Rain Beetles (Pleocomidae)

 excerpt from:

Evans, Arthur V. and Hogue, James H. 2004. pp. 158-162. in Introduction to California Beetles. University of California Press, Berkeley, CA. 299 pp.

Plate 30. The Southern Rain Beetle (Pleocoma australis, Pleocomidae) (males 24 to 28 mm) is known from portions of the Transverse and Peninsular Ranges and flies during the first fall rains in October and No· vember, usually during a light drizzle at dusk or just after dark. The larvae feed on the roots of canyon live oak (Quercus chrysolepis).

Plate 31. Female rain beetles, such as Pleocoma badia badia (32 to 43 mm) are flightless and much larger than their male counterparts. 

The family Pleocomidae contains the single genus Pleocoma, whose species are distributed from southern Washington southward to Baja California Norte, Mexico. Previously published records of rain beetles from Alaska and Utah have proved to be erroneous. California's rain beetles occur throughout the mountainous regions of the state, except in the deserts. Small, isolated populations also occur in the Sacramento Valley and the coastal plain of San Diego County. The known modern distribution of these apparently ancient beetles is restricted by the flightless females and is more or less correlated to areas of land that have never been subjected to glaciation or inundation by inland seas during the last two or three million years.

Rain beetles are large, robust, and shiny. The thick layer of hair covering the undersides is remarkably ineffective as insulation, especially for flying or rapidly crawling maleswho must maintain high body temperatures in cold, damp weather. Males can attain an internal temperature of 95 degrees F, although the mechanism by which they do this remains unclear. The thick pile probably functions to protect both sexes from abrasion as they burrow through the soil. Males and females dig with powerful, rake-like legs and a V-shaped scoop mounted on the front of the head. Males are fully winged and capable of flight, whereas the hind wings of the flightless females are reduced to small flaps of tissue. Femalesgenerally are much larger and more heavy bodied than males.

Lacking functional mouthparts, adult rain beetles are unable to feed. They must instead rely on fat stored in their bodies while they were root-feeding grubs. Because of their limited energy stores, adults are active for only a short time. On average, males of some rain beetles have only enough energy stored as fat to give them about two hours of air time and live only a few days. The more sedentary females require less energy and may live for months after fall and winter storms. In most species of rain beetles, male activity is triggered by weather conditions that accompany sufficient amounts of fall or winter rainfall or snowmelt in late winter or early spring. Depending upon circumstances, males may take to the air at dawn or at dusk, or they may fly during evening showers. Others are encountered flying late in the morning on sunny days following a night of pouring rains, or during heavy snowmelt.

Males fly low over the ground, searching for calling females releasing pheromones from the entrances of their burrows. Amorous males are capable of tracking females over considerable distances, often through dense vegetation. Dozens of malesmay descend upon a single female, clambering over one another as they jockey for position to mate. Mating takes place on the ground or in the female's burrow. After mating, the males leave and may find another mate before coming to the end of their short lives. During their nuptial flights malesare frequently attracted to lights or shiny pools of water. Females crawl back down their burrows and may wait up to several months for their eggs to mature. The femaleeventually lays 40 to 50 eggs in a spiral pattern at the end of the burrow as much as 3 m (10 ft) below the surface. The eggs hatch in about two months.      

Upon hatching, the small grubs use their powerful legs and jaws to tunnel deep in hard and compact soils to follow the root systems of their host plant. They feed upon roots of grasses, shrubs, and trees. In Oregon, the larvae of some rain beetles are considered pests when they attack the roots of strawberries, pears, apples, and cherries. Unlike most scarab beetles that have three instars, Pleocoma larvae molt seven or more times and may take up to 13 years before reaching maturity. Pupation occurs in a simple, elongate chamber. Rain beetles have numerous enemies, below and above ground. Predatory fly maggots thought to be robber flies (Asilidae) attack both the larvae and pupae. Coyotes, foxes, skunks, raccoons, and owls feast upon the adults. Agile coyotes and foxes, as well as some other birds, even snatch the slow-flying males out of the air. At the height of the flight season it is not unusual to find the droppings of these predators filled with the indigestible bits of rain beetle legs and wing covers. One record describes a female infested internally by nematode worms.

CALIFORNIA FAUNA: 20 species and three subspecies in one

genus.

IDENTIFICATION: Adult California rain beetles (plate. 30) are among the largest beetles in North America, resembling large June bugs in overall body shape. The males are usually reddish brown to black, whereas females are generally reddish brown. Their bodies are robust and broadly oval. The upper surface is shiny, whereas the underside is densely clothed in hair-like setae. The winged males (16.5 to 29 mm) are easily distinguished from the much larger, flightless females (19.5 to 44.5 mm) (plate 31). The head is armed with a horn and bears antennae consisting of 11 segments tipped with four to eight fanlike segments that can be folded tightly into a club. The club is larger in males than females. Adults lack functional chewing mouthparts and do not feed. The prothorax is broad and unique among other scarab beetle families in that the cavities into which the front legs are inserted are open toward the back. The scutellum is visible. The elytra are smooth, shining, or distinctly grooved and almost completely conceal the abdomen. The powerful forelegs are equipped with rake-like teeth. The tarsal formula is 5-5-5, with tarsal claws equal in size and simple. The abdomen has six segments visible from below. The larvae are creamy white, C-shaped grubs. The tip of the abdomen may be darkened by waste material inside the body. The head is shiny yellowish or reddish brown. The antennae are three segmented, and the eyes are absent. The legs are four segmented. The lO-segmented abdomen does not have any projections.

SIMILAR CALIFORNIA FAMILIES: Their large size, hairy undersides, horned heads, 11-segmented antennae, and their fall and winter activity periods easily distinguish rain beetles.