American Snout

(a.k.a. Snout Butterflies)

click to lift image
Note: male snouts (lower) have highly reduced front legs.

Buck Hollow Ranch, southern Real County, Texas
28 July 2006 (Bill Carr)

Adults - Adults - Immature stages per BOA

Media Reports 2006

Distribution records per BAMONA

Return to Texas Entomology - Compiled by Mike Quinn

South Texas Snout "Migration" Ecology

Mass movements of snout butterflies are spectacular for their density, duration and geographical extent.

Periodic snout oubreaks are one of the most phenominal reoccurring south Texas entomological events.

In late September 1921 an estimated 25 million per minute southeasterly-bound snout butterflies passed over a 250 mile front. Gable and Baker (1922) noted that this flight lasted 18 days. It may have involved more that 6 billion butterflies.

Caterpillar Food Plants

Celtis spp. - Elm Family Ulmaceae

American Snouts (range per BAMONA) host on various species of Hackberry (Celtis), but Spiny Hackberry (Celtis (=pallida) ehrenbergiana) provides the fuel for the tremendous periodic population explosions across the arid southwestern United States. Spiny hackberry, a.k.a. Desert hackberry or Granjeno, is one of the more common shrubs of the Tamaulipan thornscrub (or South Texas brushlands). Note that while other species of Celtis can host snout caterpillars, the non ehrenbergiana species don't put on new leaves (which the just hatched caterpillars require) in response to significant summer rain events.

Spiny Hackberry (Celtis (=pallida) ehrenbergiana) Distribution in Texas

Spiny Hackberry range in Texas

Map source: Turner et al. 2003

C. ehrenbergiana ranges from south Texas to Arizona, south to northern Argentina.

Other native hackberry species that host snout caterpillars:

Western North America

Eastern North America

Celtis ehrenbergiana - Map
Celtis laevigata var. laevigata - Map 

Celtis laevigata var. reticulata - Map  Celtis occidentalis - Map 

(scroll in on maps for county records) Celtis tenuifolia - Map 


Life Cycle

Winter is spent in the adult stage. Opler & Krizek (1984) report that mated pairs have been seen only at night (2205-2345 hr) however John and Gloria Tveten report "many ... coupled mating pairs" at around 1300 hr. on October 14, 2004 at Santa Ana NWR, Alamo, Hidalgo Co., TX..  Females lay eggs singly on young terminal leaves of their host plants (Scott, 1986). The period from the egg deposition to the appearance of the adult butterfly is relatively short, being 15 to 17 days in West Virginia (Macy & Shepard, 1941).

Migration Ecology

Raymond Neck (1983) was the first to note that snout population size is positively correlated with the intensity and duration of dry periods immediately preceding drought-terminating rains. Larry Gilbert (1985) conducted the most intensive study yet of snout population explosions in south Texas.

Gilbert's review of published accounts of snout outbreaks in south Texas between 1912 and 1980 found that they occur from late June to mid-October.

Gilbert was the first to study snout migrations at their points of origin. During the summers of 1976, 1977 and 1978, Gilbert and his graduate students centered their snout investigations on the Chaparral Wildlife Management Area in the heart of the south Texas brushlands, approximately halfway between San Antonio and Laredo. On the 5,200 acre "Chap," they observed an extreme range of weather conditions and associated snout migrations.

In early to mid-July 1976, the Chaparral WMA received 11 inches of rain (about 10.25 inches above normal). On July 26, 1976, Gilbert found an area on the Chap where Spiny Hackberry was totally defoliated by snout larvae. On one 3 m diameter hackberry shrub, they counted 2027 snout pupae. The pupae showed a relatively low parasitoid rate of 8% (n = 658).

The pupae sampled on July 27, 1976 showed a nearly even sex ratio of 1.15 (79 males to 69 females). On July 28, the butterflies began emerging from their pupal stage and started to emigrate. The emigrating adults were sampled to determine their sex ratio. Given that the caterpillar food plants were defoliated, the migratory adult sex ratio was expected to be female biased, with the females departing to search for non-defoliated hackberries. Much to his amazement (because no one had looked at this before), Gilbert found the opposite! On July 28, the migrating adults had a sex ratio of 83.5 (167 males to 2 females). 

Close inspection of the defoliated hackberry revealed that it was actually still putting on new leaves! The females mated as they emerged and remained to lay eggs on the new shoots. On a sample of 50 shoots, they found one egg on about every other shoot. 

If the females were staying behind to mate and lay eggs, why were the males departing in mass? Gilbert deduced that many of the females at this site were mating with males from a prior generation and therefore the males emerging on July 28 were most likely departing to find females that had yet to mate. While the pupae were nearly evenly split between males and females on July 27, on the same day males were almost 2 to 1 as common as females (n = 40) on Condalia flowers. This excess of males may have been from a prior generation and better able to compete for the emerging females than were the emerging males.

Migrating snouts were re-sampled on the Chap on August 11, 1976 and the sex ratio had evened out somewhat to 7.0 (76 males to 11 females). The egg and larval density may have finally reached a high enough level to induce the females to migrate to areas of lower host competition.

From June 1977 until May 1978, the Chaparral WMA and the region generally suffered one of the most severe droughts on record. On May 7, Gilbert found a single snout during six hours on the Chap. Through the rest of May and June, rainfall was well above average, thus allowing for a rapid increase in snout populations over 2 - 3 generations. Then in late July Tropical Storm Amelia tracked up through south Texas. Amelia produced the largest three-day total rainfall ever recorded in the United States on the Manatt ranch, Medina County, where more than 48 inches of rain fell during the period August 1 to 3 (Hansen 1979, Caran & Baker 1986: Fig. 2(7)!

The response of the snout butterflies was spectacular! After approximately 100 days of conditions favorable for hackberry leaf growth, the snouts reached astronomical numbers. Extracting from Ellisor (1970), Gilbert estimated 1.56 million hackberry shrubs on the Chaparral WMA, or an average of 255 hackberry shrubs per hectare (= 91 hackberry shrubs per acre). Of these, no less than 85 % or 1.32 million were defoliated in August 1978. Using data collected in 1976, Gilbert determined that under optimal conditions, the average 1.5 m diameter hackberry shrub could produce no less than 400 snout adults. Thus Gilbert estimated the total snout population generated on the Chap in 1978 was approximately one half billion!

It must be emphasized that the snout's mass directional movements are not migratory in the traditional sense, that is, north in the spring and south in the fall. Instead, snouts move locally from patch to patch of suitable habitat. Typical uneven rainfall across south Texas creates uneven patchs of suitable hackberry shrubs. Gilbert reported snouts in 1976 moving in three different directions east of I-35 (see arrows A, B, and C in Fig. 4 below), with one flight actually reversing directions between morning and afternoon!  Gilbert reports that snouts generally fly at speeds ranging from 5 to 8 mph, hardly the speed necessary for long distance migrations.

Snout migratory directions

2004-05 American Snout Outbreaks in the Rio Grande Valley

The Valley Morning Star reported: Motorists crossing the Queen Isabella Memorial Causeway to South Padre Island drove into clouds of butterflies Tuesday [August 30, 2005]. The hit-and-run victims were American snouts.

FWS Ecologist Chris Best estimated a swarm of American Snouts on September 2, 2005 might have contained 7.5 million butterflies in the Alamo area.

Also on Sept. 2, Jan Dauphin reported the following: "Since 1500 hrs (and they are still streaming through) tens, if not hundreds of thousands, of American Snouts are streaming through our [Mission] neighborhood. We have witnessed several of these movements through the Valley in the past, but nothing like this. You literally have to brush them off your clothes before going inside. Neighbors are standing outside watching them, cars are stopping to watch. All seem to be heading northeast."

Given the above reports of snout migrations in the Rio Grande Valley, I was curious to see what the preceding environmental conditions were.

National Weather Service

McAllen, TX: October 2004 - August 2005

Blue Months: Above Average Rainfall
Red Months: Below Average Rainfall

Rainfall in
Departure from

2004 Jul: 0.45 -1.20

Aug: 3.36 +0.63 Mostly: 30th-31st

Sep: 5.31 +1.23 Mostly: 2nd-3rd, 17-18

Oct: 0.73 -1.83

Nov: 0.32 -0.55

Dec: 0.85 -0.36
2005 Jan:   0.97 -0.25

Feb: 0.96  -0.37

Mar: 0.39  -0.33

Apr: 0.02  -1.30

May: 1.77  -0.91

Jun: 0.40  -2.19

Jul: 7.37  +5.72 Mostly: 16th, 20th, 25th

Aug: 1.85 -0.88

So it appears that classic conditions of drought followed by wide-spread rains occurred prior to the snout outbreaks in 2005.

In the archives of TX-Butterfly is a most interesting report of a snout outbreak observed by John & Gloria Tveten on October 14, 2004 at the old manager's residence at Santa Ana NWR, Alamo, TX. They reported "thousands, if not tens of thousands, of larvae" on the Spiny Hackberry. In addition, snout pupae were "present in great profusion, sometimes a dozen or more hanging from a single twig." Also, adults were in the midst of a "mass emergence." The Tveten's noted (at least slightly more mature) males mating with females whose wings had yet to dry. "Many were coupled mating pairs, and in quite a few of those pairs, a fully expanded male was mating with a newly emerged female, her wings only partially expanded."

Above average rains again preceded the observed outbreak at Santa Ana NWR. Mid-October is also the latest for any published account of snout migrations in south Texas. 

As for the absolute colossal snout migration mentioned at the start of this website:

In late September 1921 an estimated 25 million per minute southeasterly-bound snout butterflies passed over a 250 mile front (San Marcos south to the Rio Grande River). Gable and Baker (1922) noted that this flight lasted 18 days. It may have involved more that 6 billion (6,000,000,000) butterflies.

The following rain event was surely a contributing factor:

The most severe rainstorm ever recorded in the continental United States occurred September 9-10, 1921, in Thrall, approximately 40 miles NE of Austin. (Jennings, 1950; Bomar, 1983; Caran & Baker, 1986 Fig 2). A total of 36.4 inches of rain fell in 18 hr, which is the world's record for this period. The 24-hr total exceed in one day the expected precipitation of an entire year (Larkin and Bomar, 1983, pg. 18). This storm, which spread over a large area of Central Texas, produced 215 deaths and 19 million dollars in property damage (Bomar, 1983).

Snout Migrations Outside of Texas

Snout migrations are not strictly limited to the south Texas brushlands. Howe (1975) writes that "On August 9, 1966, [snout] migrants were so numerous that they obscured the sun over Tucson, Arizona, and it was necessary to turn on the street lights!" and "In September and early October, 1971, hundreds of carinenta invaded Franklin County, Kansas. They were taken visiting wild asters with three and four individuals taken in a single net swing. Individuals could be observed flying in a due north-northeast direction from the south." Neck (1983) suggests that these Kansan snouts probably originated in Texas.

Gochfeld & Burger (1997) report that in New Jersey snouts are usually rare to uncommon and local as an immigrant; most records are of single individuals, but occasionally abundant as in 1994. Up to forty per day in hackberry grove near Rutgers University stadium. Maximum Fourth of July Butterfly Count total was 49 at Cape May, NJ in 1994.

Wormington (2006) reports that snouts are an annual immigrant to Point Pelee National Park, Leamington, Ontario, Canada, where they often become numerous because of the abundance of Common Hackberry (Celtis occidentalis). "Normal" period of occurrence is the middle of June to early October. Extreme dates of occurrence are May 13 (2000) to November 16 (2001) inclusive. The highest recorded snout total (800) on a Butterfly Count occurred on July 28, 1984.


Libytheana carinenta (Cramer, 1777)

Most authors (see overview by Overton) now treat bachmanii and larvata subspecies of 
Libytheana carinenta,
a single species ranging from Canada to Argentina. (Layberry et al., 1998)
Kawahara (2001) found that the reported genitalic differences between the taxa do not hold up in series.

The Libytheinae is a small subfamily of Nymphalidae containing only about eight species.
However, these few species range throughout the tropical and subtropical regions of the world. 

Snouts are frequently placed in their own family, Libytheidae,
as the larvae lack the spines and horns of most Nymphalidae
and the pupae lack the dorsal bumps of most Nymphalinae.

Please send reports of Snout migrations to:

Snout Bibliography

Adams, S. 2005. Butterfly invasion. Valley Morning Star. Sept. 2.

Bernheim, J.L. 1917. Swarms of Butterflies (Lep.). Entomological News 28(8): 339-340. 

Bomar, G.W. 1983. Texas weather. University of Texas Press, Austin. 265 pp.

Breland, O.P. 1948. Two Migrations of the Snout Butterfly, Libytheana bachmanii larvata (Strecker). (Lepidoptera: Libytheidae). Entomological News 59(5): 128-l31.

Caran, S.C. & V. R. Baker 1986. Flooding Along the Balcones Escarpment, Central Texas Pp. 1-14 in: P.L. Abbott & C.M. Woodruff, Jr. (editors). The Balcones Escarpment: Geology, Hydrology, Ecology and Social Development in Central Texas. Published for the Geological Society of America Annual Meeting San Antonio, Texas

Clench, H.K. 1965. A migration of Libytheana and Kricogonia in southern Texas. Journal Lepidopterists' Society 19(4): 223-224.

Collenette, C. L. 1928. A migration of Libythea carinenta carinenta Cr. Entomol. Mon. Mag. 64: 124-126.

Crippen, J. R., and C.D. Bue. 1977, Maximum floodflows in the conterminous United States. U.S. Department of the Interior, Geological Survey Water Supply, Washington, D.C. Paper 1887, 52 pp.

Ellisor J.E. 1970. Job Progress Report: Federal Aid Project no. W-101-R-1: Job no. 2, Vegetative Trends. (Chaparral Wildlife Management Area). Texas Parks and Wildlife Department, Austin, TX.

Fletcher, R.K. 1926. Notes on a Migration of the Snout Butterfly (Lepid., Libytheidae). Entomological News 37(4): 106-107.

Freitas, A.V.L. 1999. An anti-predator behavior in larvae of Libytheana carinenta (Nymphalidae, Libytheinae). Journal of the Lepidopterists Society, 53(3): 130-131.

Friedlander, T. 1984. Another look at snout butterflies (Libytheidae: Libytheana). Journal of the Lepidopterists' Society 38(2): 139-141.

Gable, C.H. & W.A. Baker. 1923. Notes on a migration of Libythea bachmanni Kirtl. Canadian Entomologist 54(12): 265-266.

Gilbert, L.E. 1985. Ecological factors which influence migratory behavior in two butterflies of the semi-arid shrublands of South Texas. Pp. 724-747 in: M.A. Rankin. (editor). Migration: Mechanisms and Adaptive Significance. University of Texas, Port Aransas.

Gochfeld, M & Burger, J. 1997. Butterflies of New Jersey: A Guide to Their Status, Distribution, Conservation, and Appreciation. Rutgers University Press, New Brunswick, New Jersey. 327 pp.

Hansen. E.M. 1979. Study of the reported 48+ inch rainfall in the storm of August 1-3, 1978, near Medina, Texas: Silver Springs, Maryland, National Oceanic and Atmospheric Administration, Weather Service unpublished report, 12 pp.

Howe, W.H. (editor), 1975. Butterflies of North America. Doubleday and Co., Garden City, NY 

Kawahara, A.Y. 2001. Systematics of the snout butterflies (Lepidoptera: Nymphalidae: Libytheinae) with an emphasis on morphology. Entomology Department Honors Thesis, Cornell University, Ithaca, New York. 161 pp.

Kawahara, A.Y. 2003. Behavioral Observations of Libytheana carinenta Cramer. News of the Lepidopterists' Society 45(4): 107-108. 

Jennings, A.H. 1950. World's greatest observed point rainfalls. Monthly Weather Review 78(1): 4-5.

Layberry, R.A., P.W. Hall, & D.J. Lafontaine. 1998. The butterflies of Canada. University of Toronto Press, Toronto, Canada. 280 pp.

Larkin, T.J. & G.W. Bomar. 1983. Climatic atlas of Texas. Texas Department of Water Resources, Austin. LP-192, 151 pp.

Macy, R.W. & H.H. Shepard. 1941. Butterflies, a handbook of the butterflies of the United States, complete for the region north of the Potomac and Ohio Rivers and east of the Dakotas. Univ. of Minn. Press, Minneapolis. vii. + 247 pp.

Neck, R.W. 1983. Causal analysis of a migration of the snout butterfly. Libytheana bachmanii larvata (Strecker) (Libytheidae). Journal of the Lepidopterists' Society 37(2): 121-128.

Neck, R.W. 1984. On the origin of snout butterflies (Libytheana bachmanii larvata, Libytheidae) in a 1978 migration in south Texas. Journal of the Lepidopterists' Society 38(4): 319-322.

Opler, P.A. & Krizek, G.O. 1984. Butterflies East of the Great Plains. Johns Hopkins University Press, Baltimore. 294 pp.

Opler P.A. & A.D. Warren. 2003. Scientific Names List for Butterfly Species of North America, North of Mexico. Gillette Publications, Fort Collins. 83 pp.

Parman, D.C. 1926. Migrations of the Long-beaked Butterfly, Libythea bachmani Kirtland (Lepid.: Libytheidae). Entomological News 37(4): 101-106.

Rau, P. 1941. Migrating Butterflies, Libythea bachmanii larvata Stkr., in Texas. (Lepid.: Nymphalidae). Entomological News 52(10): 277-278.

Rutowski, R.L., B. Terkanian, & O. Eitan. 1997. Male Mate-Locating Behavior and Yearly Population Cycles in the Snout Butterfly, Libytheana bachmanii (Libytheidae). Journal of the Lepidopterist's Society 51(3):197-207.

Scott, J.A. 1986. The Butterflies of North America, a Natural History and Field Guide. Stanford University Press, Stanford. 583 pp.

Shields, O. 1984. A revised, annotated checklist of world Libytheidae. Journal for Research on the Lepidoptera 22: 264-266.

Shields, O. 1985. Zoogeography of Libytheidae (snouts or breaks [sic]). Tokurana (Acta Rhopalocera) 9(1): 1-58.

Smyth, E.G. 1920. Butterfly migrations. Journal Economic Entomology 13: 259: 260.

Smith, J.C. 2006. Butterfly swarms draw attention. The Monitor, McAllen, Texas. June 28, 2006.

Turner, B.L., H. Nichols, G. Denny, & O. Doron. 2003. Atlas of the Vascular Plants of Texas. Sida, Botanical Miscellany 24, Vol. 1, Dicots. Brit Press, Botanical Research Institute of Texas, Fort Worth, Texas. 648 pp. 

Williams, C.B. 1930. The migration of butterflies. Oliver and Boyd, Edinburgh. 473 pp.

Williams, C.B. 1958. Insect migration. Collins, London. 235 pp.

Wormington, A. 1983. The Butterflies of Point Pelee National Park, Ontario. Ontario Field Biologist 37: 1-26.

Wormington, A. 2006. The Butterflies of Point Pelee National Park, Ontario. Unpublished Manuscript.

12 Oct 2009   Mike Quinn / / Texas Entomology