Rain beetles only occur in California, Oregon, and southernmost Washington.
While they live here all year, rain beetles can only be seen after the year’s first big rain, or at spring’s first big snow melt. Even then, you will probably only see the males.
Rain beetle description
Rain beetles are scarabs, making them cousins to rose chafers, hoopla beetles, and Junebugs. Rain beetles (genus Pleocoma) are robust, shiny beetles. They are over one inch long, dark brown to reddish brown, with fuzzy undersides. [The word ‘Pleocoma’ is Greek for ‘abundant hair’.] That hair can range from black and brown to yellow.
Rain beetles have clubbed antennae with 11 segments, which is more than any other scarab beetle. Being filled with eggs, female rain beetles are much larger than the males. Once those eggs hatch, 3/4-inch long, C-shaped, creamy white larvae begin feeding and burrowing.
Damage caused by rain beetles
Adult rain beetles do not eat. They have no functional mouths or digestive systems, so they do not cause any damage to the garden. Their larvae, however, are another story altogether. Rain beetle larvae feed on the roots of fruit, nut, and ornamental trees and shrubs, and grasses, along with fungi and other organic matter. Rain beetle larvae seem to prefer the roots of apple, pear, and other orchard trees.
Rain beetle behavior
Rain beetle larvae do not dig through the soil, per se. Instead, they move through it by eating it and pooping it out. Burrowing up to 3 inches each day, a rain beetle larva can be found anywhere in the top 8- to 10-feet of soil. Adult rain beetles use powerful legs and a V-shaped scoop found on top of their head to push their way through the soil.
Male rain beetles generally emerge at dusk, though they can be seen flying around in the day, especially if it is raining. Males fly low to the the ground, looking for mates. Male rain beetles are relatively good flyers, though they will bang into your windows at night, being attracted to lights and bodies of water. Flightless females stay in the ground, emitting a pheromone that attracts the males. This pheromone has a lemony scent that is so strong even people can smell it.
Rain beetle lifecycle
Rain beetles are long lived bugs. From egg, to larva, to pupa, to adult can take 12 years. [Most beetle species only live one year.] That being said, by the time you see a rain beetle, it is probably only hours or minutes from death. Male rain beetles only store enough energy as fat from their more youthful stages to fly for a couple of hours, looking for mates, before dying.
After mating, female rain beetles burrow a spiral-shaped tunnel, as much as 10-feet below the soil surface, laying 40 to 50 eggs as she goes. In 2 months, the eggs hatch and the larvae begin feeding.
Rain beetles were once found throughout California and Oregon. Now, they are generally only found in pockets of foothill and mountain habitats. Being flightless and ground dwelling, female rain beetles have been wiped out everywhere urban development has occurred.
Rain beetle controls
Since rain beetles spend nearly all of their lives underground, chemical controls are ineffective. For the most part, rain beetles are more of a nuisance than a significant pest, though you can catch males in a butterfly net, if they really bother you.
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