Rain beetle

The rain beetles are a group of beetles whose extant species are found only in the far west of North America. They spend most of their lives underground, emerging in response to rain or snow, thus the common name. Formerly classified in the Scarabaeidae (and later the Geotrupidae), they are currently assigned to their own family Pleocomidae, considered the sister group to all the remaining families of Scarabaeoidea. The family contains a single extant genus, Pleocoma, and two extinct genera, Cretocoma, described in 2002 from Late Cretaceous deposits in Mongolia, and Proteroscarabeus of Late Cretaceous China.

Rain beetles feed on various deciduous fruit trees, feeding on their roots, resulting in patches or winding bands on tree roots. This ultimately causes reduced foliage and fruit yield for trees. The larvae of rain beetles can only be found through digging to the root zone where larvae feed and observing damages. Male Pleocoma beetles, however, are attracted to light and can be monitored through that method.

Male rain beetles exhibit most mating behavior during winter nights. As the females are flightless, the males spend a large portion of time walking on snow in search of them. The male's ability to maintain a high internal temperature is critical to their mating and survival prospects, as their thoracic pile is ineffective insulation.

Extant members of Pleocoma are known from extreme southern Washington, throughout the mountains of Oregon and California, and into the extreme north of Baja California.