There are two species of asparagus beetles: common and spotted. All life stages feed upon asparagus plants, and organic controls include good fall cleanup, attracting beneficial insects, letting poultry out (if you have them), spraying adult plants with neem oil or dusting with diatomaceous earth. (Details below.)
Adult Common Asparagus Beetle
(Clemson University Extension)
Adult Spotted Asparagus Beetle
Asparagus Beetle Larva
(Clemson University Extension)
The growing asparagus plant is susceptible at all stages of growth to attack by all life stages of beetles: adults, larva and eggs. Adults feed on both newly emerged asparagus shoots, as well as the stems and leaves of the adult plant. Larva feed on the red berries.
"Who let the ducks out?!"
The best way to control beetles is to set your chickens or ducks out to gobble them up like candy! Fortunately, for those without duck or chicken helpers, there are other organic control of asparagus beetles.
Practice good fall cleanup.
It is important to remove and destroy (or dumpster) all the died-down crop waste from asparagus tops, because the adults will overwinter in the debris.
Handpick adult beetles and larva off plants.
This is tedious, but does actually work. Squish them if you're not squeamish, or drop them in a coffee can with some soapy water or molasses in the bottom.
Harvest young short spears early.
Harvesting the spears when they are only 4-7" high often gets them before the eggs can hatch and begin sharing the harvest. Wash freshly picked spears and cook right away. Harvest by cutting spears off at soil level.
Spray adult plants with neem oil.
Neem oil works in a different way than other pest controls. It is effective against asparagus beetles, but it's not an overnight cure. It disorients the beetles and disrupts their feeding and reproductive cycles. Safer brand insecticidal spray may also be effective. Spray the adult plants later in the summer, but do not use on young fresh spears you are about to harvest and eat. Read more about neem oil here.
Plant a flower insectary for beneficial insects.
Asparagus beetles are also preyed upon by parasitic wasps and ladybugs, so making a good habitat for beneficial insects is another helpful tactic. Plant dill, fennel, cilantro and other small-petaled flowers to attract beneficials, and provide them with a watering saucer filled with stones will help keep them around.
Pay attention to your plants after the harvest is over and the plants are ferned-out, because beetles stress adult plants, overwinter in debris, and will decrease or damage next spring’s harvest. Keep them in check using the control measures above.
Sprinkle asparagus with diatomaceous earth.
Diatomaceous earth is composed of the microscopic glass shells of ancient single-celled organisms called diatoms, which sank to the bottom of ancient seabeds and eventually became a sedimentary rock. This rock is mined and powdered to make diatomaceous earth. These microscopic "broken-glass shards" act on hard-shelled insects (like asparagus beetles) by getting into the spaces between their joints and cutting them up, causing the insects to dry up and die. DE is non-toxic to animals and plants, and the "food grade" variety is free of contaminants. It is widely available at home improvement stores and nurseries.
Read more about asparagus below: