Wednesday, June 23, 2010

Life cycle of the Giant Mesquite Bug, Thasus neocalifornicus

Photo by Terry Ketron

Lately I have received several inquiries about clusters of colorful 'Beetles' that are clinging to the leaves and seed pods of Mesquite Trees in Southern Arizona. Luckily, photos from omnipresent cellphones cameras accompanied the questions. It turned out that the 'beetles' were actually the nymphs of true bugs, of Giant Mesquite Bugs, Thasus neocalifornicus. (a revision of the genus by Brailovski et al in 1994 states that this is the only species of the genus in the United States, while T. acutuangulatus and T. gigas occur only south of the Mexican border).

How to tell a beetle from a true bug
Beetles and many true bugs are often confused because superficially they can look quite similar, as the picture of Paranthesis Ladybeetle and Harlequin Bug shows. But at closer investigation their morphology and life cycle are very different and they are therefore grouped in different taxonomic orders.

Beetles (Coleoptera) are the largest order of insects. From above, the body of a typical beetle shows three obvious parts:
the head with compound eyes, chewing (in most species) mouth parts, and antennae that can vary in shape and usually have more than 10 antennomeres.
the pronotum, which covers the prothorax.
the hard forewings or elytra which cover the membraneous flight wings and the rest of the body like a shell. A small scutellum may be visible where the two elytra and the pronotum come together.

The mouth parts of True Bugs (a specific group of Hemipterans) form a tubular rostrum in accordance with their piercing sucking function (true bugs suck!). The antennae have only 4 big antennomeres and are long, thin and angular. The pronotum is usually as wide as the base of the wings. A very conspicuous triangular scutellum is located behind the pronotum between the wing-bases. The forewings are hard only in front and membraneous towards the tips. The borders of the scutellum and the dividing line between those wing parts forms a conspicuous X that helps to easily recognize the group.

The Life cycles of beetles and true bugs shows how very different they really are
Both beetles and true bugs hatch from eggs. Beetle larvae are elongate and may superficially resemble caterpillars. They eat voraciously and shed their skin repeatedly to allow for growth. But they still look like grubs until they finally pupate. During the immobile pupa stage their bodies undergo radical changes to become the winged, sexually active adults that we know as beetles (complete metamorphosis).

Even newly hatched True Bug nymphs vaguely resemble their parents in shape, if not coloration. They go through a fixed number of moults ,5 in most cases. Each resulting instar is a step closer to the size and shape of the adults until they emerge from the last moult winged and ready to propagate (incomplete metamorphosis).

Timing is everything
Lets follow the development of a cluster of Giant Mesquite Bug eggs that were deposited by last year's generation, maybe in a protected spot under loose bark of a Velvet Mesquite Prosopis velutina in Sabino Canyon close to Tucson. In late April, mesquite leaves unfold explosively to shroud the somber, deciduous tree in lush fresh green within a couple of days. This is also the major blooming season of the mesquite trees.

At this time little nymphs hatch from the eggs, feed on the leftovers, and almost immediately moult for the first time. If they are not already on their food tree (we found many clusters in the shrubbery underneath the mesquites) it's now time to climb up to those fresh, juicy mesquite leaves.

First instar nymph and first molt

Second instar nymph
The second instar nymph is already recognizable as baby Giant Mesquite Bugs by the characteristic diamond shaped antennal discs of the species.

Third instar nymphs Photo by Tuan Cao
The bright red, black and white pattern becomes even more conspicuous in the third instar. The nymphs are still staying closely together as a group, projecting the appearance of a much bigger animal and probably of a very bad tasting one.

Wing buds are have appeared in the 4th instar Photo Ned Harris

They can afford to stay together because they don't have to compete for food: While the nymphs are getting bigger and hungrier, the tree has begun to put a lot of resources into the production of long, juicy mesquite beans. The pods themselves, not just the seeds, are sugary sweet. Over centuries the people of the southwest have used this abundant resource for flour and to feed countless cattle. Even Coyotes rely on the pods for 80% of their summer diet. Javelina scat is full of them. Sucking the juice of these fast growing seed pods, amazing numbers of mesquite bug nymphs are able to grow quickly to their astonishing adult size without any obvious harm to the tree.

This year spring came late to the foothills of the Catalina Mountains north of Tucson. So the mesquite beans are still growing and the bug nymphs are still in their last, 5th, instar close to the end of June.

5th instar (left)Last molt (right) Photos Ned Harris

By the time the monsoon moisture rolls up from the Golf and the July heat begins to pile up huge thunderheads over the Catalinas, the mesquite beans will be ready to fall to the soaked ground to be swept away by running washes until the pods rot and set the seeds free to germinate. By then the mesquite bugs will have turned into huge adults with strong legs that can cling to branches during afternoon storms and fly to disperse and meet their mates in the heat of the day. The adults are about 2 inches long.

Female (left) and male (right) adult Giant Mesquite Bug
mating group (below)

They often congregate to mate
A collage of the nymphal instars and an adult male


  1. These are very cool bugs. We have nothing of this size. leptoglossus occidalis is the closest and that is a new arrival from the continent. I've not seen one yet.

    ashe (on flickr)

  2. I know, after I moved here from Europe, the first Mesquite bugs that flew at me while I was searching for beetles were nearly scary

  3. Wow, fascinating stuff. Great color as nymphs, neat markings as adults...

  4. Terry said:
    Thanx. That was educational.
    I did notice that the photo was run horizontally and it should have been run vertically w/ the left end at the top.
    Some of the other pictures were great too.
    I have Horned Lizard shots and desert Iguana Shots if you do any blogs on reptiles and want some pix.
    Thanx for the link.

  5. What an awesome blog post. I always learn something when I visit your blog. Thanks so much.

  6. Very nice, Margarethe! Some of those "in-between" instars are extraordinary. Keep up the great work :-)

  7. I see your photo of the Western Screech Owls. We have owls that look just like these. Very small owls, maybe 6 inches or so. I thought these were Pygmy Owls. We have photos from several angles but no ear tufts. We have had Western Screech Owls too where we have seen distinct ear tufts. Is there a way to distinguish? Your owls do not appear to have ear tufts either. Thanks.

  8. Ours owls in the picture are still youngsters. They soon got larger ear-tufts. These little guys are probably between 7 and 8 in. Pygmies are really small, 6 inches, but a lot of that is the long tail, much longer than in Screech owls. Size is difficult to estimate. Proportions are easier. Pygmies are more day-active.

  9. I stumbled upon your blog through a friend who was doing research on some of the fauna in his yard and I couldn't be happier about it! I moved to Arizona in January after being a vagabond/pilgrim in several states for many years, knowing I wanted to end up here. When I came I decided, after actually seeing with my own eyes the remarkable animals that live here in the Sonoran desert, that I want to learn about everything Arizona. Lizards, cacti, bugs, snakes, arachnids, plants, trees, everything! Your blog is going to be very helpful to reaching that goal. Thank you again for posting all of this highly interesting and educational stuff!


  10. I saw some interesting bugs on our trip to Kartchner caverns, after stumbling around on the web for a while I found your blog. Thanks for helping identify these curious critters - the Mesquite Bugs. :-)

  11. I get at least one request a year (two so far this year) to ID this bug. I always refer people to this page. Thank you.

  12. Nicely done and thank you!

  13. I have two healthy mesquite trees in my front yard in Mohave valley, AZ, but when my husband and I were pruning back some of the branches we noticed some insects/bugs on one of the trees. The bugs are gray in color, very flat, circular and about the diameter of a pea. I've looked online and can't find any info on such a bug. Does anyone know what this could be, or if it is dangerous to the tree? Thanks!

    1. Sounds like you found some Stink Bugs or Pentatomids, maybe genus Brochymena. There are many others, your description is not specific enough. Of course your trees are part of the desert ecosystem and many insects live on it. It would be tragic if that wasn't the case. The stink bugs will suck some juice from the trees. They do no harm to them. A Mesquite tree is only decoration to you, a place to live to the bugs. They themselves are food for other insects, lizards and birds. Maybe you can watch and observe the animal life on your tree and enjoy it?

  14. Thanks to you I was able to identify this little guy, Im surprised to have found it here in central mexico

  15. Wow! I live in the SE Arizona mountains near the Mexican border, with non-native mesquite as far as the eye can see -- thanks to the cows roaming here for 100 years -- and I'd never seen one of these before yesterday, had no idea what it was.

    Thanks so much for the info. I'm up at 5,000 feet, and am guessing, from your description, that the reason I'd never seen one before now is the same reason there are lots of new tropical type birds this year -- THE CLIMATE IS DESTROYED!

    1. The Mesquite trees are not non-native, that's an error. They were selectively favored by grazing cattle, that much is true. The climate is changing but not destroyed, yet. If you have not seen Mesquite Bugs you just missed them like surprisingly many people do. No, there were all around you, as long as there were Mesquite trees. Not on every one of them of course

  16. Great to know: This blog was one of the resources, and is quoted as such, for an interesting scientific paper "Mesquite bugs other insects and a bat in the diet of pallid bats in southeastern Arizona" by Czaplewski, Menard, and Peachey" online journal "PeerJ

  17. Found these creatures in their red/white stage. Right out in my front yard last week. Very creepy (to me).

  18. Margarethe! I just came to this blog post after encountering my first GMB, and then I realized you're the one who painted the two watercolors I bought at the street fair last December (a red cactus flower and a little parrot). I still love them more than I can say, and I'm just tickled at this bit of serendipity. So lovely to find you here!

  19. Just what I needed, every detail.....came upon an "outbreak" of this true bug in a small mesquite (?) next to the corral on 62A road approaching Florida Canyon. Adults and 5th (?) instars crowded all over the seed pods, which are exuding sweetness. An amazing sight!!! A True-Bug extravaganza. Can I attach photos? ---Shirley Sekarajasingham

  20. These details are just what I needed. A couple of days ago I stumbled on an amazing true-bug extravaganza...a small mesquite (?) covered with seed pods exuding sweetness, which were COVERED with 5th (?) instar and adults. The location is the old corral on road 62A approaching Florida Canyon. How can I attach photos?

    1. As it burns out, the bugs were gathered on the pods of a Catclaw Acacia.

  21. Wonderfully illustrated and informative, Margarethe.
    By "the Golf", may I presume you mean the Gulf of Mexico?