The common asparagus beetle adult is a shiny, elongated blue-black beetle with a reddish-brown thorax and four cream-colored spots surrounded by a red border on the wing covers. The adult asparagus beetle is ¼-inch long.
The larva is a greenish-gray, wrinkled, humpbacked grub with a dark head about ⅓-inch long.
Common asparagus beetle adults and larvae strip the needle-like leaves off asparagus plant fronds inhibiting the plant’s ability to photosynthesize and store energy for the production of spears. The beetles and larvae also chew asparagus spears.
There is a second species of asparagus beetle—the spotted asparagus beetle—which has 12 black spots on the wing covers. Adult spotted asparagus beetles also feed on leaves and spears, but the larvae do not. This species is less destructive than the common asparagus beetle because only the adults feed on asparagus plants.
Adult asparagus beetles hibernate in the soil and garden litter near plants during the winter then emerge about the time asparagus spears are about 6-inches tall—about harvest time. The adults feed then lay eggs on the spears.
Asparagus beetle eggs are shiny and black and are found attached by one end to asparagus spears. They hatch in about one week. The larvae feed for about two weeks on spears then burrow into the soil and pupate. A new generation of adults appears about ten days later.
There are usually two or three generations of asparagus beetles each year.
The common asparagus beetle is found throughout North America. The spotted asparagus beetle is commonly found east of the Mississippi River.
Target Plants: Asparagus
Feeding Habits and Damage: Adults and larvae chew on green asparagus shoots leaving them blemished with black stains and misshapen. Beetles also attack older stems and leaves. They can defoliate plants.
Organic Controls: Wash eggs, larvae and beetles off plants with a strong stream of water. Handpick beetles and crush them. As a last resort spray beetles with pyrethrin.
Organic Control Calendar: Here is what you can do seasonally to control asparagus beetles:
- When spears emerge in spring: Spray new spears with seaweed extract which seems to repel asparagus beetles. Cover new spears with poly row covers to exclude beetles. Plant an intercrop of calendulas, nasturtiums, or tomatoes which also seem to repel beetles.
- While spears are developing and after harvest as ferns grow on: Knock beetles, larvae, and eggs from spear and plants with a strong stream of water. Handpicking can set back the population significantly. For serious infestations, apply pyrethrum; spray the entire plant; use two application three to four days apart. Alternatively, dust plants with 5 percent rotenone, repeating every 3 to 5 days up to 24 hours before harvest.
- After harvest: In the fall, cut off the drying fronds an inch above the soil and remove any overgrown weeds. Clean up garden debris. Cultivate the soil 2 inches deep around the plants. After a week or two, lay a 4- to 6-inch layer of straw around the plants. In early spring, about two weeks before spears are expected, remove the mulch and cultivate the first one to two inches of soil again. Leave the soil bare during harvest and then mulch for summer.
Natural Predators: Asparagus beetles are attacked by lady bugs and parasitic and predatory flies. Animal predators include chickens and ducks which can be let loose to feed in the asparagus patch before spears emerge. Songbirds that feed on asparagus beetles include Baltimore orioles, bluebirds, cardinals, and chickadees.
Scientific Name: Criceris asparagi (common asparagus beetle); Crioceris duodecimpunctata (spotted asparagus beetle).