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Lecture 6b: SEASONAL ADAPTATION - MIGRATION
I. Introduction to Movement
A. Importance
1. One of the most prevalent features of insects; "Earth is canned by millions of insects flying on
air currents, who encounter suitable and unsuitable habitats"- Johnson
2. Understanding movement critical for characterizing population dynamics
3. Types
a) emigration - movement out of an area
b) immigration - movement into an area
4. Movement is sometimes more important than natality or mortality
5. Normal behavioral/physiological movements often modified by weather
6. Can result in substantial mortality (e.g., over water, glaciers, etc.)
II. Classes of Movement
A. Passive Movement
1. Wind-aided movement with no activity on the part of the individual
2. Common in flightless arthropods: e.g., Collembola, spiders, mites
B. Active Movement
1. Trivial movement
a) displacement of insects within breeding or feeding sites
b) usually movement over short distances
c) typically does not involve displacement of entire populations
(1) butterfly feeding, lightning bug mating
2. Migratory movement
a) involves displacements of entire populations
b) displacement from breeding, feeding, and/or overwintering sites
c) Johnson (1969) "adaptive dispersal"
d) dispersal usually involves flight
(1) but, chinch bug, Mormon cricket, armyworm all show migratory behavior without
flight
e) individuals displaced hundreds of kilometers
f) individuals predisposed to flight/transport
(1) "non-appetetial behaviors" - undistracted by mates, food, or oviposition sites
g) regular feature of seasonal cycle for some insects
h) can result in substantial mortality; only a minute fraction may locate suitable habitat
III. Insect Migration Within the Boundary Layer
A. Boundary Layer
1. That altitude at which wind speed equal insect flight speed; below the boundary layer insects
can have directly flight
2. Usually only a few meters high; flight of insects may be directly observable
3. Insects control their own flight path; seem to maintain a steady course
4. Examples include migrating butterflies, dragonflies, flower flies
5. E.g., monarch butterfly, Danaus plexippus
a) famous example
b) fall: adults group and fly south and west to overwintering sites (Mexico or California);
aggregate in trees (roosts) at night and in overwintering sites
c) spring: adults fly north and reproduce along the way (May and June); recolonize ca. 1
million sq. miles in 2 months; migration times (from capture-recapture studies) between
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18 and 109 days for return trip (as much as 3465 km or 2153 miles)
d) while migrating can detect thermal updrafts but still boundary layer migrant; "in short,
they (are) experts in cross-country soaring, apparently able to detect thermal activity
close to the ground, enter it, and stay within it to reduce their need for powered flight" Wellington
IV. Migration Above the Insect Boundary Layer
A. Usually Combines Flight With Wind-Aided Transport
1. Insect may not be in control of flight vector
a) e.g., spiders, aphids
2. Insects are drawn with winds into zones of air mass convergence where they are deposited
B. Aspects Of Migration Above The Boundary Layer
1. Getting aloft
a) flight behavior patterns
(1) behavioral patterns - posturing (spiders, mites), silking
(2) bursts of activity with changes in barometric pressure
(a) pressure changes detected by Johnston's organ on antennae in Diptera
(b) towering (spruce budworm), black flies, noctuid migrants
(3) weather components
(a) wind thermals (convection) of air masses as surface is warmed (used by
gypsy moth larvae for dispersal)
(b) sand and dust storms provide convection forces (dust devils used by potato
leafhopper in south)
2. Staying aloft
a) some flight activity is usually required, because of constant downdrafts and updrafts
b) insect most likely are transported in air parcels
c) transport not possible at altitudes of about 10 to 20 thousand feet because of cooling
(natural folding of wings)
d) low-level jet in the central plains of the U.S. - 1 500 to 2000 ft. (good environment for
transport) (not exclusive to U. S.), common in spring
3. Getting down ( deposition, grounding)
a) Johnson: insects may "pile up" when wind system reaches a cold front, making insects
arrive all at once
b) may get down by cooling and being caught in downdrafts
c) may become exhausted and terminate flight response
d) may be rained out (e.g., desert locust, potato leafhopper)
C. Examples
1. Desert locust, Schistocerca gregaria
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a) found in Africa, Middle East, and India
b) ancient pest; remarkable numbers (mammoth swarms of 10 billion individuals covering
100 sq miles can occur); locust is from
c) outbreaks, swarming
(1) wet conditions provide ample food, followed by dry conditions with reduced
mortality
(2) crowding leads to changes in body form and behavior (solitary vs. gregarious
forms)
(a) with gregarious forms swarms develop, fly in Intertropical Convergence
Zone (where winds from both sides of the equator converge`), drop out with
rain where greater vegetation occurs
2. Black cutworm, Agrotis ipsilon
(1) noctuid moth, pest of seedling plants
(2) pupae overwinter in Mexico and southern U. S.
(3) adults are transported north on southern winds
(4) meteorological models and sampling establish that moths follow air parcel
trajectories (model movement of sections of air through time); can predict influx
potential for moths (confirming through pheromone traps) and a degree-day model
is used to predict larval injury
(5) movement is associated with low pressure center over the Great Plains, with a
trailing N-S cold front and a leading E-W warm front, which fosters movement from
the southern source regions (true for many migrants)
3. Spruce budworm, Choristoneura fumiferana
a) moths engage in towering, flying up in association with a conducive weather system
b) fall out with rain or in with collapse of frontal system
V. Biological and Ecological Significance of Migration
A. Important Adaptive Characteristic
1. Use wind flow to relocate and colonize favorable habitats
2. Dispersal mechanism
3. Transport insects beyond the boundaries of their old reproductive site
B. Strongly Influences Population Dynamics
1. Migrations often responsible for tremendous outbreaks
2. May change genetic makeup of population
C. Prevalence
1. As research continues, more insects are added to the migrant list
D. Evolutionary Aspects
1. Two-way migrants (obvious advantages, habitat exploitation)
2. What about one-way migrants?
a) if no remigration, then genetic dead end
b) remigration is not well understood (may be more common)
c) migration may facilitate incremental habitat gains
VI. References
Domino, R. P., W. B. Showers, S. E. Taylor, and R. H. Shaw. 1983. Spring weather pattern associated with
suspected black cutworm moth (Lepidoptera: Noctuidae) introduction into Iowa. Environ. Entomol.
12:1863-1871.
Drake, V. A., and R. A. Farrow. 1988. The influence of atmospheric structure and motions on
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insect migration. Annu. Rev. Entomol. 33:183-210.
Hendrix, W. H. and W. B. Showers. 1992. Tracing black cutworm and armyworm (Lepidoptera: Noctuidae)
northward migration using Pithecellobium and Calliandra pollen. Environ. Entomol. 21:1092-1096.
Johnson, C. G. 1969. Migration and dispersal of insects by flight. Methuen, N.Y.
Johnson, C. G. 1974. Insect migration: aspects of its physiology. pp. 270-334 In M. Rockstein, ed. The
physiology of the Insecta. Academic Press, N.Y.
Peterson, R. K. D., L. G. Higley, and W. C. Bailey. 1988. Phenology of the adult celery looper, Syngrapha
falcifera (Lepidoptera: Noctuidae), in Iowa: Evidence for migration. Environ. Entomol. 17:679-684
Rankin, M. A., and M. C. Singer. 1984. Chapter 7. Insect movement: mechanisms and effects. In C. B.
Huffaker and R. L. Rabb, eds. Ecological entomology. John Wiley and Sons, N. Y.
Sappington, T. W. and W. B. Showers. 1992. Reproductive maturity, mating status, and longduration flight
behavior of Agrotis ipsilon (Lepidoptera: Noctuidae) and the conceptual misuse of the oogenesis-flight
syndrome by entomologists. Environ. Entomol. 21:677-688.
Showers, W. B., R. B. Smelser, A. J. Keaster, F. Whitford, J. F. Robinson, J. D. Lopez, and S. E. Taylor. 1989.
Recapture of marked black cutworm (Lepidoptera: Noctuidae) males after long-range transport.
Environ. Entomol. 18:447-458.
Tauber, M. J., C. A. Tauber, and S. Masaki. 1984. Chapter 6. Adaptations to hazardous seasonal conditions:
Dormancy, migration, and polyphenism. In C. B. Huffaker and R. L. Rabb, eds. Ecological
entomology. John Wiley and Sons, N. Y.
Uvarov, B. P. 1931. Insects and climate. Trans. Royal Entomol. Soc. (London). 79:1-247.
Wellington, W. G., and R. M. Trimble. 1984. Chapter 13. Weather. In C. B. Huffaker and R. L. Rabb, eds.
Ecological entomology. John Wiley and Sons, N. Y,
Wolf, R. A., L. P. Pedigo, R. H. Shaw, and L. D. Newsom. 1987. Migration/transport of the green cloverworm,
Plathypena scabra (F.) (Lepidoptera: Noctuidae), into Iowa as determined by synoptic-scale weather
patterns. Environ. Entomol. 16:1169-1174.
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