One other thing I thought of, is Asparagus beetles. They lay those tiny eggs all over, then the tiny caterpillars hatch out and eat on the ferns. It's easy to kind of forget about the asparagus after you quit cutting, and they look fine from a distance, but if you look close, you can have lots of those caterpillars on the plants. If they eat a lot of the plants, that could cause them to decline.
I usually have to spray mine with sevin a few times through the season.
I usually have to spray mine with sevin a few times through the season.