as soon as the spears start coming up

Silly question but you are looking for larva first right?

They emerge as adults that have overwintered and then breed and lay eggs.

My local paper wasps love them as a food source (eating the larvae stage). The wasps routinely patrol the ferns for a snack. When we had asparagus the adult pests showed up after a few skinny spears had grown into the first bushy ferns. I allow paper wasp nests to grow in size all summer as long as they aren't in a high traffic spot.

Like Jack said adult beetles come out of the ground when the spears emerge. If all the spears are harvested you may find adults sitting on the top of a spear looking for mates. If a spear is allowed to grow and fern out the adults will find it and congregate for an orgy, and the best place to hunt them down early and reduce the population.