Often to identify a pest eating plants, you need to see the culprit. One idea is to lay a board on the soil nearby, and check it in the morning and even later in the day, to see if slugs and snails are hiding there. Drop those you find into a carton of water with drops of soap. Another plan is to go out after dark with a flashlight and carefully check the plant leaves and soil around the stem. Are there cut worms, larvae, anything else? Maybe weevils? There are also tiny “worms” or caterpillars that eat along the petal or leaf edge, are tiny and often the color of what they are eating. An example is budworm. You have to look closely to get them removed.
Pansies are at their weakest this late in the year and due to heat. This means they may not out-grow the damage very well.
Treatment for many pests is manual removal, checking everyday.
There can be other treatments, based on what you find. Share photos if you need help identifying insects.
Some references: EM 9155 for slug control
Budworm information from Colorado Extension
Weevils on flowers information 

I’m not having success finding a suggestion for you. Do you have a magnifying glass to look for pests on the pansies?