Asparagus Beetle

Introduction

The common asparagus beetle, Crioceris asparagi, and the spotted asparagus beetle, Crioceris duodecimpunctata, are thought only to attack asparagus. Distinguishing between the two species is important because the common asparagus beetle will cause more damage to the crop.

Biology & Life Cycle

Common asparagus beetle adults are about ¼ inch long, bluish black with large cream colored irregular shaped spots on its back. Common asparagus beetle adults overwinter in sheltered locations such as under loose tree bark or in the hollow stem of old asparagus plants. The adults appear in the asparagus fields just as the asparagus spears are emerging from the soil in the spring. The beetles can lay numerous, dark brown, oval-shaped eggs on end in rows on the spears. The eggs hatch within a week. The light gray, slug-like larvae with black heads and legs migrate to the ferns to start feeding. The larvae feed for about two weeks and then fall to ground to pupate in the soil. About a week later, adults emerge to start another generation.

Common asparagus beetle eggs on asparagus fern (Whitney Cranshaw, Colorado State University, Bugwood.org).

Common asparagus beetle larvae on asparagus fern (Whitney Cranshaw, Colorado State University, Bugwood.org).

The spotted asparagus beetle has a similar life cycle but usually appears in the asparagus fields somewhat later than the common asparagus beetle. The spotted asparagus beetle adult is reddish-orange with six black spots on each wing and is ¼ inch long. They generally lay greenish eggs on the ferns. The orange larvae typically feed on the berries, or fruit, of the asparagus. (Note: spotted asparagus beetles should not be confused with beneficial lady beetles).

spotted asparagus beetles — Spotted asparagus beetle adults (Whitney Cranshaw, Colorado State University, Bugwood.org).

spotted asparagus beetle larva — Spotted asparagus larva feeding asparagus berry (Whitney Cranshaw, Colorado State University, Bugwood.org).

Damage

Feeding on the spears by both species of asparagus beetle adults can cause browning and scarring which often makes them unmarketable. Moreover, numerous eggs of the common asparagus beetle, laid on the spears, can also make the asparagus unappealing to the consumer. When the ferns appear later in the growing season, the common asparagus beetle larvae and adults can also devour the ferns. Significant defoliation can weaken the plant, making it more susceptible to invasion by a fungal pathogen, Fusarium. Serious defoliation can also impair the plant's ability to provide adequate nutrients for a good crop in the following growing season. In contrast, when spotted asparagus beetle larvae feeds on berries, they do not damage the long-term viability of an asparagus plant.

Management

Begin sampling asparagus crops in early spring and throughout the growing season. In the spring, randomly sample at least twenty plants in each of five different locations. In summer, randomly sample at least ten plants in each of five different locations. In fall, increase the number of plants sampled to at least twenty plants in each of five different locations. The thresholds can be somewhat dependent on buyers' tolerance of damaged spears. Defoliation is more of a concern, in newly established asparagus beds. You may want to consider control measures when the following thresholds are reached:

Life Stage	Economic Threshold
Adults	5-10% of plants infested
Eggs	2% of spears with eggs
Larvae	50-75% of plants have larvae or 10% defoliation

*Adult beetles are more active in the afternoon, in comparison to the morning, due to the warmer temperatures. If you are sampling in the morning, you may want to use more conservative thresholds (i.e., 5% of plants infested with adults).

Chemical Control

Asparagus beetles need to be controlled on seedlings, during fern growth, and at harvest. To ensure proper use of insecticides, refer to the most recent edition of the Midwest Vegetable Production Guide.

Avoid spraying when asparagus or crops in neighboring fields are flowering. Although bees do not pollinate asparagus, they sometimes visit the asparagus during bloom. If sprays are necessary during flowering, spray very early in the morning or in the evening when the bees are less likely to be gathering nectar or pollen in the field or garden.

Cultural Control

Destroy crop residues to eliminate overwintering sites.

Biological Control

A tiny (< 1/8 inch ) metallic green wasp, Tetrastichus asparagi , parasitizes asparagus beetle eggs. You may notice these wasps when you are monitoring beetle populations. They can sometimes provide very effective control, parasitizing up to 70% of the eggs. Lady beetle larvae and other predators may also be active; they will consume both eggs and larvae. Most insecticides, however, will also kill beneficial predators and parasites.

References

Cornell University. 2003. Vegetable Disease ID and Management http://vegetablemdonline.ppath.cornell.edu/

Delahaut, K. 2000. University of Wisconsin Extension. University of Wisconsin Garden Facts Asparagus Beetle. http://learningstore.uwex.edu/Assets/pdfs/A3760-E.pdf

Foster, R. & B. Flood. 1995. Vegetable Insect Management. Meister Publishing Co. Willoughby, Ohio.

Metcalf, R.L. & R.A. Metcalf. 1993. Destructive and Useful Insects. 5th Edition. McGraw-Hill, Inc. New York.

Tong, C. University of Minnesota Extension Service M-1251. Revised 2009. Growing Asparagus in Minnesota. http://www.extension.umn.edu/garden/yard-garden/vegetables/growing-asparagus-in-minnesota-home-gardens/index.html

University of Minnesota Extension Service BU-7094-S. Midwest Vegetable Production Guide for Commercial Growers; https://mwveguide.org/

The Minn. Extension IPM Program is a collaboration of University of Minnesota Extension & the College of Food, Agriculture, and Natural Resource Sciences (CFANS).