San Diego Natural History Museum--Your Nature ConnectionSDNHM Field Guide
Photo of rain beetles from the SDNHM collection - Jim Melli 1999

Pleocoma
Rain Beetles

SCARABAEIDAE

Description

The genus Pleocoma has approximately 30 species, all confined to the Pacific coastal region and parts of Utah. The adult rain beetle is a moderately large insect: males measure about 1-inch in length (20 mm); females are slightly larger, measuring up to 1¾-inches (40 mm). The body is oval and slightly elongated. Males have wings and females are wingless. Depending upon species, wing cover colors range from black to reddish brown. The underside of the body is covered with hairlike bristles.

Range and Habitat

In California, rain beetles may be found throughout the foothills and mountainous areas of the state. Individual species, however, have very restricted ranges, and some may be confined to a single canyon. Two rain beetle species are common in the San Diego area: the Black Rain Beetle (P. puncticollis) and the Southern Rain Beetle (P. australis). The Black Rain Beetle may be found in coastal sage scrub and chaparral areas; the Southern Rain Beetle, in pine and oak woodlands. Both species extend their range south into Baja California, Mexico.

Natural History

After a slow soaking rain—one or more inches— male rain beetles make a sudden and abundant appearance. At dusk, in the early morning, or on cloudy, drizzly days, the males fly in slow sweeping arcs through the foothills and mountainous areas, keeping low to the ground, searching for flightless females.

Neither male nor female rain beetles feed as adults. The larvae feed on the roots of live trees and shrubs, particularly oaks and conifers.

The female rain beetle rarely leaves her burrow, but waits at the entrance, emitting a pheromone (chemical sexual attractant) that leads the male to her. After mating, the female closes the entrance to her burrow and lays her eggs. The eggs, which are deposited at the base of her burrow, do not mature until the following spring or early summer.

Rain beetle larvae are slow-developing, and appear to have a long life cycle, as much as 10 to 12 years. When they become adults, the males may wait as long as a month before the first rains bring them into the open air for their mating flight. The females dig tunnels to the surface and wait for the males to arrive.


Photo credit: Black Rain Beetles (Pleocoma puncticollis) from the SDNHM collection.
(Female above, male below.) Photo by Jim Melli 1999.

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