Friday, April 19, 2013

It always pays to watch the competition

Yesterday, when I arrived for the weekly nature walk of the Friends of Sabino Canyon the temperature was in the fifties. Much too cold for mid-April morning in Tucson and too cold for most insects and lizards that I wanted to see. In addition, a fierce wind was howling down into the canyon from the Catalinas. The other naturalists decided to look mostly at plants, I guess because those can't hide from the elements. Two saguaros, one with an imposing crest, looked indeed positively defiant.

Eastern Collared Lizard, who stayed hidden yesterday (Photo Ned Harris, April 2012)
But I don't get to Sabino often enough, so I was going to give Esperero Trail a try because that's where I had seen the beautiful Eastern Collared Lizards last year. The trail leaves the main canyon and winds up the mountain side with full exposure to the western wind gusts. It seemed like a dumb idea but I am stubborn and in Arizona the sun usually breaks through eventually.

View into Sabino Canyon from Esperero Trail
 Well, it didn't and there seemed to be very few living things willing to share the exposed mountain side with me. Until I came to some part of the trail that rose somewhat over its surroundings resulting in a low but pronounced hill top.

Ash-throated Flycatcher, who was out and about
 There was a big Ash-throated Flycatcher busily swooping from his perch as if catching - flies? Sure enough, even in this abysmal weather some Bee Flies, Tachinids, Robber Flies  and even some butterflies (Tiny Checker Spots, Marine Blues) were hill-topping, a behavior that is typical for many flying day active insects.

Some of the hill-topping flies and butterflies
 Males had staked out little territories, patrolling back and forth, chasing everything that flew by, driving off other males and trying to mate with passing females. I think I understand why the big collared lizards and the flycatcher seem to prefer this exact area. It's the place where they can be sure to find enough prey. Of course the ectotherm lizards had the luxury to take the cold day off while the endotherm bird had to keep working, especially as he was feeding nestlings that I could hear keening in a Saguaro hole. I followed the bird to an especially exposed area with some bare boulders where he seemed to concentrate his efforts.

 At first I payed little attention to a big black fly that landed as if to bask in one of the sparse sunny spots because I mistook it for a Mexican Cactus Fly, a common syrphid. But then I heard a very deep basso buzz when the fly changed to another boulder, only about 5 m from the first. That sound is one of the loudest and deepest flight noises I've ever heard here in Arizona - not easy to forget. Through my new little Papillon binoculars I could also verify that the fly was not all black but had silvery grey flanks - a bot fly in the genus Cuterebra. (Later identified by Jeff Boettner as C. austeni)

Rodent Bot Fly, Cuterebra austeni, Esperero Trail, Sabino Canyon, Pima County Arizona, April 2013 
This is only the third time that I encountered these big, fascinating insects. Some people may feel that the live history of bot flies seems like an episode from the X Files or the movie Aliens. I think, however, that the film makers borrowed their ideas from the biology of these flies whose larvae are mammalian parasites.

Deer Bot Fly, Cephenemyia jellisoni infects the alimentary tract of ungulates like white-tailed deer, mule deer, moose or elk. Photo by Philip Kline Pusch Peak,  Catalina Mountains, Pima County, Arizona, March 2009
 While some bot fly larvae develop in the alimentary tract of mammals, the big black and grey Cuterebra that I was watching yesterday belonged to the group that develops subcutaneously in mammals. Luckily they are quite species specific in their host choices so I knew that I wasn't the target - the host would most likely be a rabbit or a pack rat.

Pack rat with 2 nearly grown Cuterebra larvae Photo Jan Emmings
 The female fly will lay hundreds of eggs in rat runways. Only a few of the larvae will actually find their way into a passing rodent.There it will develop rather quickly feeding on the body fluids of the rat that this irritation draws. Inflammation and pain are controlled by the larva and the wound heals very quickly as soon as the guest drops out of the host to pupate.

Cuterebra sp. larva ready to pupate found crossing Sabino Canyon Rd. on Feb. 22, 2011. Photo by Fred Heath

Rodent Bot Fly, Cuterebra sp., Pima Canyon, Catalina Mts. Pima County, Arizona, September 2009
 The only bot flies likely to infest humans do not occur north of Southern Mexico. As I mentioned, the deep characteristic buzz of the big flies is rather recognizable and would probably warn the victim of an intended egg drop. To give their larva a chance to find their way into the body of a wary primate, female bot flies in the genus  Dermatobia  have an amazing strategy. They capture another insect like a mosquito, horsefly or tick that is drawn to warm blooded mammals for its own reasons and attach a load of their eggs to this carrier. When the released  mosquito later lands on a primate for a blood meal, the bot fly eggs hatch and the young larvae bore into the warm skin of the doubly victimized monkey or human.

 While feeding from the host subcutaneously for several weeks the larvae have to behave reasonably well. They seem to prevent inflammation even though they need to keep a hole in the host's skin open to be able to breath periodically. They probably have anti inflammatory and antibiotic capacities. I have two good friends who both got parasitized in Central America, are now in their eighties, and live and love to tell about it. If not removed prematurely, the larvae will eventually leave the host, drop to the ground and pupate. In time a new generation of mature adults will hatch. These big flies are born without mouth parts. From their time as maggots in the flesh, they are provisioned with enough energy reserves to survive for about 10 days, time enough to start the cycle again.

Catching the redeye to Amhesrt, MA 
Yesterday's bot fly, however, is not going to propagate his genes this way. Instead he is contributing them to conservation biologist Jeff Boettner's research project at UMASS - Amherst. There are many open questions concerning host relationships and phylogeny of the group. As specimens are difficult to obtain, Jeff would be grateful for specimens and info about lek sites. Here is a link to his info pamphlet.
Here is a link to photos of the removal of a human bot fly larva (Peruvian Amazon)
Arthropoda (Arthropods) » Insecta (Insects) » Diptera (Flies) » Calyptratae » Oestroidea » Oestridae (Bot Flies) » Cuterebrinae (New World Skin Bot Flies) » Cuterebra (Rodent and Lagomorph Bot Flies)


  1. Hi Margarethe,
    Bot arrived in great condition. Its a boy. Cuterebra austeni. Likely that spot is a lek site. And if you went back on a sunny day between 9 am- 11:30am you would likely get to see them battle for "King of the hill". With females mating with the bot that holds the good spot when she arrives.
    The females of these bots lay eggs in rat runways. Only the Central American Dermatobia catches mosquitos to get its eggs in the primates. Cuterebra waste a lot of eggs, females can lay thousands of eggs, but not many actually make it into a host. Thanks for the post, the fly, and the link to my request. I would love getting bots if people see them. We hope to learn a lot about parasite/host evolution by looking at bot dna. Gotta run. Thanks again. Jeff

  2. Great post. I have also only encountered them a couple times ever, once hill (mountain)-topping in the high Sierra. I love the fuzzy deer bot fly -- wonder if we have them here (we have a LOT of deer)?

  3. via email: Dear Margarethe, Excellent story as usual. I have collected a dozen or so and reared a few larvae of Cuterebra in my time. The thing I have noticed when I am in a lek site is that they fly like a bullet. In fact I think I believe I read somewhere the fastest insect was a bot fly. Some guy measured it with his car. I think there were some objections but the story is probably on the web somewhere. I know the 80 year old is Charley O’Brien. I saw him on a television show once talking about his experience with Dermatobia. I always save all the bots. I believe they don’t live very long because the adults don’t have any mouthparts. Nice blog.

    Nathan Schiff, Ph.D.