Asparagus beetles feed on garden asparagus spears and those in the wild. This common asparagus beetle does much more damage than the spotted asparagus beetle.
Europe and North America.
Asparagus beetle adults are ¼-inch long with oval-shaped bodies and moderate-length antennae. They are bluish-black with six cream-colored spots on their back. The plump larvae (1/3 inch long) are slug-like in appearance. They have a black head and visible legs and are gray or greenish in color.
- Adults overwinter in plant debris and garden trash. They emerge in May/June and lay dark brown, oval-shaped eggs in rows of three to eight on the spears, ferns, or flower buds of asparagus plants. The eggs hatch within a week.
- The creamy greyish-black larvae reach up to 10mm long (3/8in) when fully grown. The larvae move up the ferns, feeding for about two weeks before falling to the ground to pupate.
- About a week later, adults emerge to start another generation, feeding on the ferns for the rest of the growing season. In the autumn, adult beetles seek sheltered places to overwinter.
- Two life cycles are common in most regions, but as many as five can be produced in areas with longer growing seasons.
Damage and Detection
- Asparagus beetles can fly and hence can invade asparagus patches from nearby.
- Both adult and larval stages of asparagus beetles feed on spears during the spring and early summer months.
- Asparagus spears that have been attacked become brown and can bend over into a hook-like shape.
- Feeding can cause visible scarring and staining as they feed and deposit frass, their excrement. Often, spear tips will turn brown.
- In summer, they move to feed on the ferns, weakening and damaging the plant.
- Significant defoliation at this point can weaken the plants, making them more susceptible to Fusarium wilt, a fungal disease.
- Fern and foliage damage may also limit the amount of nutrients plants can take in for the next growing season, causing fewer and less vigorous spears.
Prevention and Control
Distinguishing between the common asparagus beetle and the spotted asparagus beetle is important for effective damage control. A few spotted asparagus beetles in the garden are nothing to panic about; however, the sight of even one common asparagus beetle can be (and likely is) a harbinger of more to come.
Pick beetles and eggs off plants early and often.
- Start looking for signs of beetles on plants in early May or just after asparagus plants emerge, and continue checking throughout the growing season.
- Check for asparagus beetles in the afternoon when they are most active.
- Handpicking, especially in small gardens, can be effective.
- Drop adults and larvae in a pail filled with soapy water.
- Remove the dark brown eggs from the spears.
- New adult beetles can fly into the garden, so check your asparagus regularly.
Clean your garden
- Removing plant residue in and around asparagus makes it difficult for adult beetles to survive through the winter.
- After harvest, pick up garden debris and turn the soil over around plants to disturb overwintering beetles.
Encourage beneficial insects
- Beneficial insects, especially ladybugs and lacewing, will consume eggs and small larvae. Encouraging these insects in the garden helps keep a variety of pests in check.
- A tiny wasp (Tetrastichus asparagi) is an effective ally in the garden. The wasp lays its eggs in the larval stage of the asparagus beetle, destroying it from the inside out.
- Birds pick both adults and larvae from plants and the ground. Make sure your yard offers an environment that attracts birds.
- Protect the birds and beneficial insects – do not spray indiscriminate chemical pesticides. Pesticides kill beneficial predators and beetles.
- Pesticides are generally toxic to honey bees and beneficial predators and should be avoided.
- For bad infestations, introduce beneficial nematodes to your patch. These microscopic soil organisms will destroy asparagus beetle pupae right in the ground.
- As a last resort, spot treats adult beetles with botanical insecticides.
Udo Schmidt, Flickr, thatmacroguy, Shutterstock
While every effort has been made to describe these plants accurately, please keep in mind that height, bloom time, and color may differ in various climates. The description of these plants has been written based on numerous outside resources.