Dispersal & Diapause

1. Dispersal & Diapause Escape in Space & Time

2. 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.

3. 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

4. from Chapman 1971 Oxygen consumption in eggs of a diapausing grasshopper,Melanoplus differentialis (ORTHOPTERA: CAELIFERA).

5. 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

6. 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

7. 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. Adult diapause in the Blue Orchard Bee,Osmia lignaria

8. 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.

9. 3 different host plants! 3 different winged phases! Complex dispersal in the life history of an aphid.

10. ExplosiveRange Expansion of Africanized Honey Bees (NOT “migration” as defined.)

11. Pseudoscorpions on a tropical harlequin beetle Phoresy: dispersal by hitch-hiking

12. 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.

13. Familiar migrating, winter-clustering species. Large milkweed bug,Oncopeltus fasciatus(HEMIPTERA) Convergent lady beetleHippodamia convergens(COLEOPTERA)

14. Old World Migratory Locust,Schistocerca gregaria(ORTHOPTERA) • Gigantic swarms • Long-distance migration • Environmentally modulated • Food- & interaction-stimulated • Generational phase change • Reproductive diapause

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

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

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

18. 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.

19. North American Locust Locusta migratoria • A major environmental factor influencing settlement of the American West for several decades. Once as spectacular as the Old World locust. • Abruptly went extinct. • Hypothesis: discreet sedentary-phase refuges coincided with development of agricultural land by settlers.

20. Monarch (“Wanderer”) Butterfly,Danaus plexippus • Long-distance migration • Mass hibernation • Multi-generational • Reproductive diapause

21. The monarch, Danaus plexippus, a spectacular insect migrant. Monarchs at their winter roost in central Mexico.

23. Is our concept of monarch migration correct? The traditional, dual-population monarch migration paradigm.

24. 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? Pyle’s metapopulation hypothesis.

25. ~ end ~