In looking at your picture, we wonder if maybe that stem is one that has already flowered and has produced seeds- which may be what the black dots are.
 Could that be the case? 
You can just pinch that stem off. Spent flowers on annual bloomers should be pinched off so that energy goes into making new flowers instead of making seeds.
Can you look at the black dots under magnification and make out any segments (could be frass or caterpillar droppings), legs or anything else?
The good news is the plants look healthy with lots of new growth coming which will give you flowers in time.


Christine

Hi Wanda, 

You can try to put some on a white piece of paper or background and put a ruler or pen tip next to them to help focus your camera and try resending some photos to us. That will give us a scale figure and we may be able to zoom in to see better. Just make sure the photo file size is at least 1MB.

Emily

Thank you for the additional photo. They appear to be snapdragon seeds more so than insect frass, though it's hard to be certain. While generalist caterpillars, grasshoppers, crickets, beetles, and katydids could all be responsible and capable of eating several species of unrelated plants, we think the issues with the petunia are probably unrelated. (The hornworm caterpillars that eat tomato foliage, for example, can also eat petunia, because they are related plants.)

You can check the plants after dark to see if the culprit reveals itself, since some insects only feed after dark. While some insecticides could control chewing insects, they also might render the flowers risky for pollinator health, so we would generally not recommend their use on flowering annuals that are often in bloom, like petunias. You are welcome to share photos of the petunia damage in case we recognize what is chewing those flowers/buds.

Miri