What are these yellow worms on
my plant?

These are Genista caterpillars. They a type of web worm caterpillar. Horticulturist Jason Reeves says the worm appears in mid- to late summer and can defoliate the plant, in this case baptisia. He says even if they defoliate the plant, it will be fine next year.
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Caterpillars on Parsley
How do I get my severe webworm infestation under control?

Related Resources:
Genista Broom Moth / Genista Caterpillar
Genista Caterpillar on Baptisia