The other day I had a call from a gardener who was having trouble with asparagus beetles chewing on his asparagus. These beetles don’t usually do a lot of damage but they can make the spears look a bit ragged especially at the tip. If not controlled, however, a heavy infestation of beetles and their larvae can cause defoliation of the asparagus ferns during the summer. This can weaken the plants and reduce spear production the following spring.
One of the worst parts about having asparagus beetles is that they lay their eggs all over the asparagus stems. These black cigar-shaped eggs are very prominent, sticking out at a right angle up and down the stalk like little prickers. Not very appetizing to say the least! If you have asparagus beetles, you will have the eggs and lots of them! There are two types of asparagus beetles in our area; the common asparagus beetle (Crioceris asparagi) and the spotted asparagus beetle (Crioceris duodecimpunctata).
The common asparagus beetle is the most prevalent and unfortunately is the one that does the most damage to the plant. Most of the time, unless there is a heavy infestation, the damage is purely cosmetic. The beetles feed on the stem leaving shallow grooves and scars on the surface. In some cases, the spears can become disfigured, ragged, and bent over like a shepherd’s crook. However, it’s the presence of those little black eggs sticking out all over the spears that is often the most objectionable part of an asparagus beetle invasion! Luckily, they are fairly easy to rub or scrape off when you are preparing the spears for consumption.
Control of Asparagus Beetles
Our asparagus patch is relatively small so I normally just hand pick the beetles and squish them when I find them. If you have a larger bed, this can become an overwhelming job. If you cut the spears when they are still pretty short (about 8″ or so), they normally don’t have much damage and early harvesting has the added benefit of removing any eggs before they have a chance to hatch.
Natural predators in your garden can reduce asparagus beetle eggs and the caterpillar-like larvae. A small parasitic wasp will attack and destroy the eggs. Lady beetles, which are similar in coloration to the spotted asparagus beetle but are round rather than oval in shape, will consume both eggs and larvae of the asparagus beetle. Another trick is to leave a few of the asparagus unharvested. Asparagus beetles are attracted to mature plants with a lot of foliage so these plants can become “trap” plants and the emerging spears are more likely to be left alone. In large plantings or when there are more severe infestations, pesticide applications may be warranted.
Bonide Neem Oil and Pyrethrin are good organic controls for asparagus beetles. These can be used pre-harvest or post-harvest according to the label directions.
For organic control post-harvest only, Bonide Captain Jack’s Deadbug Brew which contains spinosad is a good option.
Chemical insecticides to control asparagus beetles include Bonide Eight (permethrin) and Sevin (carbaryl). Be sure to apply according to the label instructions and ALWAYS follow the pre-harvest interval recommendations.
NEVER spray an insecticide (organic OR chemical) when the bees are active. Just because a pesticide is listed as organic doesn’t mean it isn’t toxic to bees and other pollinators. The best time to spray is in the early morning or in the evening when they are less likely to be collecting nectar. Once the foliage begins to yellow in the fall, cut the plants to the ground and throw the foliage in the trash rather than into the compost pile. Weed and rake up all plant debris around the asparagus bed. This will reduce overwintering sites and help lower populations of these beetles the following spring.