This is one of this year's brood. They tend to feed on the plant's calyx first, then chew their way into the seed capsule and eat the seeds.
The pictures below are from a brood from an earlier year.
Here you can see it chewing away at the calyx to expose the tip of the seed capsule ..
.... then it eats its way into the seeds.
The caterpillars often tend to spend the day inside empty seed capsules and ....
............if you give the capsule a gentle squeeze it will often emerge.