I don't know if there's any chemical that works, I don't use anything like that. There are three signs, the beetles themselves which are small, spotty, jump like kangaroos, and are really hard-cased (fingernail to squish, not just finger), the black eggs, which they glue to the stem, a bad attack can look like a tiny bottle brush, and if you miss those you will have revolting grubs, rather like tiny slugs.
Apparently they overwinter in the soil at the base of the plants, though I've never seen them even though I try to keep the bed really tidy. If you don't spot them, the grubs will turn your asparagus fern to asparagus lace!
I check the bed twice a day from the first sign until the fern starts to yellow....