Dispersal & Diapause

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.

DiapauseMajor 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

Oxygen consumption in eggs of a diapausing grasshopper,Melanoplus differentialis (ORTHOPTERA: CAELIFERA).

from Chapman 1971

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.

from Chapman 1971

Environmental in

fluences on diapause

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.

• 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

Important General Characteristics of Diapause

Adult diapause in the Blue Orchard Bee,Osmia lignaria

• Adults active for 6-8 weeks in spring.• Females build nests, lay eggs.• Larvae develop through summer.• Late summer: pupation, maturation in cocoon as adult.• Diapuase for 6 months as adults ready to emerge next spring.

Major phases of behavior in dispersal of an aphid.

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

(meters to many miles)

Complex dispersal in the life history of an aphid.

3 different host plants!

3 different winged phases!

ExplosiveRange Expansion of Africanized Honey Bees

(NOT “migration” as defined.)

Phoresy:dispersal by hitch-hiking

Pseudoscorpions on a tropical harlequin beetle

Migration

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.

A special type of dispersal involving long-distance movement and cyclic return

Familiar migrating, winter-clustering species.

Large milkweed bug,Oncopeltus fasciatus(HEMIPTERA)

Convergent lady beetleHippodamia convergens(COLEOPTERA)

Old World Migratory Locust,Schistocerca gregaria

(ORTHOPTERA)

• Gigantic swarms

• Long-distance migration

• Environmentally modulated

• Food- & interaction-stimulated

• Generational phase change

• Reproductive diapause

Kentromorphism: polymorphism with generational change from a sedentary to a migratory phase.

Some migratory locust swarms may contain hundreds of billions of insects, weighing thousands of tons.

Main seasonal, wind & rainfall-dependent circuits of the migratory locust, Schistocerca gregaria.

1) Sedentary phase for several generations.2) Last generation crowded, female responds to abdominal contact

by stress reaction on CA, reducing JA production.3) Eggs develop into strong-flying migratory generation.4) Mature migrants, mutually stimulated, lift off en masse with

wind.5) Fly for several hours, maintain swarm by visual contact edge

control.6) Drop to feed, keep flying.7) Finally drop for final feed, production of sedentary generation.8) Cycle continues with return migration.

Breeding grounds are associated with “convergence zones” that generate predictable cyclical air movement

Basic biology of Old World locust migration.

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.

Monarch (“Wanderer”) Butterfly,Danaus plexippus

• Long-distance migration

• Mass hibernation

• Multi-generational

• Reproductive diapause

The monarch, Danaus plexippus, a spectacular insect migrant.

Monarchs at their winter roost in central Mexico.

The traditional, dual-population monarch migration paradigm.

Is our concept of monarch migration correct?

Pyle’s metapopulation hypothesis.

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?

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