Cobalt milkweed beetles (Chrysochus cobaltinus) can be found on milkweeds (especially Asclepias species) throughout the Western United States and British Columbia. With an iridescent cobalt blue exoskeleton and clubbed antennae, this beetle, in my opinion, is rather pretty.
Female cobalts lay their eggs on milkweed leaves. Larvae eat the soft tissues of leaves in the early spring and can often leave only a skeleton of veins. Larvae also feed on the plant roots. Usually the larvae do not kill the host plant, but if enough damage to the roots occurs the milkweed may become dormant until the next spring. Adult cobalt milkweed beetles emerge in the summer and can be found feeding on the flowers and leaves of their host milkweed plants for about six weeks. If enough adult beetles infest a milkweed plant they can kill their host.
Female cobalt milkweed beetles are polyandrous (mate with many males). This polyandry, according to a 2004 paper by Schwartz, provides a reproductive advantage to the cobalt beetle. Multiple matings result in an increase in daily egg production and an increase in the number of eggs in each egg mass.
Milkweeds contain toxic cardenolides, yet cobalt milkweed beetles, like the monarch butterfly, feed on these plants. Research has shown that a single amino acid substitution in both the monarch and the cobalt allow the insects to survive the accumulation of these toxins in their bodies. In addition, before feeding on a milkweed leaf the cobalt will bite into the midrib of the leaf and allow much of the toxic sap to “bleed” away.
The accumulation of cardenolides in the bodies of cobalt milkweed beetles serves as a defense mechanism. When disturbed, a cobalt beetle will tuck its legs and fall to the ground while producing a foul smelling and tasting fluid discouraging predators.
Blue milkweed beetle is another common name for C. cobaltinus. These beetles were feeding on showy milkweeds (Asclepias speciosa) in the McArthur Swamp (Shasta County CA).