Those "tiny black pellets " are -- umm -- caterpillar poop.

The eggs are small, slightly elongated, off-white and sit on end. Look for them on the backs of the leaves. Squish when found.

And yes, remove the cats you see, then repeat.

And put row cover over the plants but be certain to check now & then for at least the first week because you will have missed some of the eggs & the very young cats.

Then resolve that next year you'll add the row cover when you plant, securing all edges.

The cabbage white butterfly population has 3 or even 4 broods each year. The current brood of egg-layers will be dead in a few weeks; they require about 60 degrees or warmer to fly, so cooler autumn weather will stop them also.

And they will start all over again in early April, but later in zone 4.

Caterpillar poop? I'll be on the lookout--it's alot easier to see than the caterpillars. Thanks for the tip!

I MacGyvered a row cover out of repurposed drip irrigation tubing, PVC slip junctions and t-connectors, bridal tulle fabric, and strips of scrap lumber. Too bad I can't post a pic, 'cause I think it's pretty slick (as well as economical). I'm calling it "Death Camp for Caterpillars," and will refine it over the winter so it can go on when I set out the new plants next Spring. Very glad to learn that the butterflies' days are numbered since first frost isn't supposed to arrive here until mid-November.

I am also trying spinosad spray from the nursery on the leaves and hearts of the plants, though I wonder how effective it will be with all of the hosing I'm also doing. And some of the caterpillars either have a disease (they're fatter, whitish, and unmoving), or they're actually something else, like slugs.

Slugs don't have legs.
;-)
Mike

fatter greyish things may also be crane fly larva, but they would be on/in the ground not up on the plants.
Caterpillars don't leave shiny slime.

...sometimes caterpillars get lethargic just before they pupate, and they would be at their fattest at this time.