American Oil Beetle: Essential Facts and Tips

American oil beetles, belonging to the family Meloidae, are a type of blister beetle due to their ability to release a toxic substance that causes blisters on the skin upon contact.

Found throughout the United States and Canada, these insects are known for their elongated, narrow, and soft bodies, with variations in size and color.

These beetles play a significant role in our ecosystem, as they feed on a variety of plant species, as well as other insects. Being opportunistic in their feeding habits, they can be pests in some agricultural areas and damage crops and flowers.

However, they also serve as natural controllers of insects that are detrimental to gardens and farms.

For those interested in learning more about American oil beetles, it is essential to understand their life cycle, habitats, and potential impacts on the environment.

Awareness of their unique characteristics and features can help individuals appreciate their role in nature and take appropriate measures when encountering them in the wild or in gardens.

Identification and Physical Description

Color and Size

The American Oil Beetle (Meloe americanus) is typically black or dark blue in color. Their size ranges from 0.4 to 1.4 inches (10 to 35 mm) in length.

Characteristic Features

• Soft-bodied
• Cylindrical shape
• Overlapping plates on the abdomen

Elytra and Shell Covering

The American Oil Beetle’s elytra (hardened front wings) are shorter than their abdomen, giving them a unique appearance. This exposes the overlapping plates on their abdomen.

Antennae and Thorax

These beetles have clubbed antennae and a wide thorax.

Comparison Table

Feature	American Oil Beetle	Other Beetles
Color	Black/Dark Blue	Various
Size	0.4 to 1.4 inches	Varies
Elytra	Short	Variable
Abdomen	Overlapping Plates	Different structures
Antennae	Clubbed	Varies
Thorax	Wide	Varies

Habitat and Diet

Natural Habitat

The American oil beetle (family Meloe) is a type of blister beetle found in various habitats across North America. They can be found in:

• Grasslands: Open fields provide an ample supply of host plants.
• Woodlands: The beetle can be found near the edges of forests, where host plants thrive.

Host Plants

These beetles are mostly associated with the following host plants:

• Wildflowers: Goldenrod, buttercup, and other nectar-providing plants.
• Grasses: Tall fescue and bluegrass are examples of suitable grasses.

Feeding Habits

American oil beetles have a diverse diet, which ranges from leaves and grasses to nectar and pollen. The adult beetles primarily feed on leaves and flowers, while larvae feed on the eggs and larvae of ground-nesting bees.

Here are some key aspects of their feeding habits:

• Adults: They consume leaves and flowers of host plants, as well as nectar and pollen, which provide energy for mating and reproduction.
• Larvae: Predatory on the eggs and larvae of ground-nesting bees, which provide essential nutrients for growth and development.

Comparison Table: Diet of Adult and Larval American Oil Beetles

Stage	Diet	Example Host Plants
Adult Beetle	Leaves, flowers, nectar, and pollen	Goldenrod, aster, bluegrass
Larval Beetle	Eggs and larvae of ground-nesting bees	–

By understanding the habitat and diet preferences of the American oil beetle, one can better appreciate the important role they play in their ecosystem.

Life Cycle and Reproduction

Eggs and Larvae

American oil beetles belong to the genus Meloe, and their life cycle begins with eggs. Female beetles lay several eggs, which hatch into larvae called triungulins.

These grub-like larvae are adapted for a unique parasitic lifestyle as they hitch a ride on a male bee, eventually reaching the beehive. Some key features of triungulins include:

• Small and mobile
• Six legs
• Grub-like appearance

Pupa and Adult Stage

Once inside the beehive, the triungulins feed on bee eggs and provisions. After this stage, they enter pupal metamorphosis and develop into adult beetles. Highlights of adult American oil beetles:

• Large and black body
• Soft, heavily segmented abdomen
• Cannot fly

Mating and Pheromones

Mating in American oil beetles occurs via pheromones. Female beetles release pheromones to attract a male for reproduction.

Male Beetle	Female Beetle
Responds to female pheromones	Releases pheromones
Approaches and courts the female	Attracts and mates with the male

After mating, the female lays her eggs, thus repeating the life cycle of American oil beetles.

Relation with Bees and Other Insects

Parasitic Relationship with Bees

The American oil beetle has a unique parasitic relationship with bees. These beetles lay their eggs near a bee nest, and their larvae, called “triungulins,” hitch a ride on the adult bees to gain access to the nest.

Once inside, the triungulins feed on bee larvae, pollen, and nectar. This parasitic behavior can negatively affect bee populations by reducing the number of healthy, mature bees that can pollinate flowers.

Example: The oil beetle larvae feeding on bee larvae results in fewer bees maturing and pollinating plants.

Interactions with Other Insects

In addition to bees, American oil beetles interact with other insects in their ecosystem. They secrete a chemical called cantharidin, which can be toxic to some insects and vertebrates.

However, the beetles themselves are immune to cantharidin and can use it as a defense mechanism.

Cantharidin:

• Toxic to some insects and vertebrates
• Used by the beetle as a defense mechanism

Here’s a comparison table of the beetle’s interactions with bees and other insects:

Interaction	Bees	Other Insects
Relationship type	Parasitic	Defense
Result	Reduced number of healthy bees	Insect deterrence
Impact	Lower pollination and crop health	Survival of the beetle species

Blistering and Defense Mechanism

Cantharidin Toxin

American oil beetles being a type of blister beetles secrete a toxic fluid called cantharidin. This defense mechanism is used by the beetles to deter predators.

When threatened, oil beetles emit a fluid containing cantharidin, which causes blistering on the skin.

Key features of cantharidin:

• Highly toxic substance
• Produced by blister beetles
• Causes blisters upon contact with skin

Effect on Humans and Animals

Cantharidin can have significant effects on both humans and animals. Upon contact with the skin, it can cause swelling, blistering, and pain. Ingestion of cantharidin may lead to more severe health problems.

For example, horses that consume hay contaminated with blister beetles may experience colic, kidney damage, or even death.

Effects on humans:

• Skin irritation, blisters, and swelling
• Painful to touch affected areas

Effects on animals:

• Horses at risk of colic and kidney damage
• Potentially fatal for pets

Comparison Table:

	Humans	Animals (pets and horses)
Skin	Blisters	Blisters
Pain	Yes	Yes
Severity	Mild to severe	Severe, potentially fatal

Pests and Control Measures

Effects on Agriculture

The American oil beetle, also known as Meloe proscarabaeus, can cause significant damage to crops like potatoes and alfalfa. They are a pest that is active year-round, posing a constant threat to agriculture.

• Damage: Feeding on leaves, stems, and roots of various plants, reduces crop yields.
• Examples of affected crops: Potatoes, alfalfa, and other agricultural plants.

Chemical and Natural Control

Farmers and homeowners may implement different control methods to manage American oil beetles. Chemical control and natural control are two common approaches.

Chemical Control

• Insecticide: One effective chemical control method is using insecticides like Sevin, which targets pests like the American oil beetle.
• Pros:
  • Rapid results in reducing the beetle population.
  • Widespread application in affected areas.
• Cons:
  • May harm beneficial insects.
  • Potential environmental impact.

Natural Control

• Pheromone traps: Farmers can use pheromone traps to attract and capture the beetles, reducing their numbers in the field.
• Pros:
  • Eco-friendly and non-toxic.
  • Targets specific pests without harming beneficial insects.
• Cons:
  • May not provide immediate results.
  • Require constant monitoring and maintenance.

Comparison Table: Chemical vs. Natural Control

Control Method	Pros	Cons
Chemical	Rapid results, widespread coverage	May harm beneficial insects, the environment
Natural	Eco-friendly, preserves biodiversity	Takes time, and requires monitoring

Classification and Related Species

Scientific Classification

The American oil beetle belongs to the following taxonomic classifications:

• Kingdom: Animalia
• Phylum: Arthropoda
• Class: Insecta
• Order: Coleoptera
• Family: Meloidae
• Genus: Meloe
• Species: Americanus

Blister Beetle Family

American oil beetles are a part of the blister beetle family, scientifically known as Meloidae. This family includes over 2,500 known species, with some common features:

• Soft-bodied beetles
• Produce a defensive chemical called cantharidin
• Can cause skin irritation or blisters upon contact

Other Species

Within the Meloidae family, there are other notable species besides the American oil beetle. One example is the buttercup oil beetle (Meloe violaceus), which shares some characteristics with American oil beetles:

• Both are flightless
• Have a similar appearance

Differences between the two include their habitats: the American oil beetle is native to North America, while the buttercup oil beetle is found in Europe.

Comparison of American Oil Beetle and Buttercup Oil Beetle:

Feature	American Oil Beetle	Buttercup Oil Beetle
Native Region	North America	Europe
Flight Capability	Flightless	Flightless
Cantharidin Production	Yes	Yes
Skin Irritation	Yes	Yes

Conclusion

This fascinating creature plays a significant role in both beneficial and detrimental ways. While they can be pests in agricultural areas, they also serve as natural controllers of harmful insects.

With this post, you understand them better and appreciate and manage their presence responsibly. Remember to be always careful while approaching them as they are harmful.

Readers’ Mail

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

Letter 1 – Oil Beetle

Big Ant with a big butt?
Location: South New Jersey
December 19, 2010, 3:22 pm
Hi,
I saw this Big ant with a big butt crawling up on the side of my house outside, really slowly. Is this a ”queen” ant that’s pregnant or something? Or is it even an ant at all?
Thanks
Signature: Joe

Dear Joe
Many people make the mistake of misidentifying the Oil Beetle as a queen ant.  Oil Beetles are Blister Beetles in the genus Meloe.

Letter 2 – Oil Beetle

Large black beetle?
Location: East-Central New Jersey
November 17, 2010 9:47 am
Found this while gardening. It’s about an inch and 1/2 long. My hand is included for scale. Looks like a cross between a large ant and a wasp. Couldn’t find it in my Mid-Atlantic field guide — what is it?
Signature: Jennifer

Hi Jennifer,
The large, black, flightless Blister Beetles in the genus Meloe are known as Oil Beetles because they are able to secrete a substance known as hemolymph. 

The hemolymph is somewhat oily and it contains a compound known as cantharidin which can cause blistering of skin.  You should avoid handling Oil Beetles as well as other Blister Beetles in the family Meloidae.

Letter 3 – Oil Beetle

ant head and torso, large beetle-type body?
Sat, Dec 27, 2008 at 8:21 AM
Found a large black bug last night. Has an ant head and torso and a beetle-type body. It’s about 1 and 1/4 inches long.
Bill
Spring Hill, TN

Hi Bill,
This is a Blister Beetle in the genus Meloe, commonly called an Oil Beetle because of the substance cantharidin that is secreted from the leg joints.  The genus, represented by 22 difficult-to-distinguish species, is found throughout North America according to BugGuide.

Letter 4 – Oil Beetle

spooky bug
Hi,
Found this menacing-looking bug at our front door in southern Vermont. The head was turning side to side, with the pinchers (?) open. It was about 1 1/2″ long. Attached are a few pictures.
Regards,
Jason Chastain

Hi Jason,
You have some good reason to call the Oil Beetle spooky. Another common name is Short Winged Blister Beetle, Meloe angusticollis. I believe you may have exaggerated the size, but the beetle is found in Southern Canada and the Northern United States.

It is usually found in crop fields and meadows where it eats herbaceous foliage particularly fond of potatoes. If disturbed, the beetle feigns death by falling on its side. The leg joints exude droplets of liquid that cause blisters.

Letter 5 – Oil Beetle

Unknown beetle (I think)
Hello… I live in Colorado, and we just recently moved out into the open range lands outside of the city for the peace and quiet, and we noticed these beetle-type insects all over the weeds/wild grass that is sprouting up behind our house.

They struck me as odd because their front end was so small, almost looking like a large ant, and the abdomen was LARGE like it was engorged, so I figured it was a queen or something, but as I started looking around, I noticed they are all over the place and they all look the same.

The small front end, LARGE abdomen. They range in length from baby ones about 1/2 inch to large ones about 1 1/2 inches. Attached is a picture of 2 different views of the same one!

My family is ready to start spraying them with insect repellants, but before they do anything, I would like to know what they are. They seem to only be feeding on the weeds/grass themselves, so not hunters.

This is a type of Blister Beetle known as the Oil Beetle, Meloe angusticollis. Be careful, if disturbed, it can exude droplets of liquid from the leg joints that cause blisters.

Letter 6 – Oil Beetle

Queen Ant Like Insect
Sun, Mar 29, 2009 at 9:27 PM
I live in the mountains of West Virginia and we see all sorts of strange bugs here. I found this insect crawling through my kitchen about a week ago. I have been trying to do some research to find out what it is, to no avail.

It looks ant-like in nature. Perhaps a wingless queen? As you can see, it is black in color, and about 3/4 an inch long. After I took some photos, I carefully released it back into the wild.
Johnsons
West Virginia

Dear Johnsons,
This is an Oil Beetle in the genus Meloe, one of the Blister Beetles.  You are fortunate you did not handle the beetle. 

The name Oil Beetle originates from the beetle’s habit of secreting hemolymph or blood from its joints when it is threatened. 

The hemolymph contains cantharidin, a substance that can cause blistering of the skin.  You can read more about Oil Beetles on BugGuide.

Letter 7 – Oil Beetle

Metallic blue bug
Wed, May 6, 2009 at 6:47 PM
Hello WTB, i have been seeing this bug around my town and have no clue what it is. It looks like an ant but has a sort of metallic blue finish on it and is often about an inch to an inch and a half in length. They usually appear in the summer months only.
squishworthy
central new jersey

Dear squishworthy,
While we don’t feel entirely comfortable with your name and its ramifications, we will nonetheless write back to you to tell you that this is a Blister Beetle in the genus Meloe, commonly called an Oil Beetle.

I see now that the name was a poor choice but a assure you that this bug and the many others that I encounter I do not harm and appreciate their respective services to nature as a whole.

Thank you and I hope to have the opportunity to submit to your site again
Steven from jersey
Sent from my iPod

Letter 8 – Oil Beetle

black beetle-like insect with a very large abdomen
November 10, 2009
We found 5 of these insects on the underside of our recycling bin. The largest was 3 inches long. We live in Portland Oregon, and it is November. The insects were sluggish and appeared semi-dormant.

I wondered if they were a larval stage of another insect.
Heather in Portland
Portland, OR

Hi Heather,
Though it looks rather like a larva, this is an adult Oil Beetle in the genus Meloe, one of the Blister Beetles.

Letter 9 – Oil Beetle

Please help my 9 yr old Bug Lover ID this bug.
April 3, 2010
My son found this very blueish bug today April 3, 2010, while helping to clean the porch. He is sure it is a beetle but I could not verify it for him on any site or book we own. Thank you for your assistance.

It was difficult getting a good picture as the thing was very quick. My young entomologist appreciates your help too.
Maria Firkaly
NorthWest Pennsylvania

Hi Maria,
We think your photograph is quite good.  This is an Oil Beetle, a Blister Beetle in the genus Meloe.  They should be handled with caution, or better yet, not handled at all. 

The Oil Beetle is capable of secreting a fluid known as hemolymph that contains cantharidin, a substance that will irritate the skin.  You may read more about Oil Beetles on BugGuide.

Thank you! We were very careful not to handle it as we did not know what it was.  My son will be very pleased with this.  Maria

Letter 10 – Oil Beetle

Large Black Articulated Bug
May 30, 2010
This was found digging in some sandy soil on the St. Mary’s River floodplain. There appeared to be another one in the hole beneath it. It was close to an inch long.
Ideaphore
Waternish, Guysborough County, Nova Scotia, Canada

Dear Ideaphore,
This large Blister Beetle in the genus Meloe is commonly called an Oil Beetle.

Letter 11 – Oil Beetle

Odd blue bug
Location:  NY
September 11, 2010, 5:43 pm
Found this while taking my dog for a walk near a trail, The only thing I can compare it to is a giant oversized ant but closer inspection shows it wasn’t.
Signature:  JQ

Hi JQ,
Your Oil Beetle in the genus Meloe is a Blister Beetle.  You may read more in our archives or on BugGuide.

Letter 12 – Oil Beetle

What is this!?
Location: Southern New Jersey
November 2, 2010, 6:50 pm
These bugs seem to have come out of an oak tree we’ve had chopped down. Between an inch to two inches long, 6 legs, large abdomen. What are they and What should we do about them?
Signature: Shannah

Hi Shannah,
This is an Oil Beetle and you should let them live.

Letter 13 – Oil Beetle

Big green bug
Location: Auburn, NJ
March 19, 2011 7:27 am
Hi,
I left a bucket by my flower garden overnight. It’s crocus blooming time here in Salem County, NJ This guy was in the bottom the next day when I went out.
I’ve assumed it is a beetle, just by size.

Though his head is so big, like a giant ant. Has a sort of metallic green/grey/blue color, though some ashes I’d hauled in the same bucket gave him a dusting of white specks.

I have no clue. Have been browsing through guides all morning, but nothing quite matches up. And though I have lived here all my life, never saw one quite like him before.

Any ideas? Appreciate your help.
Signature: Val

Hi Val,
This is a Blister Beetle in the genus Meloe, a group collectively called Oil Beetles.  They should be handled with caution as they might cause blistering to the skin.

Wow, that was fast!  Thank you so much. No, I wasn’t going to dare touch it directly, not knowing what it was, regardless. I looked at the link you sent and see clearly what I’d missed in my browsing this morning. 

Wondering now if this population is part of the bee decline in recent years.  I see more wild honey bees now than previously, though they seem to be making a gradual comeback.

Pleased me to see Carolus Linnaeus mentioned as being the first to describe it correctly back in 1758.  His student, Peter Kalm actually lived nearby here, in Swedesboro for a while, and sent many native specimens over to Linnaeus in Sweden. 

I’ll pass this link along to my friends at the Swedish Colonial Society.  Maybe they’d consider it as a mascot?  Ok, just kidding, but still, very cool.
I also shared the link to your site on Facebook. 

You’re providing a great service for those of us who may lack the scientific background, but with digital photography, and access to the web, coupled with plenty of curiosity ….could be I’ll be back again.

Thanks again,
Val

Letter 14 – Oil Beetle

Ground beetle?
Location: Northern Central NJ
October 11, 2011, 2:38 pm
I saw this guy running in the grass and managed to shoot him twice … with my camera, of course. He (she?) is fairly large, perhaps a good inch long.

I tried researching it but I’m not seeing any black long beetles with the knobby bent antennae. It seems to be some sort of ground beetle, but …?

Signature: Jackie

Ground beetle? NOT!
Location: Northern Central NJ
October 11, 2011, 7:18 pm
In searching around, I found the identification of the beetle whose two photos I sent in earlier today. It’s not a ground beetle. It’s a black blister beetle. Thank you for running this site! It’s a treasure trove of bugs!
Signature: Jackie

Dear Jackie,
Since our editorial staff is gainfully employed, we generally respond to questions early in the morning prior to heading off to work.  We are happy to learn that you self-identified your Oil Beetle or Black Blister Beetle on our site.  Thanks for the compliment.

Letter 15 – Oil Beetle

Not Even Sure What Order This is In
Location: Mt. Pisgah, North Carolina
October 23, 2011 8:07 am
We saw this insect in mid-October on a trail next to the Mt. Pisgah Inn on the Blue Ridge Parkway. It was quite active considering it was on the north slope late in the morning.
Signature: Steve

Hi Steve,
Your confusion regarding the insect order is quite understandable since this Oil Beetle is not typically “beetle-like” with its short flightless elytra and soft body.  The Oil Beetle is a Blister Beetle in the family Meloidae, and many members of the family are atypical beetles.

Letter 16 – Oil Beetle

metallic blue and black bug
Location: Rhide Island, USA
November 10, 2011 7:58 am
My kids found this in my backyard, in Rhode Island. It was taken with my Android phone using its macro setting, if you look closely you can see a smaller orangish bug on its back, just behind his head.

Could it be a baby or a little helper?
Signature: W Mcquade

Dear w Mcquade,
This is an Oil Beetle, a species of Blister Beetle.  Blister Beetles can exude a compound known as cantharidin which is a blistering agent, so Blister Beetles should not be handled.  We cannot make out the identity of the hitchhiker. 

It is not a baby blister Beetle.  It may be a Phoretic Mite, but we have not heard of any Mites that use Blister Beetles for transportation.

Letter 17 – Oil Beetle

Ant queen?
Location: Michigan
November 10, 2011, 11:00 pm
I found this in the woods and I thought it might be an ant queen. It’s a little over an inch long so I was able to spot it from a distance. Any ideas?
Signature: Weezie G.

Dear Weezie,
At this time of year, we typically get numerous requests like your request to identify Oil Beetles in the genus Meloe.  You are not the first person who has mistaken an Oil Beetle for a queen ant.