Insect migration
What Is Migration?
Migration isn't the same thing as movement. Simply moving from one place to another does not
necessarily constitute migratory behavior. Some insect populations disperse, for example, spreading out
within a habitat to avoid competition for resources within the population. Insects also sometimes extend
their range, occupying a larger area of the same or similar adjacent habitat.
Entomologists differentiate migration from other types of insect movement. Migration involves some or
all of these specific behaviors or phases:
Determined movement away from the current home range - In other words, if it looks like a
migration, it probably is a migration. Migrating insects move with a mission, making persistent
progress away from their existing range and toward a new one.
Straight movement - Relative to other kinds of movement, insects will move in a fairly consistent
direction during migration.
Lack of response to stimuli - Migrating insects focus on getting where they are going, and tend to
ignore the things that occupied them in their home range. They don't stop their movement at the
first sign of suitable host plants or receptive mates.
Distinctive changes in behavior before and after migration - Insects preparing to migrate may
suspend reproductive activities and change their feeding habits. Some will climb to the top of a
tree to assess and make use of the wind currents when they depart. Locusts, which are normally
solitary insects, become gregarious.
Changes in how energy is allocated within the insects' bodies - Migrating insects undergo
physiological changes, triggered by either hormonal or environmental cues. Aphids, which usually
lack wings, may produce a winged generation capable of flight. Over several nymphal
instars, gregarious locusts develop long wings and dramatic markings. Monarch butterflies enter
a state of reproductive diapause before their long journey to Mexico.
Types of Insect Migration
Some insects migrate predictably, while others do so occasionally in response to an environmental change
or other variables. The following terms are sometimes used to describe different types of migration.
Seasonal migration - migration that occurs with the change of seasons. Monarch butterflies in
eastern North America migrate seasonally.
Reproductive migration - migration to or from a separate breeding location. Salt marsh
mosquitoes migrate from their breeding grounds after emergence as adults.
Irruptive migration - migration that occurs unpredictably, and may not involve the entire
population. Painted lady butterflies are irruptive migrants. Their migration is often associated
with El Niño weather patterns.
Nomadic migration - migration that involves progressive movement away from the home range,
but not to a specific alternate location. Locust migration tends to be nomadic.
When we think of migration, we often assume it involves animals moving north and south. Some insects,
however, migrate to different altitudes rather than changing latitudes. By migrating to a mountaintop
during summer months, for example, insects can take advantage of the ephemeral resources in an alpine
environment.
Which Insects Migrate?
So, which insect species migrate? Here are some examples, grouped by order and listed alphabetically:
Butterflies and Moths:
American lady (Vanessa virginiensis)
American snout (Libytheana carinenta)
army cutworm (Euxoa auxiliaris)
cabbage looper (Trichoplusia ni)
cabbage white (Pieris rapae)
cloudless sulphur (Phoebis senna)
common buckeye (Junonia coenia)
corn earworm (Helicoverpa zea)
fall armyworm (Spodoptera frugiperda)
gulf fritillary (Agraulis vanillae)
little yellow (Eurema (Pyrisitia) lisa)
long-tailed skipper (Urbanus proteus)
monarch (Danaus plexippus)
mourning cloak (Nymphalis antiopa)
obscure sphinx (Erinnyis obscura)
owl moth (Thysania zenobia)
painted lady (Vanessa cardui)
pink-spotted hawkmoth (Agrius cingulata)
queen (Danaus gilippus)
question mark (Polygonia interrogationis)
red admiral (Vanessa atalanta)
sleepy orange (Eurema (Abaeis) nicippe)
tersa sphinx (Xylophanes tersa)
yellow underwing moth (Noctua pronuba)
zebra swallowtail (Eurytides marcellus)
Dragonflies and Damselflies:
blue dasher (Pachydiplax longipennis)
common green darner (Anax junius)
great blue skimmer (Libellula vibrans )
painted skimmer(Libellula semifasciata)
twelve-spotted skimmer (Libellula pulchella)
variegated meadowhawk (Sympetrum corruptum)
True Bugs:
greenbug aphid (Schizaphis graminum)
large milkweed bug (Oncopeltus fasciatus)
potato leafhopper (Empoasca fabae)
Insect migration is the seasonal movement of insects, particularly those by species of dragonflies,
beetles, butterflies and moths. The distance can vary with species and in most cases these
movements involve large numbers of individuals. In some cases the individuals that migrate in one
direction may not return and the next generation may instead migrate in the opposite direction.
This is a significant difference from bird migration.
Definition
All insects move to some extent. The range of movement can vary from within a few centimeters
for some sucking insects and wingless aphids to thousands of kilometres in the case of other insects
such as locusts, butterflies and dragonflies. The definition of migration is therefore particularly
difficult in the context of insects. A behaviour oriented definition proposed is
Migratory behaviour is persistent and straightened-out movement effected by the animal's own
locomotory exertions or by its active embarkation on a vehicle. It depends upon some temporary
inhibition of station-keeping responses but promotes their eventual disinhibition and recurrence.
This definition disqualifies movements made in the search of resources and which are terminated
upon finding of the resource. Migration involves longer distance movement and these movements
are not affected by the availability of the resource items. All cases of long distance insect migration
concern winged insects.[2]
General patterns
Migrating butterflies fly within a boundary layer, with a specific upper limit above the ground.
The air speeds in this region are typically lower than the flight speed of the insect. These 'boundary-
layer' migrants include the larger day-flying insects, and their low-altitude flight is obviously easier
to observe than that of most high-altitude windborne migrants.[3]
Many migratory species tend to have polymorphic forms, a migratory one and a resident phase.
The migratory phases are marked by their well developed and long wings. Such polymorphism is
well known in aphids and grasshoppers. In the migratory locusts, there are distinct long and short-
winged forms
The energetic cost of migration has been studied in the context of life-history strategies. It has been
suggested that adaptations for migration would be more valuable for insects that live in habitats
where resource availability changes seasonally. Others have suggested that species living in
isolated islands of suitable habitats are more likely to evolve migratory strategies. The role of
migration in gene flow has also been studied in many species. Parasite loads affect migration.
Severely infected individuals are weak and have shortened lifespans. Infection creates an effect
known as culling whereby migrating animals are less likely to complete the migration. This results
in populations with lower parasite loads.
Orientation
Migration is usually marked by well defined destinations which need navigation and orientation.
A flying insect needs to make corrections for crosswinds.[9] It has been demonstrated that many
migrating insects sense windspeed and direction and make suitable corrections. Day-flying insects
primarily make use of the sun for orientation, however this requires that they compensate for the
movement of the sun. Endogenous time-compensation mechanisms have been proposed and tested
by releasing migrating butterflies that have been captured and kept in darkness to shift their internal
clocks and observing changes in the directions chosen by them. Some species appear to make
corrections while it has not been demonstrated in others
Most insects are capable of sensing polarized light and they are able to use the polarization of the
sky when the sun is occluded by clouds.The orientation mechanisms of nocturnal moths and other
insects that migrate have not been well studied, however magnetic cues have been suggested in
short distance fliers.
Recent studies suggest that migratory butterflies may be sensitive to the Earth's magnetic field on
the basis of the presence of magnetite particles. In an experiment on the monarch butterfly, it was
shown that a magnet changed the direction of initial flight of migrating monarch butterflies.
However this result was not a strong demonstration since the directions of the experimental
butterflies and the controls did not differ significantly in the direction of flight.
Lepidoptera
Distribution map of Macroglossum stellatarum showing their migration pattern. Blue, summer;
green, year round; yellow, winter
Main article: Lepidoptera migration
Migration of butterflies and moths is particularly well known. The Bogong moth is a native insect
of Australia that is known to migrate to cooler climates. The Madagascan sunset moth (Chrysiridia
rhipheus) has migrations of up to thousands of individuals, occurring between the eastern and
western ranges of their host plant, when they become depleted or unsuitable for consumption.
In southern India, mass migrations of many species occur before monsoons. As many as 250
species of butterflies in India are migratory. These include members of the Pieridae and
Nymphalidae The Australian painted lady periodically migrates down the cost of Australia,[20] and
occasionally, in periods of strong migration in Australia, migrate to New Zealand.[21]
The monarch butterfly migrates from southern Canada to wintering sites in central Mexico where
they spend the winter. In the late winter or early spring, the adult monarchs leave the Transvolcanic
mountain range in Mexico to travel north. Mating occurs and the females seek out milkweed to
lay their eggs, usually first in northern Mexico and southern Texas. The caterpillars hatch and
develop into adults that move north, where more offspring can go as far as Central Canada until
the next migratory cycle. The entire annual migration cycle involves five generations.
The painted lady (Vanessa cardui) is a butterfly whose annual 15,000 km round trip from
Scandinavia and Great Britain to West Africa involves up to six generations.
The hummingbird hawk-moth (Macroglossum stellatarum) migrates from Africa and southern
Asia to Europe and northern Asia.
Orthoptera
Locusts (Schistocerca gregaria) regularly migrate with the seasons.
Short-horned grasshoppers sometime form swarms that will make long flights. These are often
irregular and may be related to resource availability and thus not fulfilling some definitions of
insect migration. There are however some populations of species such as locusts (Schistocerca
gregaria) that make regular seasonal movements in parts of Africa;[2] exceptionally, the species
migrates very long distances, as in 1988 when swarms flew across the Atlantic Ocean.[23]
Odonata
Pantala flavescens is the world's longest known distance travelling dragonfly
Dragonflies are among the longest distance insect migrants. Many species of Libellula, Sympetrum
and Pantala are known for their mass migration. Pantala flavescens is thought to make the longest
ocean crossings among insects, flying between India and Africa on their migrations. Their
movements are often assisted by winds.
Coleoptera
Ladybird beetles such as Hippodamia convergens, Adalia bipunctata and Coccinella
undecimpunctata have been noted in large numbers in some places. In some cases, these
movements appear to be made in the search for hibernation sites.
This article is about the insect. For other uses, see Locust (disambiguation).
Locusts are grasshoppers, such as this migratory locust (Locusta migratoria), that have entered into a
migratory phase of their life.
Garden locust Acanthacris ruficornis
Locusts are certain species of short-horned grasshoppers in the family Acrididae that have a
swarming phase. These insects are usually solitary, but under certain circumstances they become
more abundant and change their behaviour and habits, becoming gregarious. No taxonomic
distinction is made between locust and grasshopper species; the basis for the definition is whether
a species forms swarms under intermittently suitable conditions.
In the solitary phase, these grasshoppers are innocuous, their numbers are low, and they do not
pose a major economic threat to agriculture. Under suitable conditions of drought followed by
rapid vegetation growth, serotonin in their brains triggers a dramatic set of changes: they start to
breed abundantly, becoming gregarious and nomadic (loosely described as migratory) when their
populations become dense enough. They form bands of wingless nymphs which later become
swarms of winged adults. Both the bands and the swarms move around and rapidly strip fields and
cause damage to crops. The adults are powerful fliers; they can travel great distances, consuming
most of the green vegetation wherever the swarm settles.
Locusts have formed plagues since prehistory. The ancient Egyptians carved them on their tombs
and the insects are mentioned in the Iliad, the Bible and the Quran. Swarms have devastated crops
and been a contributory cause of famines and human migrations. More recently, changes in
agricultural practices and better surveillance of locations where swarms tend to originate, have
meant that control measures can be used at an early stage. The traditional means of control are
based on the use of insecticides from the ground or the air, but other methods using biological
control are proving effective.
Swarming behaviour decreased in the 20th century, but despite modern surveillance and control
methods, the potential for swarms to form is still present, and when suitable climatic conditions
occur and vigilance lapses, plagues can still occur. Locusts are large insects and convenient for
use in research and the study of zoology in the classroom. They are also edible insects; they have
been eaten throughout history and are considered a delicacy in many countries. The word "locust"
is derived from the Vulgar Latin locusta, meaning grasshopper.[2]
Swarming grasshoppers
Main articles: Grasshopper and Swarm behaviour
A desert locust ovipositing in sand
Locusts are the swarming phase of certain species of short-horned grasshoppers in the family
Acrididae. These insects are usually solitary, but under certain circumstances become more
abundant and change their behaviour and habits, becoming gregarious.[3][4][5]
Desert locusts in copulation
No taxonomic distinction is made between locust and grasshopper species; the basis for the
definition is whether a species forms swarms under intermittently suitable conditions. In English,
the term "locust" is used for grasshopper species that change morphologically and behaviourally
on crowding, forming swarms that develop from bands of immature stages called hoppers.
These changes are examples of phase polymorphism; they were first analysed and described by
Boris Uvarov, who was instrumental in setting up the Anti-Locust Research Centre. He made his
discoveries during his studies of the Migratory locust in Caucasus, whose solitary and gregarious
phases had previously been thought to be separate species (Locusta migratoria and L. danica L.).
He designated the two phases as solitaria and gregaria. These are also referred to as statary and
migratory morphs, though strictly speaking, their swarms are nomadic rather than migratory.
Charles Valentine Riley and Norman Criddle were also involved in achieving the understanding
and control of locusts.
Swarming behaviour is a response to overcrowding. Increased tactile stimulation of the hind legs
causes an increase in levels of serotonin.This causes the locust to change colour, eat much more,
and breed much more easily. The transformation of the locust to the swarming form is induced by
several contacts per minute over a four-hour period.A large swarm can consist of billions of locusts
spread out over an area of thousands of square kilometres, with a population of up to 80 million
per square kilometre (200 million per square mile). When desert locusts meet, their nervous
systems release serotonin, which causes them to become mutually attracted, a prerequisite for
swarming.
The initial bands of gregarious hoppers are known as "outbreaks", and when these join together
into larger groups, the event is known as an "upsurge". Continuing agglomerations of upsurges on
a regional level originating from a number of entirely separate breeding locations are known as
"plagues". During outbreaks and the early stages of upsurges, only part of the locust population
becomes gregarious, with scattered bands of hoppers spread out over a large area. As time goes
by, the insects become more cohesive and the bands become concentrated in a smaller area. In the
desert locust plague in Africa, the Middle East, and Asia that lasted from 1966 to 1969, the number
of locusts increased from two to 30 billion over two generations, but the area covered decreased
from over 100,000 square kilometres (39,000 sq mi) to 5,000 square kilometres (1,900 sq mi).
Solitary and gregarious phases
Solitaria (grasshopper) and gregaria (swarming) phases of the desert locust
One of the greatest differences between the solitary and gregarious phases is behavioural. The
gregaria nymphs are attracted to each other, this being seen as early as the second instar. They
soon form bands of many thousands of individuals. These groups behave like cohesive units and
move across the landscape, mostly downhill, but making their way around barriers and merging
with other bands. The attraction between the insects seems to be largely visual, but also involves
olfactory cues, and the band seem to navigate using the sun. They pause to feed at intervals before
resuming their march, and may cover tens of kilometres over a few weeks.
Also, differences in morphology and development are seen. In the desert locust and the migratory
locust, for example, the gregaria nymphs become darker with strongly contrasting yellow and
black markings, they grow larger, and have longer developmental periods. The adults are larger
with different body proportions, less sexual dimorphism, and higher metabolic rates. They mature
more rapidly and start reproducing earlier, but have lower levels of fecundity.
The mutual attraction between individual insects continues into adulthood, and they continue to
act as a cohesive group. Individuals that get detached from a swarm fly back into the mass. Others
that get left behind after feeding, take off to rejoin the swarm when it passes overhead. When
individuals at the front of the swarm settle to feed, others fly past overhead and settle in their turn,
the whole swarm acting like a rolling unit with an ever-changing leading edge. The locusts spend
much time on the ground feeding and resting, moving on when the vegetation is exhausted. They
may then fly a considerable distance before settling in a location where transitory rainfall has
caused a green flush of new growth.
Distribution and diversity
Several species of grasshoppers swarm as locusts in different parts of the world, on all continents
except Antarctica and North America: For example, the Australian plague locust (Chortoicetes
terminifera) swarms across Australia.
The desert locust (Schistocerca gregaria) is probably the best known species owing to its wide
distribution (North Africa, Middle East, and Indian subcontinent) and its ability to migrate over
long distances. A major infestation covered much of western Africa in 2003-4, after unusually
heavy rain set up favourable ecological conditions for swarming. The first outbreaks occurred in
Mauritania, Mali, Niger, and Sudan in 2003. The rain allowed swarms to develop and move north
to Morocco and Algeria, threatening croplands.[21][22] Swarms crossed Africa, appearing in Egypt,
Jordan and Israel, the first time in those countries for 50 years.[23][24] The cost of handling the
infestation was put at US$122 million, and the damage to crops at up to $2.5 billion.[25]
The migratory locust (Locusta migratoria), sometimes classified into up to 10 subspecies, swarms
in Africa, Asia, Australia, and New Zealand, but has become rare in Europe.
