The curious color of the blue fungus beetle

The pleasing fungus beetle is one of the most colorful insects in the forest. Photo by Jeff Mitton.

By Jeff Mitton

I was at Heil Ranch, working on a study of mountain pine beetles with my graduate student Scott Ferrenberg, when a brilliantly colored beetle landed on our equipment.

I picked it up and confined it until I could fetch my camera. The beetle’s color is so striking that words don’t really convey the image; you have to see it to appreciate it.

The pleasing fungus beetle, Gibbifer californicus, is native to Colorado and parts of Wyoming, Kansas, Arizona and New Mexico. Throughout this range they are most commonly encountered in moist riparian woodlands with large trees. Locally, they are found in stands of ponderosa at lower elevations and in aspen at higher elevations. The very best place to find them is around the bracket fungi that grow profusely on rotting logs.

Pleasing fungus beetles are about 1 inch long and their eyes, head, legs and underside are shiny black. The elytra or wing covers of the beetle that dropped in to join us had a gorgeous hue of blue with numerous indented black dots. Throughout the range of the species, however, the wing covers may be grey or blue or purple, but they always have black dots.

Some blue colors on animals are structural, scattering light in such a way as to appear blue. My favorite example of a structural blue is the feathers of Steller’s jays; the feathers retain their bright colors even after a feather falls out. Other blues are pigments and these tend to fade in poor health and after death. Pleasing fungus beetles lose their bright hue shortly after death, so pigments produce their blue hue.

Adult beetles feed on nectar, pollen and the bracket fungi growing on rotting logs. Larvae feed exclusively on the bracket fungi, so if you want to see adult and larval pleasing fungus beetles, search rotting logs with bracket fungi.

Beetles overwinter as adults and lay eggs on rotting logs near bracket fungi in spring. The larvae feast on fungi and grow quickly. As they grow, they begin to congregate and move to the underside of the log, where they hang and form pupae. Dense clusters of hanging pupae are reminiscent of a bat roost, but on a smaller scale. The pupae transform to unpigmented adults that synthesize and accumulate blue pigment. Adults are active for the remainder of the summer and into the fall.

Bright colors on an insect always make me wonder about the evolutionary pressures. Insects edible to a wide range of predators need to defend themselves. Some use cryptic colors and textures to hide. Others have stingers or nasty bites. Still others are highly mobile and can fly away or scramble into a crevice or burrow. Some use mimicry as a strategy and converge on the color and pattern of a chemically defended species occurring in the same environment.

Curious fungus beetles are brilliantly colored but are not particularly fast, so they would have trouble escaping from predators. They lack stingers or a defensive bite. Finally, this beetle is unique, so it is not mimicking a similar but chemically defended species in the same environment.

Chemical compounds typically defend brightly colored insects that lack other defenses. Incidentally, bracket fungi are generally edible, although humans in North America do not commonly eat them. So beetles probably do not sequester defensive compounds from fungi.

I was unable to find anything written about the chemical defenses in pleasing fungus beetles, but I have a hunch. I noticed several papers on the numerous surface glands of other species in the same group. My hunch is that they are able to synthesize their own defensive compound, just as blister beetles synthesize cantharidin, which blisters human skin a few minutes after contact.

Pleasing fungus beetles are described as harmless, so you can probably pick them up, just as I did. But handle them gently to avoid a defensive release of chemicals from the glands. Those bright elytra may have evolved to advertise defenses that remain to be discovered.

Jeff Mitton ( is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Camera.

August 2011

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