Adults - Adults - Immature stages per BOA
Return to Texas Entomology - Compiled by Mike Quinn
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.
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.
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|
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
"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).
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.
Droughts are thought to greatly diminish the abundance of parasitoids, such as Brachymeria sp. (photos), which would keep snout populations in check.
Snouts wait out drought periods in reproductive diapause.
Significant summer rains induce Spiny Hackberry, the snout's primary host and a common south Texas shrub, to put on new leaves.
Female snouts restrict egg laying to new foliar growth of host plants as the young caterpillars can only develop on tender leaves.
With low parasitoid levels, a high percentage of the snout caterpillars reach maturity. In the process, they can defoliate their hosts.
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!
Thus the American Snout is characterized by late summer migrations, region-wide movements, and male-biased sex ratio in migratory populations.
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.
LOCAL CLIMATOLOGICAL DATA
National Weather Service
Blue Months: Above Average
Red Months: Below Average Rainfall
|Sep:||5.31||+1.23||Mostly: 2nd-3rd, 17-18|
|Jul:||7.37||+5.72||Mostly: 16th, 20th, 25th|
McAllen was 8.09 inches of rain below normal between mid-September 2004 through mid-July 2005.
Between July 12 and July 25, 2005, a total of 7.37 inches fell. The greatest amount of rain, 4.25 inches, fell on July 20.
From late July to the snout swarms reported at the end of August was enough time for the build up of 2 generations of snouts.
Under optimal conditions, snouts can go from egg to adult in ~16 days!
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 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.
Most authors (see
overview by Overton) now treat bachmanii and larvata
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
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
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12 Oct 2009 © Mike Quinn / email@example.com / Texas Entomology