With their tender green stalks, the slender spears of asparagus (Asparagus officinalis) are attractive to a variety of insects. When left untreated, these pests can easily kill or severely damage your spring harvest. Constant and consistent care during the susceptible portion of the growth cycle -- when the tender young stalks are not yet harvested -- is strongly recommended. As a perennial thriving in U.S. Department of Agriculture plant hardiness zones 4 through 8, asparagus will return in the same spot year after year, so keeping the garden bed pest free is important so that regular, healthy harvests will continue. Use insecticidal soap or Neem oil regularly to control pest infestations.
Cutworms are caterpillars that emerge at dusk to feed on the tender asparagus plant. The larvae of these caterpillars is yellow-brown and about 1 1/2 inches long. When feeding, the larvae have a preference for the tender tops of the young asparagus plants as they are just emerging. As they feed, small holes appear. If too much damage is done to one side of the plant, the asparagus will curl and possibly die. To prevent cutworms from taking root in your asparagus plot, consider planting sunflowers around the edge of the garden bed. As the cutworms are attracted to these bright yellow flowers, find and root out the cutworms by hand. Other methods of control, such as plowing deeply in the fall, are not an option as asparagus plants are perennial.
When young, asparagus beetles are dark green-gray and around 1/2 inch long. When fully mature, they turn blue-black and have red-bordered, yellow-spotted wings. Both the larvae and the mature beetles will eat the stems and ferns of the asparagus plant, causing the crown of the plant to become weakened. Usually, asparagus beetles will not severely damage the spears, so they have little effect on the harvest. To distract asparagus beetles, let the outer spears pop and seed. The frothy fern-like leaves will attract the asparagus beetles away from the spears.
Around 1/8 inch long, asparagus miners are shiny and black, and produce small, white eggs that sit just underneath the outer layer of the asparagus stalk. As the asparagus miner feeds along the inside or the plant, the stalk will become weak, and the fern leaves will yellow and die. Asparagus miners can be controlled by keeping the area weed free, but also by introducing natural parasites, such as parasitic wasps, that do not harm the asparagus.
These thin, winged pests grow to about 1/16 inch long. They damage plants by sucking the juices from the foliage, attacking asparagus ferns more than the slender stalks that are harvested. Thrip damage produces yellowed ferns that eventually turn brown and die. In the case of severely damaged ferns, the plant produces less nutrition, which can affect the harvest the following year. Weed control and insecticidal soap sprayed on the ferns regularly during the late summer and early fall will help prevent thrip damage.