Thursday, April 22, 2010

Springtime Blister Beetles of the Southwest

After a year of severe drought, record heat, and parched desert landscapes, this year's spring rains brought us glorious carpets of annual flowers all over the Southwest. Yet, according to my records beginning in 2007, there still seem to be much fewer insects than usually in March and April. The reason may lay in the prolonged drought or just the cool night temperatures that we are currently enjoying...

However, the large stands of Lupines, Desert Chicory, and Brittle Bush have attracted a multitude of colorful beetles of a family that is superbly adapted to our desert environments: The Blister Beetles or Meloids.

Blister Beetles occur all over the world except in New Zealand and Antarctica. Of the 335 US species the majority lives in the Southwest. We also have the most bizzare and colorful ones right here in our deserts.

This month, you'll see them everywhere where there are plenty of fresh flower petals to eat.

At Saguaro National Park West you may come across
aggregations of Iron Cross Beetles Tegrodera aloga (right) wandering along the path in the morning or hanging suspended from weed stalks during the hotter part of the day.

Big Master Blister Beetles Lytta magister weigh down the Brittle Bush Flowers in our backyard in Picture Rocks.

A Blister Beetle is recognizable by his big square head, narrow pronotum, an elongated soft body, long legs (Tarsal formula 5-5-4) and thread-like or beaded antennae.

Meloid Poison
The bright and striking color combinations of many Meloid species are probably a warning to predators: Blister Beetles are poisonous. Their blood and soft tissues contain Cantharidin. The chemical is produced by the male, transferred to the female during mating, and can also be found in the body of the larva. Blister beetles respond to disturbances by reflex bleeding from knee-joints and other body parts.

Skin contact with this fluid can raise painful blisters.
Grazing animals like horses can suffer digestive and urinary tract damage, inner hemorrhages and even death if they are unlucky enough to ingest too many beetles with their feed. Epicauta vittata and Epicauta occidentalis reportedly cause the most damage. They are not among our spring species but occur during the summer months. The lethal dose for an adult horse is more than 150 beetles. Horse owners here in Arizona are keeping a weary eye on grasshopper population explosions because they are usually closely followed by Epicauta  mass occurrences. (Epicauta larvae feast on grasshopper eggs).  Current alfalfa harvesting techniques involving waltzing the plants before baling sometimes trap hundreds of the beetles in hay bales. Alfalfa harvesting methods that avoid crushing the beetles and letting them instead escape before the hay is baled are available and are currently researched at the University of Colorado.

In folk medicine of many cultures extracts from Blister Beetles have been used for their presumed potency as an aphrodisiac and are still in use a remedy against warts.

Red-eared Blister Beetle Lytta auriculata (left) and metallic green Lytta stygica (right) devour Lupine and Cream-cup flowers in Catalina State Park

Red and black banded Elegant Blister Beetles Eupompha elegans pose openly on white Desert Chicory flowers in Sabino Canyon.

The big flight-less Oil Beetles Meloe barbarus hide in shady spots on a cow pastures of Picture Rocks. They hide so well actually, that the female I found is only the 5th AZ state record since the species was described.
read more about the amzing life cycle here:

Inflated Beetles Cysteodemus armatus carry insulating air pockets under their wings. They are at home mainly in the Mojave and the Colorado Desert but they have been found as far east as the White Tank Mountains close to Phoenix. (I went looking for them last weekend - no luck - so this photo is by Phillip Ruttenbur, copyright 2005)

Phillip Kline (copyright 2010) found this beautiful Pleuropasta mirabilis in Arivaipa Canyon in Pinal County and sent me the picture for identification. He graciously allowed me to add it to this blog.

Dull orange beetles of the genus Nemognatha love the thick pink heads of New Mexico Thistles. Their elongated maxillae form a sucking tube that allows them to compete with bees for the nectar. On the flower heads they also find mates and lay their eggs.

Life Cycle
While the adults are vegetarians, Meloid larvae are predatory or parasitoid. The females of several Meloid genera position their eggs on flowers. Highly mobile, long-legged larvae emerge and hitch a ride on a visiting bee to get to its nest. From the BugGuide: 'In at least one Meloe species, the larvae climb to the top of a grass or weed stalk as a group, clump together in the shape of a female solitary ground bee, exude a scent that is the same as, or closely resembles, the pheromones of the female bee, and wait for a male ground bee to come along. When he does, he tries to mate with the clump of larvae, whereupon they individually clamp onto his hairs. He then flies away, finds and mates with one or several real female bees, and the larvae transfer to the female(s) and with them to the bee's nest.' There the larvae change into short-legged grubs that consume the bee off-spring as well as the pollen and nectar provisions. The winter diapause is spent in form of legless coarctate larvae which in early spring change again into short-legged but mouthless creatures only to prepare the pupal chambers. There they pupate and the adult beetles hatch ready for the next spring flower season to start the cycle over. This highly complex cycle is called a hypermetamorphosis.

A-triungulin; B-caraboid stage; C-coarctate larva;D-scarabaeidoid stage; E-pupa
F-adult beetle (imago) of a Striped Blister Beeltle, Epicauta vittata

The best is yet to come
Here in Arizona, the monsoon rains will trigger another burst of fresh vegetation in summer, and with it many new species of Meloids will emerge, mostly in the genera Epicauta and Pyrota.Three species of the genus Pyrota that we'll find again in Arizona from July to October.


  1. Very nice! Welcome to the "blogosphere," Margarethe :-)

    1. I have found a large black with red stripes beetle what kind is it

      Does it look like this?

  2. Wow - nice photos, and glad to see you sharing them in blog format.
    best regards--ted

  3. These are excellent photos indeed. I love blister beetles.

  4. Wonderful pictures! Glad that winter is over and flowers are booming everywhere. Thanks for sharing Margarethe.


  5. Dear Margarethe,

    I was looking up Iron Cross Blister Beetles when I happened across your blog. Anyone who "loves" blister beetles, doesn't have horses lol. My Vet was out today to give spring vaccinations and found an Iron Cross Blister Beetle. I'd seen one at the place where I buy my hay, but this beetle was not near my hay and I don't feed alfalfa. I'd thought I was safe. He said the Iron Cross variety isn't as dangerous as another variety found in alfalfa in southern Arizona. We live in Northwestern Maricopa County and our bermuda is baled within this county. Are my horses at risk? The name "Blister Beetle" causes horse lovers to have cold sweats so we're fighting panic here.


  6. A horse has to eat many Blister Beetles to get sick. Greater congregations of beetles are probably found only on Alfalfa hay because the beetles eat leaves and flowers of Legumes. Your Bermuda should be rather save, even if there is an occasional beetle in it.

  7. Thank you Margarethe! My horses are miniatures so it would not take that many bug parts to devastate their digestive tracts. If the bugs only eat the leaves and flowers of fresh growing legumes, I too would imagine my baled grass hay would not tempt them. Thanks for the information. Do you think the blister beetles would be tempted by the dried leaves and flowers of previously baled alfalfa hay? I have 3 neighbors that feed alfalfa and they have concerns about the bugs infesting their hay.

  8. The beetles do not infest dry hay. But they may have been on the alfalfa plants when they were harvested, so the dead, dried beetles are in the hay. I know it's a nuisance, but concerned horse owners should just visually inspect the hay in small portions when they throw to the horses. Most of the beetles are at least about 1/2 inch long and should be noticeable.

  9. The Inflated Beetle appears to be wearing an old-fashioned bustle and long skirt. I've never heard of that one before. Thanks for writing about it.

  10. Nice pictures! I find the Iron Cross Beetle to be particularly stunning.

    What kind of equipment are you using?

  11. Thanks, I checked out your blog, David. My Macros aren't quite in your league. Yours are very impressive. I'm using The Olympus E-500 with the 50mm Macro. Quite a workhorse and the lens is easier to use in the field than the Canon 65mm macro. I'm thinking about switching, but I'm still shopping around.

  12. Fantastic photos!

    Do continue sharing your findings.

  13. I'm wondering if a large red beetle that lives in the Albuquerque Sandia mountain area sounds familiar to you. This beetle basically looks like an oversized ladybug, 1/2" total body length. Shiny red carapace (no spots) & wing covers (they appeared to be wing covers anyway, though it did not fly), black legs & underside. The red was a lovely dark tomato red. No camera, sigh.

  14. BTW, on the same hike today, and it is still monsoon season here, there were many flowering bushes, including one with tiny yellow flowers that had a wonderful perfume, and some of these same bushes were covered with beetles very similar to the last of the "Three species of the genus Pyrota" picture in this blog!

  15. Hi rms
    Is this your red beetle?
    Then it's a red Potato Beetle
    The ones on flowers could be Blister Beetles like the one in my blog, but are you familiar with Soldier Beetles? they have similar colors (many different species) and they congregate on flowers
    this is an example

  16. I'm sure you're right about the potato beetle, and here is another very cool beetle for you:
    This was 9/12 sunday on a jog/hike to the volcanos west of town. Looks like an Inflated Beetle from an earlier post. Other pics from the day in that folder: coyotes, coyote petroglyph, etc !

  17. Hello! I hope you're still monitoring this blog.

    Bug people intrigue me . . . I'm a plant biologist.

    Anyway, here in the Prescott area, I have seen probably a half dozen different blister beetles. This year seems to be a banner year for these bloated oil beetles. We have huge all-black ones and the red and black striped ones. Please feel free to come up and take photos, and take the bugs home with you! We raise alpacas and the thought of blister beetles causes us as much or more anxiety as the horse people; these animals only weigh a couple hundred pounds at the most.

    1. I expect that there are many more spp in the Prescott area than just 6. Not all blister beetles are really a problem - some have rather small amounts of toxin. Also, it's the alfalfa hay and the way it's harvested that causes the poisonings. No horse, and even less the more carefully nibbling alpaca will eat blister beetles while grazing. The bugs leave when the grazer approaches. But some modern haying procedures involve breaking the alfalfa down in a way that traps the beetles, and then the dead ones are getting into the hay-eaters diet. .., a problem that could be avoided

  18. I know they are mainly crop bugs but I have an infestation of what look like blister beetles in my kitchen. They are all different sizes and I fear they may be hatching in waves. Could they be coming in from outside or is it possible they are under the foundation and coming in through cracks? I see about 10 a day. Can anyone help me?

  19. Blister Beetles would not hatch in your kitchen, except if you have bee nests there which is not very likely. You do understand that your description is much too vague to give you any answer? I suggest that you take a picture and post it on or collect specimens and take them to your local university or USDA office.

  20. My son and I saw our first Oil Beetle yesterday here in Santa Fe, New Mexico, and wondered what it was. I'm glad we didn't touch the stuff coming out of it! What a fascinating bug!

  21. I live in Buckeye, AZ and had an infestation of blister beetles in my back yard a few weeks ago. This morning, another blister beetle was spotted along with a gray inflated beetle. Needless to say, exterminator is coming tomorrow morning.

    1. An exterminator to take care of blister beetles in a back yard? Needless to say, the toxins that are used to kill those harmless beetles will get in contact with you through skin contact, the air you breathe and the fruit you harvest, while it would have been easy to not touch the beetles that are only around for a very short time.