California Prionus: Essential Facts and Insights

The California Prionus, also known as Prionus californicus, is a large beetle species found primarily in the western United States.

Its larvae are known to cause significant damage to a variety of plants, making it a pest of concern for both agricultural and horticultural industries.

One key feature of the California Prionus is its wide host range, including deciduous trees, shrubs, conifers, grapes, hops, fruit trees, and caneberries, making it a highly adaptable species in urban and natural landscapes.

California Prionus

Its prevalence has grown in recent years, particularly affecting fruit tree populations.

As the larvae of the California Prionus feed on plant roots, this can cause damage and ultimately lead to plant decline or death.

It is crucial for gardeners, landscapers, and farmers to be aware of the potential threat that the California Prionus poses and implement management strategies to protect their plants and crops.

California Prionus Overview

Adult Beetles

California Prionus, scientifically known as Prionus californicus, is a species of insect belonging to the Longhorn beetle family, Cerambycidae.

These beetles, also known as round-headed borers, are commonly found in the American West. The adult beetles can be as large as 2.5 to 5.7 cm.

Here are some features of the adult beetles:

  • Large size
  • Long, segmented antennae
  • Reddish-brown body color

Larvae

California Prionus larvae are known for their broad host range, which includes most deciduous trees and shrubs found in both urban and natural landscapes.

They can also attack some conifers, brambles, grapes, hops, fruit trees, and caneberries. They have become a prominent pest of fruit trees.

Characteristics of the larvae include:

  • Creamy-white color
  • Segmented, C-shaped body
  • Up to 2.5 to 5 cm in length

Comparison between Adult and Larvae:

Adult Beetles Larvae
Size 2.5 to 5.7 cm 2.5 to 5 cm
Color Reddish-brown Creamy-white
Shape Elongated, cylindrical Segmented, C-shaped
Host range Do not feed Broad: fruit trees, shrubs, conifers, and more
Pest status Minimal Significant

Habitat and Range

United States Regions

The California Prionus, also known as the California root borer, is native to North America.

It can be found in various regions across the United States, from Alaska all the way down to Mexico.

These beetles are particularly prevalent in the Intermountain West region.

Trees and Crops Affected

California Prionus beetles have a broad host range and are known to infest several types of trees and crops. They commonly affect:

  • Deciduous trees: These beetles feed on a wide array of deciduous trees, including fruit trees such as apricot, peach, and sweet cherry.
  • Conifers: Their host range also includes some conifers, which are mostly evergreen trees.
  • Shrubs: The California Prionus can infest various shrubs found in urban and natural landscapes.
  • Brambles: This insect also attacks brambles, such as blackberries and raspberries.
  • Agricultural crops: The California Prionus is known to infest several perennial crops, including hops, grapes, and caneberries.

A comparison between some of their common host plants is shown in the table below:

Tree/Crop Type Examples
Deciduous Trees Apricot, Peach, Sweet Cherry
Conifers Pine, Douglas Fir
Shrubs Various species found in urban landscapes
Brambles Blackberries, Raspberries
Agricultural Crops Hops, Grapes, Caneberries (like raspberries)

It’s essential to monitor these host plants for signs of infestation, as the California Prionus can cause severe damage to both trees and crops in an orchard.

Life Cycle and Reproduction

Eggs

The California Prionus (Prionus californicus) lays its eggs mainly in the soil where the larvae will hatch and start to feed on tree and shrub roots.

Females can produce up to 200 eggs in their lifetime. The eggs are generally deposited near the base of the host plants.

Eggs hatch between 10 to 20 days after being laid.

Pupal Stage

After hatching, the larvae spend most of their life cycle underground. Their life cycle can take from 3 to 5 years to complete, with the majority of the time spent as larvae.

They feed on roots, playing a vital role in the ecosystem by breaking down and recycling nutrients in the soil.

File:Prionus californicus.jpg

Prionus californicus adult and larvae. Dorsal view. Laboratory photo.

Source: David Gent, USDA Agricultural Research Service, Bugwood.orgCC BY 3.0, via Wikimedia Commons

Pupal cells are often formed from the soil material by the pre-pupa, which then molts into a pupa inside the cell.

Adult Stage

Once the larval and pupal stages are completed, the California Prionus emerges from the ground as an adult.

Adults are active during the night and are attracted to lights. They are relatively short-lived, lasting only a few weeks.

During this time, males and females will mate to ensure the continuation of their species.

Mating Disruption: In some cases, mating disruption techniques can be used to control the population of the California Prionus.

These methods include the use of synthetic pheromones to reduce the chances of successful mating among the insects.

Here are some characteristics of the California Prionus at different life stages:

Life Stage Characteristics
Eggs Laid in soil near host plants
Pupal Pupal cells formed from soil material
Adult Nocturnal, short-lived, attracted to lights

Detection and Monitoring

Signs of Infestation

California Prionus infestations are typically identified by observing various symptoms in affected trees. Some key signs to look for include:

  • Canopy dieback: Gradual or sudden loss of leaves and branch health
  • Tree stress: Overall decline in tree vigor

An important aspect to keep in mind is that California Prionus larvae feed on live root material, particularly girdling and consuming root cambium.

This can lead to a weakened tree foundation and hinder the tree’s ability to absorb water and nutrients.

Pheromone Monitoring

Researchers are currently working on developing a monitoring tool for California Prionus infestations through the identification of female sex pheromones.

Here’s a brief overview of pheromone monitoring:

  • Female California Prionus produce a volatile pheromone that attracts males for mating
  • Pheromone traps can be employed to capture and document male California Prionus presence

A successful pheromone monitoring system could eventually lead to effective management of these infestations using mating disruption or mass trapping techniques.

Control and Management

Biological Control

Although general soil-dwelling predators like ground beetles and fungi may offer some level of control, no specific natural enemies have been identified as effective biological controls.

Nocturnal vertebrates such as rodents may consume adult beetles and help reduce their population.

Cultural Practices

There are several cultural practices that can be employed to reduce the risk of prionus infestations or minimize their impact:

  • Rotate crops
  • Implement deep plowing
  • Avoid planting susceptible crops in sandy soils
  • Remove potential host plants

These practices can limit the availability of ideal breeding sites for adult beetles and reduce root damage to host plants.

Chemical Control

Chemical control options for California Prionus are limited due to the subterranean nature of the larvae and the nocturnal habits of the adults.

Some soil treatments, such as insecticides, may provide temporary control, but their effectiveness can vary depending on soil type and application timing.

Pros:

  • Offers short-term protection
  • May reduce the population of adult beetles

Cons:

  • Limited effectiveness due to beetles’ habits
  • Potential negative environmental impact
Control Method Pros Cons
Biological Natural predators No identified effective control
Cultural Practices Reduce breeding sites & root damage Requires consistent implementation
Chemical Short-term protection, population reduction Limited effectiveness, environmental concerns

By employing a combination of these control and management strategies, the damage caused by California Prionus can be minimized, protecting valuable vine crops and other plants from this destructive pest.

Conclusion

In conclusion, the California Prionus, a native longhorn beetle of the American West, poses significant threats to a variety of plants, particularly fruit trees, through its extensive larval root feeding.

With a broad host range and increasing prevalence, especially in the Intermountain West region, vigilant monitoring and diverse management strategies are essential.

Understanding its distinct life stages, identifying signs of infestation, and employing a combination of biological, cultural, and chemical controls are pivotal in mitigating the impact of this pest.

Reader Emails

Over the years, our website, whatsthatbug.com has received hundreds of letters and some interesting images asking us about California Prionus’. Scroll down to have a look at some of them.

Letter 1 – California Root Borer

big bug
Location: little rock california
June 28, 2011 4:04 am
What is this thing looks like a huge beetle with pincher’s of death and hisses like a roach
Signature: idk

California Root Borer

Dear idk,
This beauty is a male California Root Borer,
Prionus californicus, and while we agree that he is a huge beetle, females of the species can be almost twice as large.  See this BugGuide image for a size comparison between the sexes. 

What you are calling “pincher’s of death” are the well developed mandibles that can be used for defense, though the primary function is to enable an adult beetle to chew its way out of the wooden chamber where it pupated.  Larvae bore into wood, primarily roots, feeding on the wood for several years before maturing. 

The mandibles can probably snap a small twig and adult California Root Borers should be handled with caution to avoid being pinched.  The hissing is called stridulation and it is a sound produced by many beetles upon rubbing together specific body parts.

Letter 2 – California Root Borer

Big Beetle
Hello Bug Man!
My husband and I came across this beetle while camping in San Diego county. It was early August, 2005 and this bug flew into our camp two nights in a row, but we only saw it at night.

I didn’t see anything that resembled this one in your “Beetle” section. It was also very hard and heavy… at least it sounded that way when it would land.
BTW… LOVE this website!!
Excitedly Awaiting a Response!
San Diego, CA

California Root Borer
California Root Borer

Hi Excited,
Your large beetle is a California Root Borer or California Prionus, Prionus californicus. The antennae on your specimen indicates that it is a male. Our edition of Charles Hogue’s Insects of the Los Angeles Basin indicates that adults emerge in early summer.

The late appearance of this specimen might be a sign of impending climactic changes. The California Root Borer is attracted to lights. The large grubs were eaten by Native Americans and there is a growing interest Entomophagy, of the consumption of insects, so we will also file your letter under Tasty Morsels, our Edible Insect section.

Letter 3 – Another California Root Borer

Is this a beetle?
Location:  Southern Oregon
August 13, 2010 2:19 am
Hi. There is a huge beetle on my window screen. The body measured 1.5” and the antennae are probably the same. I live out in the woods of southern Oregon and keep getting weird bugs trying to get in but this one is creepy. I can’t get a good picture of the head or mandibles (which are huge!). Can you please tell me what this is? Thanks. 🙂
Anna

California Root Borer

Hi Anna,
This impressive beetle is a California Root Borer,
Prionus californicus.  The mandibles are necessary so that the adult beetle can chew its way to the surface after pupation.  The larvae bore in rotting wood that is in contact with soil and they pupate inside the wood. 

Adults, especially males, are attracted to lights.  Your specimen is a male judging by his antennae.  This is the second image of a California Root Borer we are posting this morning.

Letter 4 – California Prionus

Knock, knock. This really bugs me!
Kept hearing some thumping noise the other night ago by the kitchen back door. The door has a window and screen, and the window was wide opened. So, go take a look-see and see this buggar of a bug! eek! There were two at first, but the other one flew the coop, so to speak, and probably off to gather reinforcements.

The ‘thumping’ was these two airborne of unknown squardron knocking up against the window screen. Emanating from this creature was a kind of scratching sound. I peered closer and, Swear To Beetlejuice, I saw his bicuspids gnawing away at the screen in attempt to enter the premises, to therefore I presumed, start gnawing on away on yours truly.

He even stopped for a moment and looked up, eyeing this reporter with a salivating, hungry grin, accompanied with a most evil and intent. After licking his chops, said invader then resumed to eat away at the screen once more with renewed and zealous relish.

Well, nonchalantly I backed off and moseyed over to the den, (at a dead heat run, mind you), to retrieve a fly swatter… in this case in the form of my trusty 12 gauge double-barrel shotgun. Catching my eye, though, as I hurriedly was loading up a couple of double-ought buck shotshells, was the ol’ digital camera sitting upon the table.

Hm, better first take a photo of this beastly bug, then there be proof of attack by this man-eating Thing so the insurance company will therefore remit reimbursement for the two big, smoking holes that were momentarily about to appear in the window screen.

Too, identification should be in order, which may assist in saving All Mankind, if your brave writer here should not. Such the reason for rushing off a photo or two to your well admired web site. Rather quickly, I might add, for… wait, let me check, oh yes, this Monster nearly has chewed a hole big enough for his head to squeeze though!!! yikers! Quick reply needed. Imminent! Regards,
Jim L.

P.S. Vitals: Where; Tulare County, Calif. Western Sierra Nevada Foothills. 2000′ elev. Mere yards from the North Fork Tule River. Time Of Event; Night time, (eerily dark). Kitchen porch light on. Circa July 01, 2008. Creature of Uncomfort; Winged. Just under 3″ length. (Thumbnail photo to soon follow, as I took a photo with a tape measure next this abomination, but he moved, (and barked!), thus the pic was blurred and ruined. And, No , it was not from my shakingly steady hand, hrmph!!). ;o) jim

Hi Jim,
A query this verbose (actually, we are not certain there is even a question here) demands a concise answer: California Prionus, Prionus californicus.

Letter 5 – California Prionus

Hello,
Thanks to your well-designed site, I was able to identify the large beetle in my driveway as a Prionus californicus. This was in Orange, California on July 2nd, 2004. Thought I’d share my photo with you, in return for the quick and detailed information.
Thanks again!
Marc McNaughton

Thank you for your kind words Marc, and also for the high quality photo. The species, according to Essig: “ranges along the Pacific Coast from California to Alaska and is also reported from Arizona, New Mexico, colorado, and Nevada.

Adults are nocturnal and fly in midsummer to fall. They are readily attracted to lights. The adults make a loud humming noise on the wing and often strike the windows at night with an impact that almost breaks them.”

Letter 6 – California Prionus

Prionus californicus I presume
Hello there from Palomar Mountain, California (elevation approx. 5,600 ft.)!
My husband and I found this beauty on our front porch last night. It made a wonderful quick “hiss!” to let me know that getting petted was not an appreciated activity, but it was not sufficiently annoyed to wander off. So, here is a photograph of it—an entire 5 centimeters of Prionus californicus goodness .. . and more evidence that yes, these fellows make noise when pestered.

I also note that its antennae and legs appear a bit thinner than those of the other large Prionus californicus picture posted on your site . I’m wondering if you could comment on whether that is a variation which occurs with location, gender, age, randomly, or with something completely different.

I know you’re busy so no worries if you have no time to respond. Also, if your email service won’t display the photo, I can resend as an attachment.
Thanks for your lovely website. Be well,
Laura

Hi Laura,
You are correct in your identification of a California Prionus. There is often individual variation within a species, but camera angle could also account for distorted perspective in some photographs which may explain your observation about antennae and leg girth.

Letter 7 – California Prionus

CA Prionus
Dear Bugman,
Thanks for such a fun and educational site – now that I’ve found it (a friend forwarded the link) I’m adding it to my Favorites list. Thought you might like a couple of photos of one of the California prionus beetles that visited our carport last year. We’re in northern CA, in the Auburn area and they’re fairly frequent visitors in the summer.

I love the screeching sound they make – creepy and cool at the same time. I can’t recall exactly, but that sound is either made by them rubbing wing against wing or hind leg against wing – I’ll have to gently harass one this year to find out 🙂
Ann Ranlett
Nature Illustrations & Pet Portraits
http://www.annran.com/

Wow Ann,
Your photos are beautiful. He sure is a handsome male specimen. We have been trying to find information on the stridulation of Prionus Beetles, but have been unable to locate any information quickly. We eagerly await your assessment. Also, we linked to your awesome site.

Letter 8 – California Prionus

What’s this bug?
Hi there!
The only posts I found on this cool site were from 2004 and 2003 – are you guys still identifying bugs? I found one tonight while on a walk with my dog. It was on the cement under a bright light, and it never walked anywhere (though its body was moving while I was taking photographs).

I photoshopped the phone closer to the beetle than I was willing to actually put it myself… can this be a California Prionus? What do they eat, and are they beneficial or harmful?
Mountain View , CA

Hi there Mountain View,
This is very exciting. We have never had an entire town write to us. We agree that this is a female California Prionus. The larvae eat live, dying and decomposing wood from trees, shrubs and woody vines. The grubs can be injurious to trees, including fruit trees. We cannot locate any information on adult food preferences.

Letter 9 – California Prionus

What bug is this?
We found this bug (beetle?) on the front step of our home in Southern California. It was about 2 inches long. What is it? When we moved it, it made a loud “swishing noise”. We think the noise came from it’s back legs. What a cool BUG!
Gage

Hi Gage,
This majestic beetle is a male California Prionus, Prionus californicus. The beetle grubs are root borers that, if plentiful, can be very injurious to native trees like oak, madrona, and cottonwood as well as some fruit trees.

Letter 10 – California Prionus

Big Beetle
This was on my AC unit at about 11pm in Klamath Falls Oregon. I’ve never seen anything like it. I’m guessing it’s some type of borer from the huge antenna but if I was sure, I wouldn’t be asking the bugman. Hope you can, hell I know you can, let me know what this monster is. Thanks
Dave

Hi Dave,
This is a male California Prionus, Prionus californicus. It is one of the borer beetles in the family Cerambycidae.

Letter 11 – California Prionus

yikes!
hi bugman,
i came across this fine creature late one evening in ahwahnee, california (about 30 minutes southwest of yosemite). i threw my keys in the photo for scale. the owners of the b&b i was staying at where as clueless as i, they had never seen one. i was about to put on my overshirt when i noticed the bug – it looks like some kind of beetle to me, but i am not versed in bugs at all. is it possible for you to identify this & solve our collective mystery? thank you,
barbara

Hi Barbara,
What a magnificent male California Prionus, Prionus californicus, you encountered. He was probably attracted to the lights on in your room at night.

Letter 12 – California Prionus

California Prionus
We found this at night in El Cajon CA. It was about 2.5 inches minus the antennae. Thanks to your site for helping us identify it as a California Prionus. In the jar it made a squeaking sound. We let it go the next day. The pictures are big but they came out pretty nice. (We named it Cruliette. ) Thanks,
Kylie, David & Emily

Hi Kylie, David and Emily,
Thank you for sending in your wonderful image of a California Prionus, Prionus californicus, also known as the California Root Borer.

Letter 13 – California Prionus

Large Beetles
Fri, Jun 12, 2009 at 6:54 AM
Could you help tell me a bit about this type of bug? I found this one on my patio, and have never seen this type of bug here. I have seen something similar to this farther east in Barstow, only it was about 6″ in length. This one was only about 2″. Have heard them called date bugs, and large cockroaches farther east, but this one was smaller and about 50 miles east of Barstow, CA.
JR
Apple Valley, CA

California Prionus
California Prionus

Hi JR,
This is a California Prionus, Prionus californicus.  We just finished posting an image of an eastern relative, the Tile Horned Prionus.  The Prionids are a group of Longicorns or Long Horned Borer Beetles. 

The California Prionus has grublike larvae that bore in the wood of oaks, madrone, cottonwoods, fruit trees and Eucalyptus trees according to Charles Hogue in Insects of the Los Angeles Basin.  Your specimen is a male.  The larger female has less impressive antennae.

Letter 14 – California Prionus

What kind of bug is this?
May 30, 2009
This little fella was layin on his back around 1AM on my porch. I talked him into the jar (with air holes), snapped some shots, then took him back to my porch. When he was in the jar, my wife and I would gently get him on his feet, but he’d always flip back over. The spot where I released him on the porch, he’s still at, but dead. 🙁
He’s got 6 legs, 2 antannae, pincers on the front. He’s bigger than my thumb (adult male), maybe 2 inches long.
Myk
Oakley, CA USA

California Prionus
California Prionus

Hi Myk,
Your beetle is a magnificent California Prionus.

Letter 15 – California Prionus

Subject: Root Borer
Location: San Marcos, CA
September 27, 2012 1:52 am
From what i gathered from your site, this appears to be a Broad Necked Root Borer? I guess that would make sense since it crawled out of my lawn. Have to say its strength was impressive as it powered its way back into my thick tall fescue.
Signature: redfive

California Prionus

Hi redfive,
The Broadnecked Root Borer is an eastern species.  This is a California relative, the California Prionus, and it is a male judging by his impressive antennae.  It is a little late in the season for a sighting, so we are speculating that this image is from your archive.

Daniel,
Yes, this is from my archive, but still not too long ago. August 7 of this year to be exact.  Thanks and I’ve enjoyed your website for awhile. Keep up the great work!

Letter 16 – California Prionus

Subject: Domestic or foreign
Location: Santa Rosa, Ca
July 5, 2015 8:27 am
Found this one in our dining room in Santa Rosa, Ca. This is a first sighting.
Signature: Pat

California Prionus
California Prionus

Dear Pat,
This is a native species known as a California Prionus or California Root Borer.  According to BugGuide:  “Adults active summer through early fall; fly at dusk or in the evening.  Food Larva feed primarily on living deciduous trees (oaks, madrone, cottonwood) and are also recorded from roots of vines, grasses, and decomposing hardwoods and conifers.

Will also attack fruit trees growing on light, well-drained soils (e.g. apple, cherry, peach).”  Especially males are attracted to lights, and your individual is a male as evidenced by his highly developed antennae. 

Wonderful!  We have all those trees you mentioned.  Bad enough we have to contend with Sudden Oak Death.  But, thank you very much for your response.
Pat

Letter 17 – California Prionus

Subject: Cockroach?
Location: Corona, CA, USA
July 13, 2015 12:06 pm
I found this bug dead in the gutter in front of our house. We are in Corona, CA and are backed up to the Cleveland National Forest (5 feet passed our gate ). The weather here has been sunny, low to mid 80s for a few days now.

There was a sprinkle of rain a few days ago. I put my foot next to the bug for size reference, it was maybe 2.5 to 3 inches long. What is this thing? It almost looks like a cockroach but I can’t find any that look like this.
Signature: Kate

California Prionus
California Prionus

Dear Kate,
This magnificent beetle is a California Prionus, and according to BugGuide:  “Adults active summer through early fall; fly at dusk or in the evening.  Food  Larva feed primarily on living deciduous trees (oaks, madrone, cottonwood) and are also recorded from roots of vines, grasses, and decomposing hardwoods and conifers. Will also attack fruit trees growing on light, well-drained soils (e.g. apple, cherry, peach).”  Your proximity to the Cleveland National Forest is likely a factor in your sighting.

Letter 18 – California Prionus

Subject:  Big Beatle
Geographic location of the bug:  United states southern California, Rancho Cucamonga
Date: 06/10/2020
Time: 12:35 AM EDT
Your letter to the bugman:  Hello,
We came home to find this guy in our driveway. He wasabout 2 inches long with long antennae and a dark red/marrone/brown color. Do you know what kind of Beatle he is?
How you want your letter signe:  The Davies

California Prionus

Dear Davies,
June and July are the months we receive most North American Prionid sightings, a subfamily of especially large Long-Horned Borer Beetles.  This is a California Prionus or California Root Borer,
Prionus californicus, and according to BugGuide

“Larva feed primarily on living deciduous trees (oaks, madrone, cottonwood) and are also recorded from roots of vines, grasses, and decomposing hardwoods and conifers. Will also attack fruit trees growing on light, well-drained soils (e.g. apple, cherry, peach).”

Letter 19 – California Prionus Grubs

Possible Prionus californicus
Got a good one for ya…I have found many photo references to the adult California Prionus and not many pics of the larvae. My husband and father in law were removing the last bits of a long since dead oak tree and found 2 gargantuan grubs in the midst of the rubble. The larger of the two measured 4 inches (it was not fully extended either!) and the smaller was just over 3 1/2 inches.

They have some nasty looking choppers and a reddish pink “tongue” looking thing that would come out and retract on the top of it’s black head. I’m certain this should be turned over to Spielberg or Wes Craven for their next horror movie!
EW!!
Thanks for any help.

Hi Sara,
We are in agreement with your identification, and we are happy to post your photos and letter.

Letter 20 – California Root Borer

Large beetle in E. Washington state
July 18, 2009
We found this large black/brown/red beetle with reddish legs and a red underside on our driveway on July 18 in Spokane Valley, WA. It was discovered just after dusk. It is 3-4 inches in length.

When touched on the back, it would raise it’s back legs to make a scraping/hissing sound. What is it? What does it eat? Are they common in this area (we’ve never seen one before)
Lobdells
Spokane Valley, WA

California Root Borer
California Root Borer

Dear Lobdells,
This is a California Root Borer, Prionus californicus.  It ranges from Canada to Mexico on the West Coast, as far inland as Nevada and Montana.  According to Charles Hogue in Insect of the Los Angeles Basin, the larvae bore into roots of oaks, madrone, cottonwood, fruit trees and eucalyptus. 

Adults emerge and fly in early summer and are attracted to lights.  The beetle is not considered rare, but populations may be very localized. Your specimen looks like a female as males are smaller and have more exaggerated antennae.

Letter 21 – California Root Borer

I have found this hanging it out, didnt like pics taken
Location:  Selah,WA
August 13, 2010 3:29 am
Hey Bugman 🙂 glad i found the website, i came home at 12am and found this beetle hanging out in front my neighbors door
Thank you Lucas Clark

California Root Borer

Hi Lucas,
You have encountered a California Root Borer,
Prionus californicus, a member of the Longhorned Borer Beetle subfamily Prionini.  There are sixteen known species in the genus in North America that look similar and you may read about them on the BugGuide information page for the genus.  Your specimen is a male as evidenced by his well developed antennae.  The males are frequently attracted to porch lights.

thank you i appreciate the speedy reply, i guess i have never seen them before 🙂

Letter 22 – California Root Borer

Huge Beetle
Location:  Oregon, USA
August 17, 2010 12:29 am
Could you please Identify this monster for me?
Thank you
Robert

California Root Borer

Hi Chris,
What a stunningly beautiful specimen of a male California Root Borer,
Prionus californicus.  Your photo is the third we have received in the past week.

Hello Daniel,
Thank you so much for the identification. Amazing! We carefully put it in a cup and released it so it wouldn’t get hurt.
Thanks again

Letter 23 – California Root Borer

Prionus pocularis ?
Location: Westside of Okanagan Lake near Kelowna BC
August 2, 2011 2:19 pm
Just wondering if this may be the Prionus pocularis
Signature: oklakebc

California Root Borer

Dear oklakebc,
Prionus pocularis is a species found in eastern North America.  In our opinion, this is Prionus californicus, the California Root Borer, which ranges from Mexico to Canada in the western states.

California Root Borer

Letter 24 – California Root Borer

Subject: What is this?
Location: Kelowna, BC
August 19, 2014 8:44 am
A friend posted a picture of this and nobody knows what it is. She lives in Kelowna BC.
Signature: Courtney

Male California Prionus
Male California Prionus

Dear Courtney,
Though is it named for the Golden State,  the range of the California Root Borer,
Prionus californicus, extends well beyond California as you can see from the sighting data on BugGuide.  According to BugGuide

“Larva feed primarily on living deciduous trees (oaks, madrone, cottonwood) and are also recorded from roots of vines, grasses, and decomposing hardwoods and conifers. Will also attack fruit trees growing on light, well-drained soils (e.g. apple, cherry, peach).”  This individual is a male which can be distinguished from the female by his developed, distinctly sawlike antennae.

Letter 25 – California Root Borer

Subject: Large beetle looking thing?
Location: Newberg, OR
August 20, 2014 9:47 pm
Hello, Bugman! Maybe you could tell me what bug this is? It’s very large (see penny in photo for scale)
It doesn’t appear to be a cockroach due the the antennae, so I’m stumped.
Signature: Buggily Yours

California Root Borer
California Root Borer

Dear Buggily Yours,
This is a male California Root Borer, and we posted another example just yesterday.

Letter 26 – California Root Borer

Subject: What is this?
Location: Roseville ca
May 16, 2016 6:42 am
I found this in the hallway this morning. It didn’t move at all when I passed by and was easy to catch with Tupperware container. It’s body is about an inch long.
Signature: Debbie

California Root Borer
California Root Borer

Dear Debbie,
This is a male California Root Borer,
Prionus californicus, which you can verify by comparing your individual to this image on BugGuide.  We thought this May sighting was a bit early and unusual, but BugGuide includes data on sightings in California as early as April. 

According to BugGuide:  “Adults active summer through early fall; fly at dusk or in the evening” and “Larva feed primarily on living deciduous trees (oaks, madrone, cottonwood) and are also recorded from roots of vines, grasses, and decomposing hardwoods and conifers. Will also attack fruit trees growing on light, well-drained soils (e.g. apple, cherry, peach).”

Thank you Mr. Marlos –
We do, in fact, have an apple tree and many oak trees in the field behind our home.  My boys were cutting wood yesterday.  I’ll bet it hitched a ride in with one of them or the dog.
The apple is dying a slow, mysterious death.  Could this critter’s activity be part of the problem?  Don’t worry, I won’t use pesticides.
-Debbie Mulligan

They don’t normally attack healthy trees, but they will attack a compromised tree. The drought has been tough on trees.

Letter 27 – California Root Borer

Subject: Large black beetle
Location: Southern California- beach area
June 2, 2016 1:59 pm
Hi- could you identify this bug for me? It was found in Southern California climbing through some wood chips at the local school. It didn’t move very fast but was really large! The second photo is when he flipped upside down. Thanks for your help!
Signature: Lacy

California Root Borer
California Root Borer

Dear Lacy,
This is a female California Root Borer, Prionus californicus, and you can compare your individual to this image of a pair of California Root Borers on BugGuide as well as this image of the ventral surface of another individual pictured on BugGuide

According to BugGuide:  “Larva feed primarily on living deciduous trees (oaks, madrone, cottonwood) and are also recorded from roots of vines, grasses, and decomposing hardwoods and conifers. Will also attack fruit trees growing on light, well-drained soils (e.g. apple, cherry, peach).”

California Root Borer
California Root Borer

Letter 28 – California Root Borer

Subject: 3 inch brown beetle Sonoma County
Location: Geyserville, Sonoma County CA
June 5, 2016 10:43 pm
This big beetle banged against our window 6/5/2016 and then sat quietly at the bottom of the sill. I caught it in a small vase without a fight, checked it out and let it go. What is it? It was really cool! Its claws clicked against the glass I caught it in and it turned itself into its back. It had a had time turning itself back over- I don’t think it was defense.
Signature: Jeff

California Root Borer
California Root Borer

Hi Jeff,
This is a California Root Borer, and we just posted our first sighting of the year two days ago.

California Root Borer
California Root Borer

Thanks! Makes perfect sense surrounded by grapes and deciduous trees.

Letter 29 – California Root Borer

Subject: Beetle at our door
Location: Santa Barbara, CA
June 12, 2016 9:27 am
Hi, I recently (June 2016) accidentally stepped on this bug (beetle?) as it was sitting on our front door mat. It was dark outside and the front porch light was on. I have lived at my home in Santa Barbara, CA for 30 years and have never seen one like this. A photo is attached showing it’s size with respect to a dime. Any idea as to what it might be? Thanks, Jim
Signature: Jim

California Root Borer
California Root Borer

Dear Jim,
This is a California Root Borer, and we posted several images of individuals in early June.

Letter 30 – California Root Borer

Subject: Roach or Beetle?
Location: Blackfoot, Idaho
July 22, 2016 8:37 am
We live in southeastern Idaho and have for almost 40 years and we have never seen anything like this. We found one in our garage last week (Mid-July) and another one last night in our garage as well. It is a little over 2″ long and makes a clicking-hiss sound when disturbed.

Initially we thought it was a cockroach and panicked at the thought of a cockroach invasion, but after doing some research, we think it is a beetle of some kind. Based on photos we have seen, it looks similar to the Palo Verde Borer. What do you think it is?

And will it cause any damage to our home, trees, etc.? We took several photos but it was on the move so most of them were blurry. Thank you for your help!
Signature: Christine

California Root Borer
California Root Borer

Dear Christine,
You are correct that this is NOT a Cockroach, and though it is not a Palo Verde Root Borer, it is a member of the same subfamily Prioninae.  This is a California Root Borer,
Prionus californicus,  though that name is somewhat deceptive as the range includes much of western North America and is not limited to California. 

According to BugGuide:  “Larva feed primarily on living deciduous trees (oaks, madrone, cottonwood) and are also recorded from roots of vines, grasses, and decomposing hardwoods and conifers. Will also attack fruit trees growing on light, well-drained soils (e.g. apple, cherry, peach).”  In our opinion, there is rarely enough of an infestation to damage healthy trees.

Letter 31 – California Root Borer

Subject: What’s this beetle?
Location: Mica, WA
September 5, 2016 8:45 am
We had a hatch of these beetles back in late July and I’ve never seen them before or since. The cat was swatting this guy and he hissed at her and she jumped straight up in the air! I came over and rescued him and he hissed at me too. Shot some pics and the released him back into a pine tree unharmed. Saw a couple of them in the next few days.
We live in Mica, WA 99023
Signature: Keep on buggin,

Male California Root Borer
Male California Root Borer

This is a California Root Borer, Prionus californicus, and despite its common name, it is found in many western states according to BugGuide.

Letter 32 – California Root Borer

Subject:  Beetle
Geographic location of the bug:  Ramona, CA
Date: 09/04/2017
Time: 11:29 PM EDT
My 6 year old found this dead beetle in my mothers succulent garden. What is it? Sorry he is missing an antenna.
How you want your letter signed:  Jennifer Diaz and Future Bug Doctor Harrison

Male California Root Borer

Dear Jennifer and Harrison,
This impressive beetle is a male California Root Borer, and males of the species have much more exaggerated antennae than the female.  The female releases pheromones and the male locates her with his antennae.  According to BugGuide citing Evans & Hogue: 

“Adults:  Robust, reddish-brown to almost black; Three sharp spines on each side of pronotum; Saw-toothed antennae with 12 segments (scape, pedicel, and 10 flagellomeres…pedicel very short, 1st flagellomere longest, then decreasing in size apically).  Males with antennae distinctly sawlike, more than 2/3 length of body.  Females with more slender antennae, about 1/2 length of body.” 

Thank you Daniel for the identification.  He is now apart of our bug collection:)  Take care.
Sincerely,
Jennifer Diaz and Harry

Letter 33 – California Root Borer

Subject:  WT Heck is this big
Geographic location of the bug:  California
Date: 06/11/2018
Time: 03:54 AM EDT
Your letter to the bugman:  We found this bug on the porch, our house is in the country on an orchard. I’ve looked everywhere and still can’t figure it out. It’s definitely a big, but I’ve never seen anything like it before. Any ideas?
How you want your letter signed:  Great big bug

California Root Borer

Your great big bug is a California Root Borer, Prionus californicus, and according to BugGuide:  “Larva feed primarily on living deciduous trees (oaks, madrone, cottonwood) and are also recorded from roots of vines, grasses, and decomposing hardwoods and conifers. Will also attack fruit trees growing on light, well-drained soils (e.g. apple, cherry, peach).”  The antennae on your individual indicates it is a male.  We suspect the porch light attracted it.

That’s completely fascinating! Are they harmful to humans? Will they bite?

They have powerful mandibles and they might bite, possibly even drawing blood, but they are not venomous.

Letter 34 – California Root Borer

Subject:  A beetle I’m assuming…
Geographic location of the bug:  San Francisco Bay Area (East Bay)
Date: 06/10/2019
Time: 03:31 AM EDT
Your letter to the bugman:  Hello! I’m finding these large, 1 1/2-2” beetles hanging out completely still in my driveway during the summer months. I’m not looking to get rid of them, just like to know a bit more about what scares the crap out of me when I’m taking the trash out late at night! (They’re HUGE!). Thanks.
How you want your letter signed:  Kara W.

California Root Borer

Dear Kara,
This is a California Root Borer, one of the largest Beetles native to California.  They are attracted to lights, which might be the reason you are finding them in your driveway.  Though they are not aggressive, they do have very powerful mandibles, so you should handle with caution to avoid a nip.  Here is a BugGuide image for comparison. 

Letter 35 – Female California Root Borer

Huge Moth is northern California
July 23, 2009
I live in Northern California, and i found this insect at the place i work today, it’s huge and can fit in my palm and it the size of a small mouse. It somewhat looks like a giant moth – hornet hybrid.
random
Northern California – Tahoe

Female California Root Borer
Female California Root Borer

Dear random,
Your female California Root Borer, Prionus californicus, is actually a beetle, and more about the genus can be read on BugGuide.

Letter 36 – Female California Root Borer

Beetle like with big jaws red/copper color and 3-4 inches in size
July 23, 2009
Was camping in Northern california last weekend. Mendocino national forrest to be exact and we found this scurrying on the ground. It was huge and vicious. I would love to know what kind of bug this is? and how common these are? Thanks for your time.
Max Weisman
Mendocino county northern california

California Root Borer
California Root Borer

Dear Max,
This is a female California Root Borer, Priunus californicus, and common is a relative term.  The species is not uncommon, but populations may fluctuate from location to location.  We just returned from camping in the Mendocino woodlands and we were not fortunate enough to see a California Root Borer.

Letter 37 – Female California Root Borer

Large Beetle
Location: Ochoco National Forest, Oregon
September 11, 2011 3:51 pm
Found this magnificent (and large) beetle in the lawn in Central Oregon. Found Sep. 2011 at 4400’ during a typical 80o fall day. It was the only one like it that we’ve seen here. My searches on other beetle sites didn’t uncover anything close,
Signature: JDinOR

California Root Borer

Dear JDinOR,
This lovely beetle is a female California Root Borer,
Prionus californicus, and you can see a matching photo on BugGuide.

California Root Borer

This beetle’s ovipositor is protruding from the end of the abdomen.

California Root Borer

Letter 38 – Female California Root Borer Carnage

Giant beetle thing
Location: Southern California, summer
July 1, 2011 2:18 pm
This bug was flying around at night and you could hear it in the distance and getting closer, it landed and it had a dark, black striated shell and a reddish orange underbelly. It was over an inch long easily and after I threw boiling water on it and a glass of bleach it was still alive. It looks kind of like the broad necked root boarer on here but I would like to know for sure. Because it was the grossest thing I have ever seen and am hoping to not see another one.
Signature: Sara

California Root Borer Carnage

Hi Sara,
Your dead beetle is related to the Broad Necked Root Borer.  It is the California Root Borer and your individual is a female.  Grossness should not be considered a criterion for killing a creature, so we have to consider this posting as an example of Unnecessary Carnage and today is turning out to be a carnage heavy day.

Letter 39 – Male California Root Borer

Subject:  What kind of beetle is this?
Geographic location of the bug:  Boise Idaho on a screen
Date: 07/30/2018
Time: 12:23 AM EDT
Your letter to the bugman:  Please advise as to the kind of beetle is shown in the attached picture.
ThanksW!
How you want your letter signed:  Best find of the day

Male California Root Borer

Dear W!,
This wondrous beetle is a male California Root Borer,
Prionus californicus, and despite its name, it is reported from numerous other western states on BugGuide, including Arizona, Colorado, Idaho, Montana, Nebraska, Nevada, New Mexico, Oregon, Texas, Utah and Washington as well as British Columbia.

Authors

    by
  • Bugman

    Bugman aka Daniel Marlos has been identifying bugs since 1999. whatsthatbug.com 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.

  • 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.

21 thoughts on “California Prionus: Essential Facts and Insights”

  1. Thanks! I just had one of these bad boys hit the screen behind my head! Glad to be able to identify it before I take it into the kid school tomorrow. Thanks you guys!

    Reply
  2. We just found a dead California Root Borer in our driveway this morning in the Santa Cruz Mountains off Old Santa Cruz Hwy. about 7 miles from Los Gatos. We have a lot of coast live oaks, madrone, Douglas fir, and redwoods in this area.

    Reply
  3. I live in West Kelowna. I sent a photograph taken here July 14th, 2016, of a very large beetle (photo available to anyone) to a daughter in Ontario who got an opinion from a knowledgeable friend, identifying the beetle as the California Root Borer. The beetle was found dead on the sidewalk, outside a 4 storey building, probably attracted to lights at night.

    Reply
  4. I am responding to the initial posting about the sighting of a California Root-borer beetle in Kelowna, BC. I have read the one responce by Lars Karstad, I know Lars quite well as we both live in the same building, I heard about the beetle that Lars found on 14th July 2016, and a short time after I to found a dead specimen by our covered parking area which also is well lighted, this was on the 23rd July, and then on 26th I found a second one which was just alive, on the 29th July I found a third one, and a fourth one on 30th July; the third and fouth ones were very much alive and thought to be females, also the first one. All four were found in a covered parking stall which measures 16′ x 9′. the location is: Lat_ 49.825315 Long – 119.631909. I hope this is of some interest; I have photos of all four specimens.

    Reply
  5. Does this bug kill trees?
    Do trees (willow, cedar, pine) need to be cut down if they have small round holes in their bark- I don’t have the Asian Longhorn beetle.
    Thank you!

    Reply
  6. Does this bug kill trees?
    Do trees (willow, cedar, pine) need to be cut down if they have small round holes in their bark- I don’t have the Asian Longhorn beetle.
    Thank you!

    Reply
  7. I have a bug flying around my apartment that looks like the California prionus but smaller and is a steel grey color. It knows enough to hide when I turn on a light and is driving me crazy because I’m afraid it will land on me in bed. I swatted it with a magazine and thought it was dead but a few minutes later it was buzzing around my bed again. It’s about half an inch long but without a long body and looks like a beetle. The closest one pictured is the prionus, but it looks too big.

    Reply
  8. These bugs are at Comanche south shore me and my kids called it a night after theses bug bugs came flying hissing at us lol

    Reply
  9. Just found one at the bottom of our apartments stairs in Missoula Montana. Got on here to make sure it wasn’t an invasive species and released it next to a tree after finding out it is not. He seemed to be injured but maybe he’ll make it. Was considering picking him up bare haded to see if he would bite but decided against it. Very odd sounds it made. I have never seen one in Montana.

    Reply
    • I am from Missoula, and your comment caught my attention.
      I just found a huge female at our weekend house in Plains, MT.
      The size of this amazing bug made me fearful it wasn’t from around here. Glad to hear others in Montana have seen this bug!

      Reply
    • I am from Missoula, and your comment caught my attention.
      I just found a huge female at our weekend house in Plains, MT.
      The size of this amazing bug made me fearful it wasn’t from around here. Glad to hear others in Montana have seen this bug!

      Reply
  10. On 06/02/2020, about 8:00 pm, there was a large beetle behind my house. I only had a phone camera so the photo I took is only fair.
    It’s been identified as a borer, but I’m hoping to specifically identify it.
    How can I submit a photo here?

    Reply
  11. On 06/02/2020, about 8:00 pm, there was a large beetle behind my house. I only had a phone camera so the photo I took is only fair.
    It’s been identified as a borer, but I’m hoping to specifically identify it.
    How can I submit a photo here?

    Reply
  12. Is it possible they live in Northern Michigan? I found a beetle similar to this in my basement about an inch and a half long or more. I have a picture but not great and don’t know where to post it for description

    Reply
  13. My wife found one in lake county and it was huge. We were very careful with it and moved it over from our dirt project and set it into the neighbors yard. We left it alone and it scurried off.

    Reply

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