June 10, 2010 at 8:54 a.m.
It's important to know the difference between the common beetle and spotted beetle because the common beetle is more numerous and more destructive. Both asparagus beetle adults are one-fourth inch long with oval-shaped bodies and moderate length antennae. The common adult beetle is bluish-black with six cream colored spots on its back, while adult spotted beetles are reddish-orange with 12 black spots. Spotted asparagus beetles should not be confused with beneficial lady beetles. Lady beetle adults have broadly oval to nearly rounded, dome-shaped bodies with a varying number of spots. They also have heads that are partly to completely concealed when viewed from above, and short antennae.
Common asparagus beetle adults overwinter in sheltered locations such as under loose tree bark, the hollow stems of old asparagus plants, or in weeds in the patch.
Adults appear at the same time as the asparagus does. The beetles lay numerous dark brown, oval-shaped eggs in rows on the spears, ferns, flower buds, and again, weeds. The eggs hatch within a week. 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 beetle has a similar life cycle, but usually appear in gardens somewhat later than the common beetle, appearing in mid-May and disappearing in late July. They generally lay greenish eggs on the ferns. The orange larval typically feed on the berries, or fruit of the asparagus.
Feeding on the spears by both species of asparagus beetle adults can cause browning, scarring, and may cause the spears to bend over into a shepherd's crook. When the ferns appear later in the growing season, the common asparagus beetle larvae and adults can eat the entire fern, which will weaken the plant. The feeding of the spotted beetle larvae on berries does not affect the health of asparagus plants over the long run. Numerous eggs of the common beetle laid on the spears can make the asparagus unappealing.
Handpicking, especially in small gardens, can be effective, but don't think it's like picking potato bugs. Drop the adults and larvae in a pail filled with soapy water. Also remove the dark brown eggs from the spears. New adults can fly into the garden so be sure to check them regularly, especially in the afternoon.
If you detect a heavy infestation, you may want to use an insecticide. Look at the label and look for active ingredients that have pyrethrins, malation, carbaryl, or permethrin.
The Chisago County Master Gardeners sold many bunches of asparagus this spring, so watch these new plants very carefully. Since both types of the asparagus beetle can easily kill new young ferns, and since you shouldn't harvest new asparagus for at least two years, you may want to use an insecticide when you first notice them.