Asparagus beetles are insects that belong to the family Chrysomelidae. Like the name suggests, these beetles can cause damage to asparagus plants. The two most common species of asparagus beetles are the common asparagus beetle (Crioceris asparagi) and the spotted asparagus beetle (Crioceris duodecimpunctata). Both species are native to Europe but have now spread to other parts of the world, including North America. Here, they have become a major pest in commercial asparagus fields. This past summer, the spotted asparagus beetle showed up on the asparagus plants in my garden.
Characteristics of the Spotted Asparagus Beetle
The common asparagus beetle is a reddish-brown insect with black stripes and spots. In contrast, the spotted asparagus beetle is a bright red beetle with black spots. The beetles feed on the foliage, stems, and buds of the asparagus plant, causing the plant to become stunted and weakened. In severe infestations, the beetles can even cause the plant to die. In addition to feeding on the asparagus, the female beetles also lay their eggs on the foliage, which can cause further damage as the larvae feed on the plant. Fortunately, in my case there were only a few adult beetles on the plant and it seemed to do well over the course of the summer. Some of the asparagus spears were distorted, especially towards the end of the growing season. However, most of the beetle activity was on the fern-like stems.
Controlling the Spotted Asparagus Beetle
While one of the most effective ways to control asparagus beetles is through the application of insecticides, I had no interest in using this method. There are several chemical insecticides that are specifically designed to control asparagus beetles, including neonicotinoids and pyrethroids. However, the use of insecticides has negative impacts on the environment and other non-target species. Additionally, asparagus beetles can develop resistance to insecticides, making the chemicals less effective over time.
Another way to control asparagus beetles is through the use of cultural methods. This includes planting asparagus in well-drained soil and keeping the garden free of weeds, which can serve as a breeding ground for the beetles. In addition, removing and destroying any infested plant material can help to reduce the population of beetles. If the beetles are damaging spears, consider letting some plants on the edges of the patch produce ferns which the beetle prefers. The larvae of asparagus beetles are dark, green-gray grubs about 9 mm long when fully grown. They feed inside of the plant’s berries.
Other Natural Methods of Controlling the Asparagus Beetle
To me, a more appealing method of controlling asparagus beetles is through the use of natural predators. There are several species of predators that feed on asparagus beetles, including lady beetle larvae, green lacewings, and parasitoid wasps like the metallic green wasp. Fortunately, my carrots overwintered and the following year I let them go to flower and then seed. The flowers attracted a wide array of ladybugs, wasps and other insects. By encouraging the presence of these predators in your garden, you can help to reduce the population of asparagus beetles and other pests.
Understanding the life cycle of the spotted asparagus beetle is helpful to control this garden pest as well. Adults overwinter in the hollow stems of asparagus and under bark. The adults emerge in May and begin to feed on new asparagus spears. At this time, harvesting the spears regularly will reduce the number that will become stems. Once the ferns begin to develop, the adults lay eggs on the stems. The larvae feed inside the asparagus berries and drop to the ground to pupate when fully grown. Removal of female plants will also eliminate the food source for the larvae. Another alternative is to allow some asparagus on the edges to develop ferns to act as a trap crop. Beetles can be manually removed from these plants if so desired.
Living With a Beautiful Pest
Even though asparagus beetles can cause significant damage to asparagus plants, they are beautiful insects and well worth watching. In the case of the spotted asparagus beetle, while the adults can damage the emerging spears, the larva feed on the berries of the plant rather than the leaves of the fern. As a result, this beetle is less problematic than the common asparagus beetle. If you want to protect your lovingly cultivated asparagus crop, it is important to think about the potential impacts on the environment and other non-target species when choosing a control method. Consider attracting predators to deal with the problem and tending the plants carefully. With proper management, gardeners can simultaneously control asparagus beetles, enjoy the presence of the beetles, and protect their hard-grown crops.