Dispersal & Diapause

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Dispersal Diapause
Escape in Space Time
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• Main Points
• Insects can escape unfavorable conditions /or
  await favorable ones, even over thousands
  of kilometers and over many months.
• Diapause is a genetically programmed state of
  arrested development found in many
  insects.
• Control of diapause is usually via hormones and
  the environment.
• Dispersal is any physical relocation under some
  control of the insect.
• Dispersal may be controlled by hormones,
  environment, other factors.
• Migration is a special type of dispersal
  involving long-distance movement and
  cyclic return, often of a subsequent generation.

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Diapause

• Major Features
• Arrested growth development
• Temporary
• Endocrine controlled
• Indirectly influenced by environment (set,
  triggered, turned off)
• Genetically programmed
• Can be partial, i.e. only one system (usu.
  reproductive) shuts down
• Other considerations
• JH usually involved, ecdysone usually suppressed
  
• May be obligatory (many univoltine insects) or
  facultative (multivoltine spp.)
• Occurs in various developmental stages,
  depending on species
• Influencing factors temperature, moisture,
  environmental chemistry, food
• NOT
• quiescence or torper, direct response to
  environmental conditions
• hibernation, prolonged winter quiescence
• aestivation, prolonged summer quiescence

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from Chapman 1971
Oxygen consumption in eggs of a diapausing
grasshopper,Melanoplus differentialis
(ORTHOPTERA CAELIFERA).
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Environmental influences on diapause
Influence of photoperiod on egg diapause in two
moth speices.
Influence of food quality and day length on
diapause behavior in the Colorado potato beetle.
The diapause cue may be experienced by the
previous generation, so the mother insect may be
cued to lay eggs that will diapause or not.
from Chapman 1971
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Important General Characteristics of Diapause

• Controlling hormones basically the same as the
  most important molting hormones JH, PTTH,
  ecdysone plus diapause hormone.
• The insect is sensitive to diapause induction
  during a brief period during a particular
  stage of development, sometimes in the
  previous generation.
• The environmental cue is other than the
  disagreeable condition from which diapause
  will allow escape.
• Some conditions from which insects, as an
  adaptive response, may escape via
  diapause low/high temperatures, dehydration,
  dearth of food, parasites/predators.
• Some environmental cues switching on diapause
  day length, food quality, temperature,
  moisture, pH, chemicals (esp. in water).
• Diapause may occur in any developmental stage,
  depending on species egg, larva,
  pupa, adult. most common

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Adult diapause in the Blue Orchard Bee,Osmia
lignaria
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Dispersal

• Physical relocation under some control of the
  insect.
• Along with reproduction, the main function of
  the insect imago.

(meters to many miles)
Major phases of behavior in dispersal of an aphid.
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Complex dispersal in the life history of an aphid.
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ExplosiveRange Expansion of Africanized Honey
Bees
(NOT migration as defined.)
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Phoresy dispersal by hitch-hiking
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Migration
A special type of dispersal involving
long-distance movement and cyclic return

• NOT trivial movement, e.g.
• local change of food patch plant feeding
  bug moving to next bush,
• range expansion, e.g. Africanized Honey
  Bee,
• Aquatic insect drift in lotic environments.

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Familiar migrating, winter-clustering species.
Large milkweed bug,Oncopeltus fasciatus(HEMIPTER
A)
Convergent lady beetleHippodamia
convergens(COLEOPTERA)
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Old World Migratory Locust,Schistocerca
gregaria(ORTHOPTERA)

• Gigantic swarms
• Long-distance migration
• Environmentally modulated
• Food- interaction-stimulated
• Generational phase change
• Reproductive diapause

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Kentromorphism polymorphism with generational
change from a sedentary to a migratory phase.
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Some migratory locust swarms may contain hundreds
of billions of insects, weighing thousands of
tons.
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Main seasonal, wind rainfall-dependent circuits
of the migratory locust, Schistocerca gregaria.
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Basic biology of Old World locust migration.
Breeding grounds are associated with convergence
zones that generate predictable cyclical air
movement

• Sedentary phase for several generations.
• Last generation crowded, female responds to
  abdominal contact by stress reaction on CA,
  reducing JA production.
• Eggs develop into strong-flying migratory
  generation.
• Mature migrants, mutually stimulated, lift off en
  masse with wind.
• Fly for several hours, maintain swarm by visual
  contact edge control.
• Drop to feed, keep flying.
• Finally drop for final feed, production of
  sedentary generation.
• Cycle continues with return migration.

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North American Locust Locusta migratoria

• Once as spectacular as the Old World locust.

• A major environmental factor influencing
  settlement of the American West for
  several decades.

• Abruptly went extinct.

• Hypothesis discreet sedentary-phase
  refuges coincided with development of
  agricultural land by settlers.

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Monarch (Wanderer) Butterfly,Danaus plexippus

• Long-distance migration
• Mass hibernation
• Multi-generational
• Reproductive diapause

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The monarch, Danaus plexippus, a spectacular
insect migrant.
Monarchs at their winter roost in central Mexico.
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Is our concept of monarch migration correct?
The traditional, dual-population monarch
migration paradigm.
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• Old hypothesis does not explain
• Rapid re-esablishment of California colonies
  following natural decimation,
• Migrants heading S and SE toward Mexico from
  western states.
• Can population genetics solve the problem?

Pyles metapopulation hypothesis.
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