The natural environment can cause many things to change over time.
Sunlight can change the color of objects if they are left outside.
For example, a sheet of colored paper will start to fade if you leave it in the sunlight in a few days. Similar to how paper changes color in sunlight, chemicals like methoprene can also change when they are in a natural environment. Chemicals can change into another form when they get exposed to light, heat, or other chemicals in the environment. Because chemicals can change like this when they are in the environment, it is possible that methoprene is changed into a new form after it gets sprayed. The new form of methoprene might cause deformities in frogs, even if methoprene in its original form does not.