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Anjana Duwal, Carlton, Christopher E.
Grape colaspis aedeagus comparison showing internal sac structures (arrows) (Chris Carlton, LSU AgCenter).
Louisiana colaspis aedeagus comparison showing internal sac structures (arrows) (Chris Carlton, LSU AgCenter).
Grape colaspis adult, museum specimen (Louisiana State Arthropod Museum).
Louisiana colaspis adult, museum specimen (Louisiana State Arthropod Museum).
Grape and Louisiana colaspis beetles are small, brown, leaf-feeding beetles in the large family Chrysomelidae (leaf beetles). Adults of Louisiana colaspis are, on average, slightly larger than those of the grape colaspis, but considerable size overlap exists. Grape colaspis adults range in length from 3.7 mm to 4.2 mm and those of the Louisiana colaspis are typically 3.7 mm to 5 mm long. Adults of both species are nearly identical oblong beetles, light to dark brown in color, and possess obvious punctures and ridges on the shield-like forewings (elytra). In some specimens the ridges alternate between light and dark brown.
Distinguishing the two species from each other requires specialized techniques to dissect and examine the male sex organs (aedeagus). The aedeagus of the grape colaspis is slightly less curved, with an internal structure called the internal sac that is difficult to observe. The aedeagus of the Louisiana colaspis is more curved, and the internal sac appears as an oval, mitten-shaped structure visible through the wall of the aedeagus. The internal sacs are everted during mating but usually remain withdrawn into the aedeagus in preserved specimens.
The eggs are white or yellow, smooth, and around 0.6 mm long and 0.25 mm wide. Larvae are grayish white, with dark brown heads, somewhat C-shaped and approximately 7 mm long when mature. Pupae are pale yellow, around 5 mm long and possess exposed adult appendages tucked tightly against the body.
At least 28 species of colaspis beetles are found in the U.S., including several within species complexes that are extremely difficult to identify, even by specialists. Approximately 200 species are described worldwide. The two species covered in this article are members of the brunnea species group, which includes two additional species found in Florida and Arizona.
The two colaspis species undergo one generation per year. Larvae overwinter in soil, typically in the third instar. During spring and early summer, the overwintered larvae begin feeding on roots and stems of host plants. This process continues through five additional instars, followed by pupation in upper layers of soil. The pupal stage lasts three to seven days. After emerging, adults begin mating and laying eggs within two weeks. Adults feed on the plant leaves, and their populations usually peak during July. Females prefer legumes for egg laying, depositing as many as 35 eggs per clump near the soil surface. Larvae hatch one to two weeks later feeding on roots before burrowing deeper into soil as winter sets in.
Colaspis beetles primarily damage corn and soybeans, but they are polyphagous and may also affect rice, grapes, white and red clover, beets, snap peas and other crops. Damage is usually worse in poorly drained soil or in situations where corn and soybean intercropping occur. Most damage results from larvae feeding on the roots and stems of seedlings, with relatively minor damage from the adult leaf feeding. Root feeding by the larvae causes stunting, wilting, and, in corn, brown leaves or yellowing. During 2012, some rice stands in Concordia Parish experienced a 40% reduction due to colaspis larvae feeding on seedings during early spring. Colaspis damage can be difficult to diagnose, as digging is necessary to confirm the presence of larvae. Scouting for adults can provide early warning of possible infestations the following spring, but no action thresholds have been established.
Understanding the population dynamics of colaspis beetles in Louisiana is complicated by the co-occurrence of two extremely similar species. The relative impact of the two species as pests is unknown as they are rarely properly identified in field situations, and infestations are typically attributed to the grape colaspis.
Cultural control in soybeans includes rotating with cotton and practicing best cultivation methods to promote vigorous root development during spring, with particular attention to proper drainage and fertilization. Colaspis larval survival is higher in moist soil, so infestations are usually less severe during years when drier conditions occur during late summer and fall when young larvae are present.
Seed treatments can prevent larval damage to seeding roots during spring, and insecticide applications may be necessary when adult populations are high during July. Formulations of the microbial insecticide Bacillus thuringiensis (Bt) show some promise in corn and soybeans against colaspis beetles. For the most current insecticide recommendations, consult the latest issue of the LSU AgCenter Field Crops Insect Pest Management Guide.
Colaspis life cycle (LSU AgCenter).
P3991
6/13/2025
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