Asparagus beetles overwinter in plant debris, so removing fronds in winter will reduce their numbers.
Illustration By Keith Ward
This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
The most common of asparagus pests, asparagus beetles weaken plants when fed upon by adults and larvae. Organic methods of asparagus beetle control include handpicking, and gathering up and composting old asparagus foliage and berries. Originally from Europe, asparagus beetles are now found throughout North America, wherever asparagus is grown.
What Are Asparagus Beetles?
The common asparagus beetle (Crioceris asparagi) is the most prevalent asparagus pest that damages young asparagus spears. Overwintering as an adult, this slender, elongated beetle bears four white or yellowish spots on its wings, is reddish underneath and on the wing edges, and has a dark-red thorax. It is 1/4 to 1/3 inch long.
The spotted asparagus beetle (Crioceris duodecimpunctata) is also quite colorful. In the western United States, the spotted asparagus beetle is bright pumpkin orange with tiny black dots on its wings. In other areas, the background color of spotted asparagus beetles can be red to dark red. The antennae of both asparagus pests measure about half its body length.
Asparagus Beetle Damage
In early spring, just as asparagus spears are breaking the surface, adult asparagus beetles emerge after overwintering in plant debris. They begin feeding on the earliest spears, causing them to crook like a shepherd’s crook. Soon after mating, the females lay tiny, dark, elongated eggs on the little spears and new foliage, which stick out at right angles. Adults dine on the main spears, while the grayish-green, soft-bodied larvae feed on spears and foliage. Indentations caused by the feeding are brown in color and will decrease the vigor and size of affected spears. Severe feeding reduces the vigor of the asparagus plants.
Adult spotted asparagus beetles emerge one to three weeks later than common asparagus beetles. Late in the spring, these asparagus pests may be found all over asparagus plants, but usually they do only minor damage. The egg-laying cycle is timed to coincide with the formation of asparagus berries. Adults lay eggs on or near berries, the larvae bore inside to feed, and then drop to the ground to pupate into adults. The larvae feed only within the berries. Spotted asparagus beetle larvae are yellowish with black heads and legs.
Asparagus Beetle Reproduction
Soon after emerging in spring, asparagus beetles lay eggs on emerging spears, which hatch in three to eight days. The small soft-bodied, grayish grubs feed on the tender new growth in spring. Later in summer, eggs and larvae can be found feeding on asparagus foliage. When the grubs mature after about two weeks of feeding, they drop to the ground and pupate inside an earthen cell for seven to 10 days before emerging as adults. In most climates, there are two generations each year, but in warm climates there may be as many as five generations of this asparagus pest.
Spotted asparagus beetles emerge in midspring and promptly begin eating asparagus foliage, but they wait until asparagus plants develop berries to lay their eggs. After the eggs hatch, the larvae bore into the berries to feed.
Natural Enemies of Asparagus Beetles
Birds often eat asparagus beetles and larvae, which appear during nesting season when birds need more protein. Lady beetles eat young larvae, but the most effective predator is small black Eulophid wasp (Tetrastichus asparagi),which eats asparagus beetle eggs early in the season, and lays its eggs in asparagus beetle eggs in summer. This beneficial insect, which is so helpful with asparagus beetle control, was identified in Massachusetts in 1915 and is now found wherever asparagus is grown. It is not currently offered for sale commercially.
Organic Asparagus Beetle Control
When dealing with the asparagus beetle, organic control methods can be quite effective. One key is to remove plant debris from the patch in fall and compost it in an active compost pile. Larvae and adults can be removed by hand. When handpicking, place a pail of water on the ground, and many beetles will drop into it when knocked from the plants. Beneficial wasps will parasitize beetle eggs. Chickens also do a good job of gathering asparagus beetles when allowed to forage in the patch in late summer. In severe situations, you can get good organic control of asparagus beetles by spraying plants with neem early in the season, when populations are modest. Do not use neem or any other pesticide if small black wasps are present in your asparagus patch.
Spotted asparagus beetles will be discouraged if the asparagus planting is male-only cultivars that do not form berries. If your plants do produce berries, gather and compost all asparagus berries if spotted asparagus beetles have been seen. This is the best organic asparagus beetle control.
More Organic Pest Control Advice for Asparagus Beetles
Reduce populations of overwintering adults of both asparagus pests by keeping the surrounding area lightly tilled and by eliminating plant debris. This forces asparagus beetles to hibernate in soil, where subsurface predators eat them.
In early spring, cut spears promptly at ground level with sharp knife. Attentive harvesting will keep many asparagus beetle eggs from hatching.
Both kinds of asparagus beetles can be easily handpicked. A beetle's instinct is to drop to the ground when it feels disturbed, so hold a container of water under the plant before gently shaking it, and the bugs will drop into the water. A sheet or newspaper or pizza box works well, too.