When thinking of asparagus pests, we normally worry about early season feeding on the developing spears. However, late summer and fall scouting is important to get ahead of some of the spring insect problems. Two of the insects that we recommend scouting for are the common asparagus beetle (Figure 1-A) and the twelve-spotted asparagus beetle (Figure 1-B). Although asparagus plants are not producing spears at this time, they are collecting nutrients that will be used to grow spears during the next season. In addition, both of these beetles overwinter as adults near gardens or even in the hollowed stems of older asparagus plants. This means that scouting and managing adults now could mean fewer to manage next year. In addition, both species are capable of having multiple generations per year, so management can greatly reduce population increases.
Common Asparagus Beetle
Common asparagus beetles have a black head and legs, an orange thorax with two black spots, and orange and black hardened forewings (elytra). Each elytron has three cream colored spots on it (Figure 2). However, the size and shape of the cream-colored spots and the two black spots on the thorax are highly variable.
These beetles have black antennae. The common asparagus beetles are approximately ¼ of an inch long and oval shaped. The adults emerge in early spring and are active until the first hard frost.
The eggs of the common asparagus beetles are dark brown and cylindrical shaped (Figure 3-B).
The eggs will be placed on the ferns in rows with a space between each egg.
Twelve-Spotted Asparagus Beetle
The twelve-spotted asparagus beetle (also referred to as the spotted asparagus beetle) has an orange head, thorax, and abdomen. The elytra are covered with a total of 12 black spots, hence its name (Figure 4).
The beetles have black antennae and orange legs that have black coloration around the joints. The adults are approximately ¼ of an inch long. These adults emerge in mid-May and can be observed through the fall.
The twelve-spotted asparagus beetle larvae have light gray to cream colored bodies with orange heads and black legs (Figure 5).
The twelve-spotted asparagus beetle larvae feed on the asparagus ferns’ berries.
Twelve-spotted asparagus beetle eggs are green in color and laid on the ferns.
Injury and Management
When either beetle feeds on the asparagus spears in the spring, the spears will become brown and will bend over into a shepherd’s hook (Figure 6).
Once the ferns appear, larvae and adults of both the common asparagus beetle and the twelve-spotted asparagus beetle will feed on the leaves and will weaken the plant (Figure 7).
The twelve-spotted asparagus beetle larvae also feed on the berries but feeding has little impact on the plant’s health.
The best management strategy for asparagus beetles is to scout the asparagus both in the spring and throughout the growing season and remove the observed beetles and larvae by hand and destroy them. It is important to scout for these pests later in the afternoon as that is when they are most active. If the eggs are observed on the spears or the ferns remove them and destroy them. There are also parasitoid wasps that attack the larvae of the beetles, which can reduce the populations of additional generations. Other natural enemies such as ladybug larvae will also eat the asparagus beetle eggs and larvae.
For large gardens, or when large populations are present, insecticides can be used to reduce the populations. The threshold is when 10% of plants have one adult of either species present or 50 to 75% of plants have larvae present. Before applying an insecticide product, read the label and do not harvest before the listed pre-harvest interval period has expired.
An important fall management strategy is to clean your garden. Do not use grass clippings, mulch or leaves around asparagus plants as the adults can use these materials to protect them for overwintering.