C. cobaltinus has an iridescent cobalt-blue exoskeleton. Both sexes range from 6–9 millimetres (0.24–0.35 in) in length, with clubbed antenna. The elytra usually have more or less evident epipleura, changing within the varying species within the Chrysomelidae. It rarely has an exposing pygidium. The body is oval, although the ventral is not prominently convex.
C. cobaltinus is similar to click beetles in being able to launch themselves a couple times the length of their body.
Both adults and larvae feed on milkweed plants. Adults typically feed on the foliage and flowers of the newly developed milkweed plant. When a large community appears, they consume a large portion of the leaf tissue from the plant, which causes significant damage to the plant. The adults eat holes in the leaves of milkweed, appearing in spring as the days become warmer and the milkweed leaves begin to develop. During the spring they disperse in large numbers on various plants within the same area of distribution.
Adults emerge in early summer and persist on milkweed plants in patches for approximately six weeks. Females are highly polyandrous; males engage in extended periods of post-copulatory mate guarding.
The adult females lay their eggs on the leaves of the milkweed plants; the larva consumes the leaf tissue between the veins, leaving nothing but a skeleton. On occasion the larva also eats the root system of the plant. C. cobaltinus larvae in large numbers can consume all the plant's leaves. This apparently does not kill the plant; it goes dormant until the following year, although if the larvae consume the root system of the plant, it will eventually wither away.
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