most important thing to do is read the label to see if the products that you're using are compatible. As pesticides get older, they lose their effectiveness.
Many pesticides carry caution statements about applications during hot weather.
Water can evaporate from the spray droplets, concentrating the product and causing it to burn foliage. If a spray mix is to concentrated, it may harm the plant that it's applied to. On the other hand, if it is diluted, it won't control the targeted pest. Read and follow the label directions. Generally, pesticides must come in contact with the pest in order to be effective.