The common and spotted asparagus beetles are annual pests of asparagus in Wisconsin. The common asparagus beetle is the most prevalent and the only one that causes economic damage to asparagus.

Appearance

Both species of adult beetles have a slim, long shape with hard wing covers. The common beetle is bluish-brown with cream spots, while the spotted beetle is orange with black spots. Both are about ¼-inch in size. Eggs of both species are shiny dark rods 1/8” long. Those of the common asparagus beetle are found in groups and oriented in rows. Eggs of the spotted asparagus beetle are oviposited singly. Larvae are plump, resembling slugs, wrinkled and have visible legs. Those of common asparagus beetles are cream to grey-colored with a dark head and legs, while those of the spotted asparagus beetle are orange.

Symptoms and effects

Adults of the common asparagus beetle feed on the plant’s spears and ferns. Disfigured and unmarketable spears can result when the beetles feed or lay eggs on the spears. Spotted asparagus beetle larvae feed more on the berries rather than the ferns of asparagus.  Larvae secrete a black fluid onto the plants. Spring spear feeding reduces crop quality (browning, scarring, staining, and bent growth).  Summer fern feeding can cause defoliation and reduces yield of subsequent years.  Eggs laid on spears are unattractive to consumers, though harmless. Large populations of asparagus beetles, if left unchecked, can defoliate the plants.

Life cycle

Asparagus beetle overwinters in plant debris and brush as an adult.  Adults become active in spring when new spears emerge. The spotted asparagus beetle becomes active later in spring than the common asparagus beetle. Common asparagus beetles lay eggs on spears while spotted asparagus beetles lay eggs on ferns. About a week later eggs hatch. The larvae feed for about two weeks on asparagus and then pupate in the soil. About one week later the next generation of adults hatch.  Two to three generations occur in a growing season. Most larvae and adults are more active in the afternoon when the temperature and sunlight are at their peak.

Scouting

Begin in early spring and continue throughout the growing season. In spring, sample twenty plants each at five different locations, and in summer, sample ten plants each in five different locations. See scouting thresholds for when to begin treatment. Spring sampling thresholds are designed to reduce spear damage while later-season thresholds are designed to reduce long-term damage caused by defoliation.  Scouting should occur in the afternoon when the beetles are most active.

Degree-day modeling

The development of many insects can be modeled using degree-days, which are calculated from daily high and low temperatures. Visit the Vegetable Disease and Insect Forecasting Network (VDIFN) website, which features a convenient map-based interface, to view the asparagus beetle model and many others.

Management

Cultural control

Destroying crop residues will eliminate overwintering sites for asparagus beetles. Floating row covers can be applied early in spring to keep beetles off of spears but should be removed after harvest.

Biological control

A tiny parasitic wasp (Tetrastichus asparagi) is an egg parasite of the common asparagus beetle and is capable of reducing the population by up to 70%.

Chemical control

If insecticides are needed to reduce beetle populations below threshold levels, it is not necessary to treat the entire planting. Rather, you can spot treat those areas where threshold levels have been exceeded. New plantings tolerate less injury than established plantings.

Refer to the UW-Extension publication Commercial Vegetable Production in Wisconsin (A3422) for a list of registered insecticides and management recommendations.

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