There are many insects that can chew leaves, often after dark (hiding away from the plant during the day), so you can check the foliage with a flashlight at night to see if anything can be caught in the act. Make sure to check the leaf undersides as well since leaf-eating insects often rest there between or during feeding. Culprits may include beetles, caterpillars, sawfly larvae (which look fairly caterpillar-like), young grasshoppers and katydids, crickets, earwigs, and even slugs/snails. Beetles are one of our prime suspects given the pattern of chewing and simply because of how numerous and diverse leaf-feeding beetles are this time of year. If you do find caterpillars, it may be worth identifying them first before removal, since at least one or two species of local butterfly uses sunflower-family plants as host plants for their young.

Fortunately, plants can withstand this degree of leaf damage and be fine without intervention, and since these plants will be popular with pollinators and other wildlife later this season when they come into bloom, we do not recommend the use of an insecticide given how relatively minor the damage currently is. If necessary, you might be able to forego pesticide use and protect the plants with an insect netting cover until they are ready to flower as a way to block access to most leaf-chewing insects, but  unless damage increases significantly this may be more trouble than it's worth. Bear in mind that leaf injuries cannot heal, so even if the culprit is long gone (either having matured and left or been eaten by a predator), the easiest way to tell if damage is ongoing is to look at the youngest growth for indications of fresh chewing. Many insects will also leave behind frass (insect poop), dark pellets that sometimes accumulate in the juncture of leaves and stems or on the leaves themselves, so new frass residue can indicate an insect is still active and visiting the plant. If you find a suspected culprit but aren't sure if it's the one causing damage, feel free to send us photos for ID. Sometimes beneficial insect predators hang out on damaged plants and can be confused for the culprit themselves.

Miri

This does appear to be frass, though it's hard to be certain. We can't quite tell what insect may have left it, but a caterpillar or beetle is likely. We are starting to see question submissions of other chewing damage left by insects like May/June Beetles, Masked Chafers, and Asiatic Garden Beetles, and these beetles are also often attracted to night lighting (porch lights, landscape lighting, etc.) so also might be drawn-in to flowers close to homes due to this behavior.

Miri