Inside the World of Rain Beetles: Understanding Their Significance

Rain beetles are a fascinating and lesser-known group of insects that play a unique role in our ecosystems. These elusive creatures are best known for their tendency to emerge during rainy weather, which is believed to be an adaptation to the moist environments they prefer. Rain beetles are typically found in the western United States and exhibit some interesting behaviors that set them apart from other beetle species.

One key characteristic of rain beetles is their preference for living underground, where they spend most of their lives. This subterranean lifestyle requires specialized adaptations, such as strong body structures and the ability to survive in low-oxygen environments. Additionally, their emergence during rainy periods helps to provide optimal conditions for breeding and dispersal.

Rain beetles are not only intriguing due to their lifestyle, but also because they play an essential role in their ecosystems. As decomposers, they help break down organic matter and contribute to soil health. By understanding and appreciating these remarkable creatures, we can better recognize their importance and the need to protect their habitats.

Rain Beetle Overview

Species and Distribution

Rain beetles belong to the Pleocoma genus in the Pleocomidae family. They are primarily found in North America, specifically in the western regions of the United States, such as California, Oregon, and Washington.

Physical Characteristics

  • Size: Small to medium-sized beetles
  • Color: Dark brown or black
  • Antennae: Modified antennae with lamellae (plate-like structures)
  • Wings: Contains hard, protective wing covers called elytra

These beetles remain underground for most of their lives but can be found surfacing during the rainy season.

Life Cycle of the Rain Beetle

Eggs and Larvae

Rain beetles, belonging to the families Scarabaeidae and Geotrupidae, have a unique life history. Females lay around 40 to 50 eggs in the soil near tree roots during winter rains. These eggs hatch into larvae, also known as grubs, which feed on organic matter like fungi, tree roots, and fruit. They thrive in both foothill and mountain habitats.

The key features of Rain Beetle larvae include:

  • Creamy white color
  • Feed on organic matter
  • Found in soil near tree roots

Pupae and Adults

After a period of growth, the larvae transform into pupae, buried within the soil. This stage can occur during droughts, and the beetles remain protected until sufficient rain returns. The pupae then emerge as adult rain beetles, which are typically active during the winter months, particularly in December.

Adult rain beetles are known for their black or yellow color and their flying abilities, which they use primarily at dusk and dawn. They are drawn to lights and often gather around illuminated trees and shrubs. Males and females engage in mating activities, after which they die off, completing their life cycle.

Some defining characteristics of adult rain beetles are:

  • Males have prominent horns
  • Active at dusk and dawn
  • Attracted to lights
Larval Stage Pupal Stage Adult Stage
Feeds on organic matter Occurs during droughts Active during winter months
Found in soil near tree roots Protected underground Males have prominent horns
Creamy white color Transforms to adult beetle Yellow or black color

While rain beetles may cause some damage to trees and shrubs due to their feeding habits, they’re not typically considered pests. Their unique life cycle, with roots in the soil during winter rains and Baja California mountain habitats, makes them an intriguing species to study and observe.

Behavior and Ecology

Reproduction and Pheromones

Rain beetles (Pleocoma sp) have a unique reproductive process. Male beetles fly in search of females after a soaking rain, while females are flightless and wait underground for the males. A key factor in mating is the use of pheromones by females to attract males.

  • Males have wings and can fly
  • Females are flightless and rely on pheromones
  • Mating occurs after a heavy rainfall

The male rain beetle’s behavior resembles that of a bumblebee, using the fat stored in their large abdomen to power their flight. This flight period is brief and occurs primarily in higher elevations like the Northern California region.

Diet and Tree Root Association

Rain beetles have a strong connection to tree roots for their diet and habitat. As scarab beetles, they feed on the roots of various orchard and forest trees, such as apple, nut, and Douglas fir trees.

Example of trees associated with rain beetles:

  • Apple trees
  • Nut trees
  • Douglas fir trees

Though feeding on tree roots, rain beetles are not considered as pests and do not harm populations of orchard and forest trees. As these beetles mainly inhabit areas of higher elevations, they do not pose a significant threat to trees in lower areas.

Rain Beetle Bumblebee
Flight Limited flight Longer flight
Diet Tree roots Nectar
Habitat Higher elevations Various

In summary, the behavior and ecology of rain beetles are characterized by their unique mating process, flight patterns, and diet related to tree roots in higher elevations. Understanding these aspects of their lifestyle can help in the study and conservation of this fascinating species.

Evolution and Ancestry

Relation to Scarab Beetles

Rain beetles belong to the Scarabaeidae family, a large and diverse group of insects. Their physical features include:

  • Brown color
  • Shiny appearance
  • Hairy body

These beetles share some similarities with the Geotrupidae family, commonly known as earth-boring dung beetles. However, they are classified as distinct families within the Coleoptera order.

Fossil Record

The earliest known ancestors of scarab beetles, including rain beetles, date back to the Late Cretaceous period. Fossil evidence of ancestral species such as Cretocoma and Proteroscarabeus have been found in regions like Mongolia and China. These prehistoric beetles were likely similar to their modern-day descendants in terms of size and appearance.

Conservation and Human Interaction

Rain Beetles and Pest Control

Rain beetles, belonging to the Pleocoma genus, are native to the western United States, specifically in regions like California, Oregon, and Washington. These beetles are known to inhabit moun habitats, including those at higher elevations, such as Douglas fir forests.

While not directly considered beneficial for pest control, they do play a role in the natural ecosystem. Some features of rain beetles include:

  • Active during the rainy season
  • Larvae feed on plant roots, while adults consume little to nothing
  • Attracted to lights at night

Adapting to Climate Change

As climate change affects the environment, species like rain beetles may also experience changes in their populations and habitats. Factors to consider include:

  • Alterations in precipitation patterns
  • Changes in availability of plant roots for larvae
  • Shifts in temperature affecting the distribution of foothill and mountain habitats

Rain beetles’ adaptation to climate change may have broader implications for other species in the ecosystem, such as the bumblebee, which shares habitat with the Pleocoma.

Rain Beetles Bumblebees
Primarily nocturnal Active during the day
Prefer wet conditions Prefer warm weather
Feed on plant roots (larvae only) Feed on nectar and pollen

Understanding the effects of climate change on rain beetles and interconnected species can help in developing effective conservation efforts for maintaining healthy ecosystems in areas like northern California.


  • Bugman

    Bugman aka Daniel Marlos has been identifying bugs since 1999. is his passion project and it has helped millions of readers identify the bug that has been bugging them for over two decades. You can reach out to him through our Contact Page.

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  • Piyushi Dhir

    Piyushi is a nature lover, blogger and traveler at heart. She lives in beautiful Canada with her family. Piyushi is an animal lover and loves to write about all creatures.

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25 thoughts on “Inside the World of Rain Beetles: Understanding Their Significance”

  1. Maria Smith is correct, this is a female rain beetle. The large size, reddish color and most importantly, the structure of the tarsi and antennae will separate the females from the males. Sometimes the females will leave their burrows and walk around. Female rain beetles are always a great find. Males are much easier to find, generally, as they fly and are attracted to lights. Based on the locality, I strongly suspect this to be the species Pleocoma badia. Please consider posting a photo on I’ve been running a rain beetle thread over there and this would be an interesting sighting.

    • Thanks so much for the correction. We will write to Maria and suggest that she post to BugGuide. Meanwhile, feel free to link to our posting.

  2. Weather was clear skys , breezy, and very warm with almost all snow melted , except for north side slopes.. 2 to 4 inch snow packs there in spots , going fast. gst

  3. I have a number of these beetles flinging themselves into my window tonight, after the rain started. Very unusual. November 1, 2015

  4. I have moved back to the Hood River valley after being gone for many years. I have seen alot of these beetles around here and had no clue what it was.

  5. Dawn and Daniel , I believe that there is a strong probability that this Rain Beetle is in fact a male Pleocoma staff Schaufuss 1870 , from the photo assessment and locality association . The larvae have been found to be feeding on Oak , Buck Brush, and anything close by and close in taste and age ,when the first two food sources are not available. Cheers, Gene St. Denis – Sierra Nevada Research –

  6. Daniel, it appears to me that you have pictures of Pleocoma staff males , given the limited local information . I collected a nice series last week and will post pics and narrative soon, once I recover from the week straight rain beetle trip thru northern California . If Crystal gets any pictures of the Hood River beetles, then I could ID them . Cheers ! Gene St. Denis Sierra Nevada Research

    • Thanks Gene. Your submissions over the years means that WTB? must have one of the most comprehensive archives of Rain Beetles available on the internet. We eagerly await your new submissions.

  7. Daniel and Paul, it appears to be an early flying Pleocoma hoppingi male . They normally fly in December/ January around 4,000 feet in that eco- zone of the Sierras . Is that the only one that you found Paul ? Cheers ! Gene St. Denis Sierra Nevada Research

    • I found a coppery orange beetle in Arnold, California (4000′)on January 19, 2020. The one I found seemed to have less “furry” legs than the pictured earlier but the coloring (no black on the face) is similar to the /Users/Sandy/Desktop/Beetle, Arnold, CA, 2-11-2020.jpegother beetle reported in Arnold.Don’t know how to insert photo in this message. 🙁

  8. Daniel , the normal elevation range is 5,000 to 6,000 feet . However, they have been found on occasion below that level in the past . Cheers ! Gene St. Denis Sierra Nevada Research

  9. Daniel and Stuart , after reviewing the provided images I believe that you have some wonderful male Pleocoma staff Beetles ! The larvae will eat Pine , Oak, Ceanothus Buck Brush , Deer Brush , Etc. , what ever is the Prevalent Food Source and Long Lasting / Lived in that Exact Area Environment . Cheers ! Gene St. Denis Sierra Nevada Research

  10. Daniel and DR , it appears from the angled photo that you have a fine male specimen of Pleocoma staff – Named by Schaufuss 1870 . Cheers ! Gene St. Denis Sierra Nevada Research


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