Frequent flyer miles
May Berenbaum
few months ago, a phone call from
Robert Krulwich, a well-known radio
and television reporter who has won
a number of awards for science journalism,
sent me on a search through the entomological literature to find out just how high insects
can get. The query was for a story idea
he had, not on insect hallucinogen abuse
(although such a query would have been
equally probable from Robert Krulwich, who
has a knack for finding science stories with
broad public appeal) but rather on insect
traffic patterns. As it happens, the question
has intrigued entomologists for the better
part of the past century and has generated
a diverse array of publications.
Until heavier-than-air
aircraft were
invented, nobody really had a clear idea of
how high insects could fly. Early on, there
was general recognition of the ability of
some species to take advantage of passive
transport by wind. Because wind is slowed
down near the earth's surface by friction, the
layer of air nearest the ground (the boundary layer) moves relatively more slowly. In
the boundary layer, insect flight speed can
exceed air speed, and as a consequence,
insects can control their movements. But
beyond the boundary layer, active flight
presents a real challenge to small creatures, and insects that escape the boundary
layer are at the mercy of the wind. Intrepid
mountaineers
scaling Himalayan peaks
often found arthropods awaiting them on
the snowfields; the Mount Everest expeditions of 1921 and 1924 reported flies at
4,900 m, butterflies and moths at 6,400 m,
and spiders as high as 6,700 m (Holzapfel
and Harrell 1968). On the other side of the
world, hover flies were found occupying the
summit of Tacana, on the border between
Mexico and Guatemala, at 4,090 m (Spalding
1979). L. W. Swan (1963) even identified a
high-altitude life zone populated largely by
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insects blown in from elsewhere subsisting on other flotsam blown in by the wind.
Inspired by Greek mythology, he called this
the aeolian zone, for Aeolus, the Greek god
known as the "ruler of the winds."
But even at these high altitudes, insects
were generally encountered close to or on
the ground. How high they had to fly to get
to mountaintops remained an open question. Not long after airplanes became a
commercially viable form of transportation,
entomologists saw the value of using them to
collect specimens hitherto beyond the reach
of conventional nets. In a review of studies of
insect distribution by aerial currents, Hardy
and Milne (1938) reported that one author
"remembers Mr. E. H. Speyer emphasizing
the importance of wind drift in the dispersal
of the aphid Chermes and suggesting in 1920
the tow-netting of the air from aeroplanes:'
Professor F.V.Theobald at the Southeastern
Agricultural College in Kent, England, noticed aphids stuck to airplane parts during
World War 1, inspiring the use of flypaper
soon thereafter to quantify aphids at heights
above 300 m (Felt 1928). Beginning in
1926, Tanglefoot-coated slides were affixed
to airplanes to collect insects, with famed
aviator Charles Lindbergh contributing to
the data-collection effort by carrying sticky
glass slides on his 1933 flight crossing the
Atlantic at 750 to 1,650 m (2,460 to 5,410 ft.)
and over Greenland at 2,400-3,700 m (7,870
to 12,135 ft.) (Holzapfel 1978).
Sticky slide sampling was a haphazard
and unsystematic affair; the first successful
quantitative estimate of high-altitude insect
densities by airplane-enabled sampling appears to have been accomplished by one B.
R. Coad, who reported his findings in the
1931 Yearbook of Agriculture. Coad used
an elaborate trap consisting of a series of
one-foot-square slightly sticky screen trays
that could be extended one at a time at a
specified altitude to winnow insects out of
the air for the desired amount of time, after
which it could be pulled back into a secure
compartment for counting after landing.
No one can accuse Coad of hype in titling
his paper "Insects captured by airplane
are found at surprising heights" -he found
insects everywhere from 50 ft. above the
ground to 14,000 ft., where "parasitic flies,
wasps, plant-lice and similar light, smallbodied insects" were remarkably common.
Even more surprising than the height were
the densities.
In the sort of calculation
popular during the era, Coad computed the
number of insects in an air column "1 mile
square starting 50 feet from the ground and
extending 14,000 feet high" in the neighborhood of Tallulah, Louisiana, not known to
be remarkable for insect aerial plankton
densities. The average value arrived at was
25,000,000, ranging from a low in January of 12,000,000 to a high during May of
36,000,000, a density that would suggest
some major visibility problems for pilots, if
not air traffic control issues.
A few years later, Hardy and Milne
(1938) undertook their own study (costing
£100 pounds, generously provided by the
UK Agricultural Research Council) using
not airplanes but kites to send up nets that
could be opened and closed to sample at any
given height (supplementing their data with
catches with nets "flown from the masts of
the Tetney wireless station in Lincolnshire").
They found only 245,000 insects in a onemile-square column of air at 1,000to 2,000 ft.,
amounting to only 9 insects per million cubic
feet of air (vs. Coad's January minimum of 69
American Entomologist.
Spring 2010
insects per million cubic feet of air). Although
they had no explanation for the difference in
densities measured, they did point out that
Coad neglected to provide "details of the collection and the figures on which the estimates
are based" in his publication, making both
interpretation and replication problematic.
As airplane technology improved, airplane-enabled high -altitude sampling methods improved as well. Gressitt et al. (1961)
described an insect trap designed for use in
a Super-Constellation airplane that involved
an elaborate cylindrical affair with baffles
and funnels and a fine-meshed stainless
steel screen, along with a high-tech deicing
mechanism and air-speed indicator. The
trap was operated for 186,970 km (116,684
statute miles) to screen a total air volume of
about 1,390,000 cubic meters. Although the
number trapped wasn't very impressive (23),
the height was; the trap managed to capture
a single termite at 5,790 m (19,000 ft.).
Finding a solitary individual of a eusocial
termite species raises questions as to the
ecological significance of achieving heights
of 5,790 m; indeed, finding any insect at such
heights raises such ecological questions.
Wind dispersal at great heights can be rough
on insects. Of 1,610 insects captured by
Taylor (1960) in traps up to 5,000 ft., for example, 97% (1571) were alive and undamaged, 2% alive and damaged, and 1% dead.
It's no surprise, then, that insects have found
a way to disperse at high altitudes without
assuming unnecessary risks. Even given
the current state of air travel, with on-time
performance parameters in steep decline,
flying inside an aircraft is still a more reliable way to travel than outside the aircraft
at the whim of air currents. This is certainly
the case for insects-insects
are found at
high elevations inside airplanes rather than
outside with increasing frequency. The first
report of onboard arthropod stowaways
dates back to 1928: when the GrafZeppelin
arrived in the United States, a government
agent inspected the dirigible and found ten
species of insects secreted among the decorative bouquets on board (Kisaluik 1929).
Since that time, a staggering diversity of
species, including disease vectors, have been
found on board (Gratz et al. 2000).
The increase in human use of airplanes
has been accompanied by an increase in
insect use. Liebhold et al. (2006) reviewed
interceptions of insects in airline baggage
by U.S.D.A.airport inspectors. About 85%
of insects intercepted in baggage arrive by
plane; only 14% are found in motor vehicles,
American Entomologist.
Volume 56, Number 1
and fewer than 1% of interceptions involve
boat traffic. In the period between 1984 and
2001, air baggage interceptions in the U.S.
numbered 290,101-and
those were just
the non-native species. They arrived from
316 countries, with the majority traveling
in fruit or nuts (mangoes in particular).
About one-third of the species intercepted
were Homoptera (aphids and scales), many
of which are wingless and can't even fly.
Curiously, the gross national product of the
country of origin was negatively associated
with the number of interceptions.
Speaking ofinterceptions, there's one other way insects reach great heights-on
the
outside of airplanes. The usual outcome of
such transport, however, is more often death
and dismemberment than it is dispersal. No
one has any estimates of the frequency with
which airplanes collide with insects, but it's
frequent enough that there are patents for
systems for removing insect debris from
various and sundry airplane parts, particularly the inlet cowl of the engine (e.g., Spiro
etaI.1997). New drag reduction techniques
depend on hybrid laminar flow control, and
insect impingement can cause "the outer
shell of an insect to rupture, releasing the
body fluid on to the leading edge surface. This
causes the insect, or parts of the insect body;
to adhere to the aircraft skin, thus disrupting
the laminar flow" (Young and Humphreys
2004). Thus, aeronautical engineers share
an interest with entomologists at determining how high insects can fly. Most studies on
insect strikes (distinguishing between subcritical strikes leaving a "thin protein stain"
and supercritical strikes in which "some of
the insect body was visible") suggest that
the problem is most acute below 500 m and
between mid-April and mid-December.
The era of space travel has brought about
the greatest changes in arthropod distributions, along with attitudes about their presence in, on, or around aircraft. Instead of
having to stowaway on board, arthropods
have occupied a place of honor on each craft,
breaking new distance barriers. In 1947, on
board a United States V2 rocket, fruit flies
became the first animals to be deliberately
launched into space, as subjects of a study
designed to determine effects of radiation
exposure during space flight. They were also
among the first animals to arrive in deep
space, on board the 1968 Soviet circumlunar
voyager Zond-5, along with mealworms and
two Horsfield's tortoises. Skylab 3 carried
the first spiders in space (two cross spiders,
Araneus diadematus, named Arabella and
Anita, who, by spinning during the course of
the 90-minute trip around the planet, basically invented the World Wide Web). The
first private spacecraft to carry animals into
space was Genesis I, launched 12 July 2006
by Bigelow Aerospace with Madagascar
hissing cockroaches and Mexican jumping
beans (caterpillars of Cydia deshaisiana) on
board. Rocket propulsion has proved to be a
quantum improvement over wind power for
carrying arthropods to new heights-not
to
mention a giant leap for Mexican jumping
beans.
References
Coad, B. R.,1931.lnsects
captured by airplanes
are fond at surprising heights. Yearbook of
Agriculture 1931: 320-323.
Gratz, N. G. , R. Steffen, and W. Cocksedge
2000. Why aircraft disinsection? Bull World
Health Org. 78.
Gressit, J. L., J. Sedlacek, K. A. J. Wise, and C.
M. Yoshimoto, 1961. A high speed airplane
trap for air-borne organisms. Pacific Insects
3: 549-555.
Hardy, A.C. and P. S. Milne, 1938. Studies in the
distribution of insects by aerial currents. J.
Anim. Eco!' 7: 199-229.
Holzapfel, E. P., 1978. Transoceanic airplane
sampling for organisms and particles. Pacific
Insects 18: 169-189.
Holzapfel, E. P.and J. C. Harrell, 1968. Transoceanic dispersal studies of insects. Pacific
Insects 10: 115-153.
Kisaluik, M., 1929. Plant quarantine inspection
of the dirigible "Graf Zeppelin." J. Econ. Ent.
122: 594-595.
Liebhold, A.M., T. T. Work, D. G McCullough,
and J. F. Cavey, 2006. Airline baggage as a
pathway for alien insect species invading the
United States. Amer. Entomo!. 52: 48-54.
Ramachandran,
R. 1987. Terminal velocity of
the first instar Ectropis excursaria (Guenee)
(Lepidoptera).
Proc. Indian Acad. Sci. 96:
673-678.
Spalding, J. B., 1979. The Aeolian ecology of
White Mountain Peak, California: Windblown
insect fauna. Arctic Alpine Res. 11: 83-94.
Spiro et al. 1997. Aircraft anti-insect system.
United States Patent 5,683,062 (November
4,1997).
Taylor, L.R. 1960. Mortality and viability of insect
migrants high in the air. Nature 186: 410.
Witt, P. N., M. B. Scarboro, D. B. Peakall, and R.
Gause. (1977) Spider we b-building in outer
space: Evaluation of records from the Skylab
spider experiment. Am. J. Arachno!. 4:115.
May Berenbaum is a professor and head of the Department of Entomology,
University of Illinois, 320
Morrill Hall, 505 South
Goodwin Avenue, Urbana,
IL 61801. Currently, she is
studying the chemical aspects of interaction between herbivorous insects
and their hosts.
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