Are Oil Beetles Dangerous? A Quick Guide to Assessing Risks

Oil beetles are a type of blister beetle that may pose risks to humans due to their defense mechanism.

Belonging to the genus Meloe, these beetles are named after the yellowish oil they excrete from their joints when under threat or pressure.

This secretion contains cantharidin, an irritating substance that can cause blistering on human skin.

While oil beetles might not be life-threatening, they can still inflict discomfort on those who come into contact with them.

For example, when squeezed or distressed, the cantharidin in the excreted oil may cause skin irritation and blisters for people who have sensitive skin or allergies.

In addition to their potential dangers to humans, oil beetles can also harm the environment and wildlife.

For instance, oil spills caused by accidents in the ocean might have serious consequences for marine animals and plants.

These spills can damage the insulating abilities of fur-bearing mammals like sea otters and affect the water-repellency of birds’ feathers, leading to hypothermia and even death among these creatures.

Understanding Oil Beetles

Classification and Species

Oil beetles belong to the genus Meloe and are a part of the Coleoptera family.

There are over 20 Meloe species found in North America, with Meloe americanus being a common example. Some key characteristics of oil beetles include:

• Belonging to the genus Meloe
• Part of the Coleoptera family
• Over 20 species in North America
• Meloe americanus being one example

Physical Description

Oil beetles secrete a yellowish oil from their joints when squeezed or distressed.

This oil contains cantharidin, an irritating chemical that can cause blistering in many people.

Male oil beetles have unique kinks in their antennae for clasping females during courtship.

Distribution and Habitat

Oil beetles can be found in various habitats such as grasslands, meadows, and gardens.

Their distribution is widespread across North America, but they are more common in certain regions than others.

Life Cycle

The life cycle of an oil beetle involves several stages. The larvae first hatch and seek out suitable hosts, such as bees.

They then hitch a ride on the host to gain access to the host’s nest. Once in the nest, the larvae feed on the host’s eggs and provisions before metamorphosing into pupae.

The adult oil beetles emerge after completing the pupal stage.

The typical lifespan of an oil beetle is relatively short, although the exact duration can vary depending on the species and environmental factors.

Comparison between Meloe species and other beetles:

Feature	Meloe Species (Oil Beetles)	Other Beetles
Habitat	Grasslands, meadows, gardens	Varied
Secretion	Yellowish oil with cantharidin	Not present in some beetles
Antennae	Kinks in male oil beetles	Varies
Life Cycle	Involves parasitism in nests of host species	Diverse life cycles depending on species

Are Oil Beetles Dangerous?

Cantharidin and Blisters

Oil beetles, also known as blister beetles, belong to the family Meloidae. When disturbed or squeezed, they excrete a yellowish fluid from their joints.

This fluid contains a chemical called cantharidin, which can cause blisters on the skin.

Effects on Humans

Coming into contact with cantharidin can lead to various forms of irritation, including:

• Redness
• Swelling
• Blisters
• Pain

In some cases, a severe reaction may require medical attention. However, oil beetles do not bite or sting, and their primary defense mechanism is the release of cantharidin.

Implications for Pets

Cantharidin has the potential to be harmful to pets, particularly if ingested. Symptoms of cantharidin poisoning in pets include:

• Drooling
• Vomiting
• Abdominal pain
• Diarrhea

If you suspect that your pet has come into contact with a blister beetle, it is essential to consult your veterinarian immediately.

Characteristic	Oil Beetles
Size	Variable, typically larger than other beetles
Color	Dark and metallic
Defense Mechanism	Excretion of cantharidin-containing fluid
Bite or Sting	No
Risk to Humans	Can cause skin irritation and blisters
Risk to Pets	Can cause poisoning if ingested

Oil Beetles and Their Relationship with Bees

Interaction with Solitary Bees

Oil beetles interact with solitary bees in a unique way:

• The beetle larvae (called triungulins) climb onto flowers.
• A female solitary bee visits the flower.
• Triungulins attach to the bee.

While the bee collects pollen and nectar, the triungulins get a free ride back to the bee’s nest.

Hypermetamorphosis and Hosts

Oil beetle larvae exhibit hypermetamorphosis, which means they have different forms as they develop. The triungulins are:

• Small
• Highly mobile
• Equipped with specialized legs to grasp hairs on bees

Once inside the bee’s nest, the triungulin consumes:

• Bee eggs
• Stored pollen
• Nectar

After feeding, the triungulin transforms into a less-mobile larval stage more specialized for consuming bee provisions.

Pros of Oil Beetles’ Interaction with Bees	Cons of Oil Beetles’ Interaction with Bees
Hitchhiking on bees allows triungulins to find food and shelter easily	The presence of triungulins can harm bee populations by preying on their eggs and food

In summary, oil beetles exhibit a fascinating relationship with solitary bees.

Their larvae, known as triungulins, depend on hitching rides on female bees to find food and shelter.

However, this interaction can be detrimental to bee populations due to the triungulins’ consumption of bee eggs and provisions.

Behavior and Diet of Oil Beetles

Feeding Preferences

Oil beetles, belonging to the genus Meloe, have specific feeding habits during their life stages.

As larvae, they primarily feed on the eggs and larvae of ground-nesting bees.

Adult oil beetles, on the other hand, tend to feed on plant materials such as:

• Flowers
• Leaves
• Stems

These beetles might occasionally be found munching on garden plants, but generally, their damage to plants is minimal.

Mating and Reproduction

Adult oil beetles have a fascinating mating and reproductive process. Males use their unique kinked antennae to clasp onto females during courtship.

After mating, the females lay their eggs in the soil. Here’s a comparison of the different stages of the oil beetle life cycle:

Stage	Diet	Habitat
Larva	Bee eggs	Soil and in bee nests
Pupa	None	Underground in a pupal chamber
Adult	Plants	Gardens, meadows, and grasslands

Once the eggs hatch, the tiny, mobile larvae, called triungulins, seek out ground-nesting bee burrows.

They climb onto flowers and attach themselves to the bee’s body, hitching a ride to the bee’s nest.

Once at the nest, they consume the bee eggs and larvae and eventually metamorphose into pupae, before emerging as adults.

Oil beetles face some natural predators in their environment. For example, birds might prey on adult beetles, while other insects such as ants could attack the larvae and pupa.

However, their secret weapon against predators is a yellowish oil they secrete from their joints when threatened, containing a chemical called cantharidin that can cause blistering in many creatures, including humans.

Overall, adult oil beetles have a plant-based diet and display unique mating and reproductive behaviors.

While they do not pose a significant threat to plants or humans, their chemical defense mechanisms should be respected to avoid any potential harm.

Recognizing and Managing Oil Beetle Infestations

Identifying the Damage Caused by Them

Oil beetles, also known as blister beetles, belong to the family Meloidae and are found across the United States and Canada¹.

They have distinct elongated, narrow, cylindrical, and soft bodies². Human contact with these beetles can lead to skin irritation, as they excrete a yellowish oil containing cantharidin³.

When it comes to potential damage, oil beetles are not as harmful to humans as other pests. However, they can cause several issues:

• Skin irritation: Direct contact with the oil they excrete can cause blisters and irritation⁴.
• Crop damage: Some oil beetles are known to feed on certain crops, damaging their leaves.

Preventing and Controlling Infestations

To prevent and manage oil beetle infestations, consider the following methods:

• Insecticides: The use of insecticides can help control beetle populations, but always follow manufacturers’ instructions, and ensure it does not harm beneficial insects.
• Physical barriers: Installing screens or barriers around your home can help prevent beetles from entering.
• Regular monitoring: Early detection of any signs of infestation can help prevent severe complications.
• Encourage natural predators: Birds or other insect-eating creatures can help control the population of oil beetles.

Pros and Cons of Using Insecticides

Pros	Cons
Effective against beetles	Non-selective, may harm beneficial insects
Easy to apply	Potential issues for humans, pets, and environment

Comparison: Oil Beetles vs June Bugs

Aspect	Oil Beetles	June Bugs
Identification	Elongated, narrow, and soft bodies5	Oval-shaped and hard-bodied[^6^]
Harm to humans	Can cause skin irritation6	Mostly harmless to humans[^8^]
Damage to plants	Some species can feed on crops	Known pests for several plants7

By taking proper steps to identify, prevent, and control oil beetle infestations, we can ensure a safe and irritation-free environment.

Interesting Facts about Oil Beetles

Unique Biological Features

• Oil beetles belong to the Meloidae family and are sometimes referred to as blister beetles.
• They are called oil beetles due to the yellowish oil they excrete from their joints when distressed.
• This oil contains cantharidin, which can cause blistering on contact with human skin.
• They exhibit a peculiar life cycle called hypermetamorphosis.

Oil beetles are best known for their unique biology. They belong to the Meloidae family and are sometimes referred to as blister beetles.

These insects are called oil beetles because of the yellowish oil they excrete from their joints when squeezed or distressed.

This oil contains cantharidin, an irritating chemical that, when in contact with human skin, can cause blistering in many people.

An interesting aspect of their biology is their life cycle, known as hypermetamorphosis, which differs significantly from the life cycles of many other insects.

Cultural and Historical Significance

• Oil beetles were used in ancient times as aphrodisiacs.
• They were also employed for their blister-causing properties in medicines.
• Pheromones used by scarab beetles are used by female oil beetles to attract their unsuspecting hosts.

In ancient times, oil beetles had cultural and historical significance due to their properties.

They were used as aphrodisiacs, possibly because of the cantharidin in their excretions. Additionally, the same compounds that can cause blisters were, ironically, sometimes employed for medicinal purposes.

A fascinating aspect of their behavior relates to the scarab beetles, which are hosts to the oil beetle larvae.

Female oil beetles release a pheromone mimicking the scent that male scarab beetles are attracted to.

This lures the male scarab beetles to the female oil beetle, thereby giving the oil beetle larvae access to a new host.

Footnotes

1. UNL Beef ↩

2. UNL Beef ↩

3. Missouri Department of Conservation ↩

4. Missouri Department of Conservation ↩

5. UNL Beef ↩

6. Missouri Department of Conservation ↩

7. Gardening Know How ↩

Conclusion

In summary, the discussion around the potential danger posed by oil beetles offers a balanced perspective on these insects.

While they do produce a toxic substance known as cantharidin, which can cause skin irritation and discomfort, the risk to humans is generally low.

However, it’s important to handle oil beetles with care to avoid any unwanted contact with their defensive chemicals.

Reader Emails

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

Letter 1 – Oil Beetle

Some sort of blister beetle?
Thu, Oct 23, 2008 at 1:34 PM
We were out walking Jester again and nearly tripped over this fellow walking down the road.

Unfortunately, I didn’t have a macro lens or adapter with me so the photos aren’t all that good, but I couldn’t resist.

There do seem to be rather a lot of these beetles around lately — I don’t actually remember ever seeing them at all, before.
Have I mentioned lately how much I enjoy this site?
Thanks,
Pat
Southwest Michigan (about a mile or so from Lake Michigan)

Hi Pat,
Your Blister Beetle is in the genus Meloe, the Oil Beetles.  The common name refers to the oily substance that is emitted by the joints of the leg. 

This oily substance contains cantharidin which is a skin irritant explaining the common name Blister Beetle used for the entire family Meloidae.  The infamous aphrodisiac Spanish Fly was originally derived from a European relative Lytta vesicatoria.

Letter 2 – Oil Beetle

metallic blue wingless insect
Dear bugman,
Our family enjoys finding critters in our backyard and identifying them on your wonderful site. But this time we are stumped. My son and I found this insect burrowing very slowly under a tree near our house in southeastern PA.

By the next day, all that was left was a small pile of grit, similar to what you see next to an anthill. The insect seems to have succeeded in burying itself. Do you know what it is?

It looks just like a carpenter ant except for that amazing faceted abdomen. Thanks in advance for your help.
Regards,
Mark Vitale

Hi Mark,
You probably never thought to search our beetle pages since the Short Winged Blister Beetle or Oil Beetle is an atypical looking beetle.

Letter 3 – Oil Beetle

big black one in Wyoming
Dear Bugman,
I saw this amazing, tireless creature on the my front porch, and delayed her so I could take some pictures. Our house is on 12 acres outside Cheyenne, Wyoming, and we have Aspen and Blue Spruce in our yard, although most of the land around us is grassy prairie.

The bug was about 1.5 inches long and leathery. It looked like a very tough, very big, very pregnant ant to me. Thanks for your assistance in identifying it.
Ellen Davis

Hi Ellen,
These could well be the finest photos of an Oil Beetle, Meloe angusticollis, one of the Blister Beetles, that we have ever received.

Letter 4 – Oil Beetle

Can you help us identify this bug before science fair!!! See pic
My 7 year old son is doing a bug collection for his school’s science fair. We have identified most of our bugs on your site ( we love it!) but this one we couldn’t find. This bug was found on our doorstep.. Please help

Dear Unnamed Parental Assistant,
This is a type of Blister Beetle known as an Oil Beetle.

Letter 5 – Oil Beetle

big guy in the kitchen
Hi.
I found this guy on the kitchen floor in the middle of the night last night. He was already on his last legs, apparently (pun intended, and lame). I thought it was a beautiful bug, matte black with a huge abdomen (ready to lay eggs?).

Anyway, I’ve browsed your site and others, and am still not sure what I’m looking at. Any ideas? We’re in Northern Kentucky, and it has been pretty warm for this time of year. Thanks.
Matt

Hi Matt,
This Short Winged Blister Beetle in the genus Meloe is also known as an Oil Beetle because of the oily substance that it exudes from its joints. The oily substance may cause blisters.

Letter 6 – Oil Beetle

Subject: Odd looking bug with enormous abdomen
Location: Jackson Hole, WY
May 11, 2017 8:04 am
Hi Bugman!
Recently, I found this bug walking very unsteadily, falling and rolling, across a hillside in Jackson Hole, Wyoming.

Its enormous abdomen was soft to the touch. It was about 2″ long. Is it an ant, beetle or bug from space?! Thanks for your help. Have a Buggy Day!
Signature: Angela B.

Dear Angela,
This is a female Oil Beetle, a Blister Beetle in the genus Meloe.  Blister Beetles should be handled with caution as they can secrete a compound, cantharidin, that can cause blistering in human skin.

Letter 7 – Oil Beetle

Subject: FYI
Location: Huntly’s Cave, Grantown-On-Spey, Highlands, Scotland
May 30, 2017 1:58 pm
Hi,
Not sure if you’re interested in this but thought I’d show you an Oil Beetle I found today. I had no idea what it was but thanks to the wonders of Google and your site I think i’ve got the right name.
Signature: Thanks, Bill

Dear Bill,
Thanks so much for sending in your image of your Scottish Oil Beetle.  We don’t know how many species in the genus Meloe are found in the UK, but Meloe proscarabeus is mentioned on UK Safari where it states:  “When alarmed, the adult oil beetles can release a foul smelling oil – hence the name.” 

There does appear to be a secretion on the palm of your hand.  According to Bug Life:  ” There are five species of oil beetles in the UK, the Black oil beetle (Meloe proscarabaeus), the Violet oil beetle (Meloe violaceus), the Short-necked oil beetle (Meloe brevicollis), the Rugged oil beetle (Meloe rugosus) and the Mediterranean oil beetle (Meloe mediterraneus). 

The first three can be found in the spring and we need your help to monitor their distribution to aid our conservation work.”  We would encourage you to post your sighting there.

Letter 8 – Oil Beetle

Subject:  I’ve been looking for a while, can figure this out
Geographic location of the bug:  Chelsea Michigan
Date: 09/27/2017
Time: 11:51 PM EDT
Found in sandy soil about 30 yards from a medium sized lake. A white pine forest surrounds this lake with the occasional clearing.

I figure this is a queen of some sort, I’ve been looking around myself for a while. I can’t find a match.
How you want your letter signed:  Tyler

Dear Tyler,
We have received several requests in the past few days to identify Oil Beetles in the genus Meloe, but your image is by far the best, hence it is the only one we are posting to our site. 

Letter 9 – Oil Beetle

Subject:  Black iridescent possum beetlely bug
Geographic location of the bug:  Central Michigan
Date: 10/10/2017
Time: 05:56 PM EDT
My cat found this one.  If you mess with it, it plays possum. The pictures don’t do the iridescence justice- it’s a subtle blue.
How you want your letter signed:  Curious in MI

Dear Curious in MI,
This is a Blister Beetle in the genus Meloe, commonly called an Oil Beetle.  According to BugGuide:  “the common name refers to the habit of exuding yellowish oily liquid from the joints when molested” but we have always speculated that Oil Beetle might be a reference to the iridescence you noticed, because it does look like a thin layer of oil on the surface of water.

Letter 10 – Oil Beetle

Subject:  Help scary bug
Geographic location of the bug:  Michigan
Date: 10/10/2017
Time: 08:33 PM EDT
Please let me know what kind of bug this is
How you want your letter signed:  Charlee

Dear Charlee,
We just posted another image from Michigan of an Oil Beetle.  According to BugGuide:  “the common name refers to the habit of exuding yellowish oily liquid from the joints when molested” which your image illustrates. 

Letter 11 – Oil Beetle

Subject:  Black insect with large abdomen
Geographic location of the bug:  Atlanta GA
Date: 11/04/2017
Time: 03:59 PM EDT
I found this fella on my gravel drive in ATL today, which is a warm (75) day in November. I have never seen anything like it.

Its antennae are quite segmented and the abdomen huge. About one inch in total length. Can you tell me what it is? It was alive but slow.
How you want your letter signed:  A little country in the middle of the city

This Blister Beetle in the genus Meloe is commonly called a Oil Beetle.

Letter 12 – Oil Beetle

Subject:  What is this?
Geographic location of the bug:  Central NJ (Edison)
Date: 11/26/2017
Time: 02:30 PM EDT
I saw this creature as I was taking a walk. It was fairly large in size – about 1 1/2 to 2 inches in size. I saw it just now right after Thanksgiving. I did a reverse image search.

Google proclaimed it a stag beetle. Bing returned a lot of pages from Japan. The shapes and colors are correct, but the wings are too small.

The two wings on this picture are tiny stubs. Beetles have full wings that cover the abdomen. Hornets have transparent wings.
How you want your letter signed:  David W.

Dear David,
Your claim that “Beetles have full wings that cover the abdomen” is true in most but not all cases.  This is an Oil Beetle, a species of Blister Beetle in the genus Meloe.  Oil Beetles are flightless. 

There are other species of Blister Beetles with vestigial wings like this Spanish individual, and many species of Rove Beetles like the Devil’s Coach Horse also appear wingless, though their wings are described on BugGuide as “elytra short (about same length as pronotum, or only slightly longer; wings are functional in most), typically exposing 3-6 (usually 5-6) abdominal segments.”

Thank you for your reply. I was beginning to wonder if this was an invasive species.

I did an image search on Bing (Google said it was a stag beetle) and it pointed me to a bunch of Japanese pages (in Japanese, of course) with pictures of very similar looking bugs.

One site (via Bing translate) identified it as a “miyamatsuchihanmjou” which wasn’t much help, but there was a note attached calling it a type of blister beetle and warning me not to touch it. (Not that I had any desire to do so).

I guess the Japanese maybe more attuned to nature, but seeing all of these Japanese pictures and none in America, I feared it was an invasive species.
I see that these live in my area of New Jersey and are mainly active in the spring.
Again, thank you for the quick ID.
David Weintraub

Hi again David,
North America also has Oil Beetles that are active in the fall.

Yes. I saw they’re active all year. By the way, this specimen was between 1 1/2 to 2 inches in length – much larger than the ones you have posing by the quarter. I was hoping to have a quarter or some object to put in the picture in order to judge its size, but didn’t have anything.

I also resisted the urge to pick it up and move it somewhere with better contrast. I’m glad I did.

I found one species called a “short winged oil beetle”. This specimen was about the size of the one I saw and was also found in New Jersey during freezing weather. https://bugguide.net/node/view/37966

I see the larvae live in flowers, hitch a ride on a passing bee, and live in the hive eating honey, pollen, and bee larvae.
David Weintraub

Letter 13 – Oil Beetle

Subject:  Ant or wasp family?
Geographic location of the bug:  Ludlow, Vermont
Date: 12/16/2017
Time: 03:27 PM EDT
Curious as to what this is, having never seeing one before!
How you want your letter signed:  Gary Stevens

Dear Gary,
Though many folks mistake them for queen ants, this is actually an Oil Beetle in the genus Meloe.  We will be postdating your submission to go live to our site at the end of the month when our editorial staff is away on holiday.

Letter 14 – Oil Beetle

Subject:  Big black bug!
Geographic location of the bug:  Northeast Pennsylvania
Date: 04/11/2019
Time: 08:45 AM EDT
Your letter to the bugman:  Hello! I found this while doing yard cleanup in one of my flower beds, under mostly dried grasses, and some damp leaves.

It is about an inch to an inch & a half long & Was relocated to a far corner of the yard. Any idea what this is?
How you want your letter signed:  Knitwit in the poconos

Dear Knitwit in the poconos,
This is a Blister Beetle in the genus Meloe, commonly called an Oil Beetle.  Blister Beetles should be handled with caution as some species are capable of secreting a compound known as cantharadin that can cause blistering in sensitive individuals.

Wow! Thank you for the quick reply and the great info!
~pj~

Letter 15 – Oil Beetle

Subject:  Is this an invasive bug?
Geographic location of the bug:  Jay Vermont
Date: 06/15/2019
Time: 10:43 PM EDT
Your letter to the bugman:  Found today Jun 15, 2019 in short grass along driveway, close to a wooded area of Vermont.
How you want your letter signed:  Heather

Dear Heather,
This is an Oil Beetle in the genus Meloe, and it is native, not invasive.

Thanks so much!  This is our new favorite bug.  He did squirt some yellow stuff on my garden tool and I assumed it was hurt so just put it back where I found it.  Awesome.
Heather

Hi again Heather,
We would advise you to handle Oil Beetles as well as other Blister Beetles in the family Meloidae with caution because, according to BugGuide:  “Pressing, rubbing, or squashing blister beetles may cause them to exude hemolymph which contains the blistering compound cantharidin.

Ingestion of blister beetles can be fatal. Eating blister beetles with hay may kill livestock. Cantharidin is commercially known as Spanish Fly.”

Letter 16 – Oil Beetle

Subject:  Blue beetle?
Geographic location of the bug:  West Virginia
Date: 04/05/2021
Time: 08:09 PM EDT
Your letter to the bugman:  What is this guy?!
How you want your letter signed:  Marion Sophia

Dear Marion,
This is a Blister Beetle in the genus Meloe commonly called an Oil Beetle.  According to BugGuide, there are 22 North American species and we do not have the required qualifications to provide you with a species. 

We do know that some species are found in the spring and others in the fall.

Letter 17 – Oil Beetle

Subject:  What is this!?
Geographic location of the bug:  Trenton, Ontario, Canada
Date: 10/25/2021
Time: 09:33 PM EDT
Your letter to the bugman:  While out with my kids at the park we found a few of these bugs and have no idea what they could be !
How you want your letter signed:  The bug man

This is a Blister Beetle in the genus Meloe, a group commonly called Oil Beetles.  We are posting this on the seventh anniversary of this previous Oil Beetle posting from Ontario. 

Letter 18 – Oil Beetle

Subject:  Maybe a beetle
Geographic location of the bug:  Southwest Washington state
Date: 10/19/2021
Time: 12:30 AM EDT
Your letter to the bugman:  Initially looks like a beetle but what looks to be wings aren’t shaped and only go down half the body
How you want your letter signed:  Doesn’t matter. Thanks for the identification

This is indeed a Beetle.  More specifically, it is a Blister Beetle in the genus Meloe, commonly called an Oil Beetle.