I finally got rid of the catepillars but the flowers are gone. Will they come back? Is there something I can do to help the flowers return? What is the best way to prevent catepillars from ever showing up?
Caterpillars have a life cycle the same as every other insect /creature, they are all different in as much as the Butterfly that lays it's eggs only do so when they know there chooses / preferred plants are in season as this offers there eggs a place to hatch into the caterpillars and then butterflies they need to keep producing.
I personally don't like spraying my plants to rid them off these very damage causing insects, BUT I do know people who get so disturbed about any insects / bugs that come close to there patch, they will go buy the most powerful / poisonous chemicals permitted for sale to the public, so really maybe trying to reach a happy medium is the way to go.
I use a water spray to get rid of some types of Caterpillars but if that cant be done, I go hand pick them off the plants , cabbages, or other types of plants depending on the type of butterfly / moth has hatched, I drop them in a plastic container with lid and take them a walk to where we allow Nettles to grow as this is also a host plant for over wintering butterflies.
Around May time here in UK is when a lot of these are about looking for their host plant to lay the eggs so at that time I go around looking at the back of the leaves to find the eggs, usually laid in batches of a dozen or more on each leaf, they can be so tiny for some type of butterfly or a bit larger for others, once you find them all you do is either hose them off (at the egg stage they cant crawl back onto the plants and sometimes the birds follow me around to grab the eggs they forage for, OR crush the eggs on the plant, between finger and thumb, if squeamish wear a glove,
If you go to the Library or book store you will pick up a hand / pocket book of all the type of insects including their individual larvae state, when the lay eggs, what they feed on, and the beautiful type of Butterfly they grow into all in a season BUT sometimes it can take 2 seasons for the butterfly to unravel from the pupa it was woven into. Hope this helps you realise that there are ways to deal with these insects AND not always with Chemicals.
Good Luck and Best regards.
Tobacco Budworms are very common pests of Petunias.
Spray with Bacillus thuringiensis before the adults lay the eggs, and when the eggs hatch the babies will just eat a few bites and get a lethal dose.
Now, I would trim off the flower buds that have been eaten, cut the plants back just a bit (get rid of raggedy, half eaten leaves) and use a fertilizer with moderate nitrogen and somewhat higher phosphorus and potassium. Maybe a 5-10-10 or similar. You want to feed the leaves (nitrogen) so they make a come-back, and can support more flowers, and you want to feed new flowers.
Petunias are quite durable, and can make a spectacular come-back. Just take care of the caterpillars before they reach the point of eating all the flowers!
Are there seed pods forming? Some Petunias get so busy making seeds, they don't continue to make flowers unless those are removed periodically.
I try to find out what kind of caterpillars are present before deciding what to do about them. (In this case, I'd search "petunia host plant" to find appropriate pics and match the pictured results to the caterpillar on my plant.) If they're going to be a pretty butterfly, they're probably welcome to munch on the plants. If they're something less desirable, I've never seen one that can survive being halved with the scissors.