Wednesday, October 10, 2012

Wasp Wednesday: The "Horse Guard"

Heidi and I were away last week in Cape May, New Jersey for a belated honeymoon, celebrating in the same place we had our first “date” back in October, 2010. We went a week earlier this time, and visited some locations we had missed on our last trip. Consequently, we saw some different birds and insects, including a wasp that was on my “bucket list.”

The Triangle Point Butterfly Garden, a tiny park near Cape May Point State Park, is planted with a variety of native and exotic flowers that attract many kinds of insects. We visited in the early afternoon of October 5, and found another party looking at the butterflies, but wondering what the big black and white wasps were.

”Oh, my God, that’s a Horse Guard!” I couldn’t contain my enthusiasm as I frantically focused my zoom lens on the wasp. I had forgotten that Stictia carolina, the largest of our “sand wasps” aside from cicada killers, ranged this far north, and persisted into the autumn months.

The Horse Guard is found from New Jersey and Pennsylvania south to Florida and west to Illinois, Kansas, and New Mexico (barely). It is most abundant in the southern Great Plains. Though solitary, each female excavating her own nest, many wasps may nest in a small area of sandy soil.

How did the Horse Guard get its name, you might ask? Also known as “cowfly tigers” and “insecto policia,” the wasps are specialist predators that chiefly attack horse flies. Naturally, the best place to hunt horse flies is around equines. The female wasps will fly around a horse, up and down each leg, searching for their prey. They can even fly backwards in front of a moving horse. This hunting behavior is frequently interpreted as aggression by “hornets” by the average, intimidated horseback rider or ranch hand. Once people understand the intent behind the wasps’ persistent hovering, the usual reaction is one of relief and elation.

”That’s fantastic! How can I encourage more of these wasps for horse fly control?”

Short of importing a sand dune, as the late Howard E. Evans writes in his book Wasp Farm, there is very little one can do to ensure the presence of horse guards at any given farm or ranch. The best thing to do is simply not kill them or accidentally obliterate their nesting areas.

Males search for females by engaging in “sun dances,” which are level flights in circles, figure eights, or sinuous patterns, occasionally perching on low herbs, dung, stones, or on the ground. These patrol beats are most often in the vicinity of female nesting areas, and usually in the morning hours. They will actively chase each other, or molest other large insects that pass through their individual territories.

Females take a surprisingly long time to complete a burrow, taking an average of 22-30 hours. Burrows are usually initiated in the morning, with frequent breaks as the sun becomes more intense, with digging activity increasing again in the late afternoon. The wasp closes the tunnel each time she leaves the vicinity. The burrow is a diagonal excavation, averaging 35-51 centimeters in length, and to a depth of 18-24 centimeters. Soil properties influence these numbers. The tunnel terminates in a single cell, where the wasp deposits a single egg.

Once the nest is completed, the wasp begins hunting, feeding her larval offspring in the progressive fashion that birds do. Between visits, the wasp makes both an inner closure that seals off the terminal cell, and an outer closure that obscures the nest entrance at the surface. Fifteen to thirty-five flies may be fed to the average wasp larva during its lifetime. Larger flies (like horse flies), mean fewer flies are necessary to feed a growing larva. Still, the hunting tactics of the wasps can be very effective in protecting livestock from blood-sucking flies.

The double closure of the burrow between the frequent visits by the mother wasp helps eliminate many of the opportunities parasites usually use to gain entry into sand wasp nests. “Satellite flies” in the family Sarcophagidae (subfamily Miltogramminae) do deposit their own tiny larvae on the prey carried into the nest by the female Horse Guard, but those larvae are generally content consuming the leftovers of prey, rather than attacking the wasp larva itself.

Interestingly, while I was employed by the Cincinnati Zoo in 1988, we received pupae of horse guards from Mississippi that yielded both the expected wasps, but also “Cow Killer” velvet ants, Dasymutilla occidentalis. This would imply that occasionally the Horse Guard offspring fall prey to larvae of velvet ants.

There are two other species of Stictia in the southern U.S., but even more in the New World tropics (28 species total).

Look for the Horse Guard if it ranges in your part of the U.S. All the specimens imaged here are females, by the way. Males, active much earlier in the season, have the white markings confined to the front half of the abdomen only.

Sources: Bohart, R.M. and A.S. Menke. 1976. Sphecid Wasps of the World: A Generic Revision. Berkeley, California: University of California Press. 695 pp.
Evans, Howard E. 1966. The Comparative Ethology and Evolution of the Sand Wasps. Cambridge, Massachusetts: Harvard University Press. 526 pp.
O’Neill, Kevin M. 2001. Solitary Wasps: Behavior and Natural History. Ithaca, New York: Comstock Publishing Associates (Cornell University Press). 406 pp.
Pulawski, Wojciech J. 2011. “Catalog of Sphecidae sensu lato,” California Academy of Sciences

23 comments:

  1. Seeing this, especially flying, I'd share the ranch hand's concern! How do you quickly separate this from a bald faced hornet?

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    1. The Horse Guard has more white markings on the abdomen than a Bald-faced Hornet. Remember, though, that Bald-faced Hornets are *also* fly predators (not scavengers like other yellowjackets), so both species should be welcomed in any rural, urban, or suburban setting.

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  2. "Look for the Horse Guard if it ranges in your part of the U.S."

    I checked bugguide.net and eol.org but I could not find range information except for their very small number of record reports. Is there a better place to look online?

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    1. I give the ranger right here in the blog post: NJ and PA south to FL and west through TX, KS, IL. It requires sandy soil, even in those states, in which to nest.

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  3. Yes, I would have loved these guys when I worked as a horse-back tour guide in Europe after school. Did you know that the horse flies attach themselves nearly exclusively to the lead animal of a group? Too bad for the guide! Great blog as usual!

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  4. Love those little guys! Do they exist on this side of the pond?(Ireland)

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  5. Enjoyed your article - helped me identify the little wasps frequenting a small pile of sand at our horse facility in mid-state Massachusetts. Perhaps they hitched a ride on one of our east coast hurricanes or came in on the sand? Regardless, these little hard working horse protectors are very welcome!

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    1. Thank you for the compliments...I suspect, however, that what you are observing up that far north are stink bug hunters in the genus Bicyrtes, rather than Horse Guards. I found Bicyrtes to be very common when I spent one summer around Amherst, Massachusetts.

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  6. No horses, or black flies, but for a week now one of these has been flying and landing on the approximately same spot on a paved pathway that is surrounded by grass. It acts like a guard dog. No others around. No sand. No evidence of a nest being built. Is there an explanation of this behavior? Will they attack and sting people?

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    1. That sounds like the behavior of a male. Without seeing the specimen, or at least an image, I also cannot tell you if it is a horse guard or something else. Could easily be a male cicada killer (Sphecius spp.). Anyway, nothing to fear.

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  7. I think I have one of these in my yard in Rhode Island. Do you think so? Is it a danger to my kids-2 and 10 months. It is seriously scary looking. I have a picture but it doesn't look like I can post it here I caught a photo of it eating a huge horse fly today ( a couple inches long)

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    1. You can post a photo in the "forum" section (see tabs at top of this page)....You may be describing some other insect, though I expect Rhode Island might be in the geographic range of the Horse Guard. Very few insects are dangerous to *anybody*, including children.

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  8. I think I might have one of these in my yard in Rhode Island. Is that possible? I caught a photo of it with a huge horse fly today- a couple inches long. How dangerous are they. I have two little kids that play in the yard daily and the wasp has about 4 nests throughout my yard

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  9. Im in Cape May County NJ and this are under my pear tree festing on the nectar of those on the ground, along with nasty cow killers.

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    1. I'm jealous! I've seen few cow-killers in the wild.

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  10. Is there anywhere to purchase horse guard wasp in a fashion similar to fly predators? Is there a way to encourage them to your equine property earlier in the season. What can you do to help encourage or breed a large robust population? The horseflies and deerflies are driving my horses bonkers this year. No wasp here yet, anxiously awaiting their arrival.

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  11. Have 4 or 5 around my horses now in Virginia June/July. First though that bot flies were getting really big. Found this article in my research. Thank you.

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  12. We live in north west florida and these little beasties are very prolific but so are the Velvet ants. I was wondering if there are any plants or other natural steps we can take to give the lovely critters an advantage over their predators without upsetting their ecology too drastically.

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    1. Not really. I am not an advocate of intervention, generally speaking. If you can expand the area they have to nest in, that might help, but short of importing a sand dune, as Howard E. Evans writes in "Wasp Farm," not sure what to recommend.

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  13. I think importing a truck of sand is feasible, how would that help encourage the horse guard?

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    1. The Horse Guard nests preferentially in sand, ideally with a gentle slope. I have also seen other beneficial wasps nesting in sand piled up for construction projects, etc.

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