Asparagus Beetles

There are two species of asparagus beetles that may cause damage to asparagus. The common asparagus beetle (Crioceris asparagi) is the most common but it is unlikely there will ever be enough of them to do serious damage. The spotted asparagus beetle (C. duodecimpunctata) do less damage because they show up a little later in the spring so the adults don't have much opportunity to feed on the spears, 

Description and Life Cycle

Both asparagus beetle adults are ¼-inch long with oval-shaped bodies. The common asparagus beetle adult is bluish-black with six cream colored spots on its back; the adult spotted asparagus beetle is reddish-orange with twelve black spots. The larvae of both species are slug-like with visible heads and legs: common asparagus beetle larvae are light gray with a black head; spotted asparagus beetle larvae are orange. 

Common asparagus beetle adults appear in gardens just as the asparagus spears are emerging from the soil in spring. The beetles lay numerous dark brown eggs in rows on the spears, ferns, or flower buds. The eggs hatch within a week and the larvae migrate to the ferns to start feeding. They feed for about two weeks and then fall to the ground to pupate in the soil. About a week later, adults emerge to start another generation, feeding on the ferns for the remainder of the growing season.

The spotted asparagus beetle has a similar life cycle, but usually appears in gardens in mid-May and disappears in late July, somewhat later than the common asparagus beetle. They generally lay greenish eggs on the ferns. The orange larvae typically feed on the berries, or fruit, of the asparagus.

Damage

Both the larvae and the adults of the common asparagus beetle damage the asparagus plants. The overwintered adults emerge and begin to feed on the tender growing  tips of newly sprouted asparagus. They chew holes in the plant and cause a brownish discoloration of the tissue. The larvae feed on the tender young tips and on foliage. The spotted asparagus beetle adults attack the growing tips and sometimes eat the buds of newly sprouted asparagus. They also feed on foliage. The larvae cause little damage because they feed inside the berries.

Management

Start checking plants for asparagus beetles in early May or just after asparagus plants emerge and continue throughout the remainder of the growing season. The best time to check is in the afternoon when they are most active. The easiest way to get rid of a few beetles is to pick the adults and larvae off the plants and drop them in a pail of soapy water. Another important step is to clean the bed  up in the fall since adult beetles overwinter in leaf and plant litter. If you notice some tiny, metallic-green wasps around the asparagus patch leave them alone; they are a natural insect enemy of the asparagus beetle. 

A rule of thumb is that if you spot an adult beetle on 1 in 10 plants or larvae on 50 to 75 percent of the plants, you should use some kind of control. However, in a small asparagus patch, those numbers might still mean  only a few beetles, which are easily handpicked from the plants. If all else fails, products containing the botanical insecticide pyrethrin, or the chemical insecticide carbaryl (Sevin) are effective against asparagus beetles. Some formulations are safe to use up until the day before harvest; read the label to make sure.

Adapted from: the Cornell Cooperative Extension, 1999; University of Minnesota Extension, 2007