The asparagus beetle is a persistent pest that can significantly reduce the yield and health of an asparagus patch. Two species primarily target this crop: the common asparagus beetle (Crioceris asparagi) and the twelve-spotted asparagus beetle (Crioceris duodecimpunctata). Both beetles emerge early in the growing season, and their feeding activity and egg-laying can severely damage the spears, making them unmarketable, while later infestations weaken the plant’s ability to store energy for the next year.
Identifying the Asparagus Beetle and Its Damage
The common asparagus beetle, the more prevalent pest, is a slender, blue-black insect about a quarter-inch long, featuring six cream or yellowish spots on its back. The twelve-spotted variety is similar in size but is reddish-orange with twelve distinct black spots across its wing covers. Both beetles emerge in early spring from overwintering sites, immediately beginning to feed on the tender, emerging spears.
The larvae are slug-like, grayish-green grubs with visible legs. The common beetle lays tiny, dark, oval-shaped eggs in neat rows along the spears or ferns, which hatch within about a week. Larvae feed for roughly two weeks, causing the most significant defoliation to the ferny growth after the harvest season is over.
Damage appears as small feeding pits and scarring on the spears, often causing them to brown or bend into a “shepherd’s crook” shape. The dark, oblong eggs glued to the spears also render the crop unmarketable. Later in the season, adult and larval feeding on the fern-like foliage reduces the plant’s ability to photosynthesize, weakening the crowns and leading to a decreased harvest the following spring.
Immediate Physical and Cultural Control Methods
Daily monitoring of the patch during the harvest season is a highly effective first line of defense, especially for smaller plantings. Adult beetles and their larvae can be easily removed by hand-picking and dropping them into a container of soapy water for disposal. The beetles are most active on warm, sunny afternoons, making that the ideal time for inspection and removal.
Any spears showing eggs should be snapped off at ground level and destroyed, which immediately prevents the next generation of larvae from hatching. The practice of continuously harvesting all emerging spears every day also reduces the number of stems available for egg-laying and prevents larvae from establishing themselves.
A strong, direct jet of water can be used to dislodge the slug-like larvae from the ferns, forcing them to the ground where many will fail to return to the plant. Ensuring the asparagus patch and surrounding area are free of weeds and plant debris during the growing season eliminates sheltered areas where adults can hide. These manual, non-chemical interventions are often sufficient to manage populations below damaging levels.
Targeted Biological and Chemical Treatments
Biological control relies on natural enemies, such as the parasitic wasp Tetrastichus asparagi, a tiny metallic-green insect that attacks the eggs of the common asparagus beetle. This wasp can parasitize and kill up to 70% of the eggs, acting as a natural population suppressor.
Predatory insects like lady beetle larvae and ground beetles also consume beetle eggs and larvae, further contributing to biological control. To support these beneficial insects, the use of broad-spectrum pesticides should be avoided, as they eliminate both pest and predator populations. Certain beneficial nematodes, such as Steinernema carpocapsae, can also be applied to the soil to target pupating larvae.
Neem oil and insecticidal soaps are effective organic options against soft-bodied larvae. Neem oil is particularly useful for control during the fern stage. Synthetic options, such as those containing pyrethrins, are fast-acting but have a short residual effect, requiring careful timing. These treatments should be considered a last resort and should be applied after the harvest season ends to protect both the crop and beneficial insects.
Long-Term Prevention Strategies
Asparagus beetles spend the winter as adults in sheltered locations, often inside the hollow, dead stems of the asparagus ferns themselves, or in nearby plant debris. After the first hard frost turns the ferns brown, it is necessary to cut down all the dead fern growth to ground level and remove it from the area.
Lightly tilling or disking the soil surface in the fall can also disrupt overwintering sites, forcing adults into less protected areas where they are vulnerable to subsurface predators. This cultural practice is highly effective in reducing the initial population pressure on new spears.
Extending the harvest period later into the season, sometimes until late June, can also help to suppress the population by removing the earliest eggs laid on the spears.
While asparagus is a perennial crop and cannot be rotated, establishing new plantings far away from older, infested beds can prevent re-infestation. Selecting all-male asparagus cultivars, which do not produce the berries that the twelve-spotted beetle larvae feed on, can also help to discourage that species.