Diapause, as described above, allows an insect to track its resources in time — when conditions become inclement, development ceases until diapause breaks. An alternative to shutdown is to track resources in space by directed movement. The term migration was formerly restricted to the to-and-fro major movements of vertebrates, such as wildebeest, salmonid fish, and migratory birds including swallows, shorebirds, and maritime terns. However, there are good reasons to expand this to include organisms that fulfill some or all of the following criteria, in and around specific phases of movement:
- persistent movement away from an original home range;
- relatively straight movement in comparison with station-tending or zig-zagging within a home range;
- undistracted by (unresponsive to) stimuli from home range;
- distinctive pre- and post-movement behaviors;
- reallocation of energy within the body.
All migrations in this wider sense are attempts to provide a homogeneous suitable environment despite temporal fluctuations in a single home range. Criteria such as length of distance traveled, geographical area in which migration occurs, and whether or not the outward-bound individual undertakes a return are unimportant to this definition. Furthermore, thinning out of a population (dispersal) or advance across a similar habitat (range extension) are not migrations. According to this definition, seasonal movements from the upper mountain slopes of the Sierra Nevada to the Central Valley by the convergent ladybird beetle (Hippodamia convergens) is as much a migratory activity as is a transcontinental movement of a monarch butterfly (Danaus plexippus). Pre-migration behaviors in insects include redirecting metabolism to energy storage, cessation of reproduction, and production of wings in polymorphic species in which winged and wingless forms coexist (polyphenism; section 6.8.2). Feeding and reproduction are resumed post-migration. Some responses are under hormonal control, whereas others are environmentally induced. Evidently, pre-migration changes must anticipate the altered environmental conditions that migration has evolved to avoid.
As with induction of diapause (above), principal amongst these cues is change in day length (photoperiod). A strong linkage exists between the several cues for onset and termination of reproductive diapause and induction and cessation of migratory response in studied species, including monarch butterflies and milkweed bugs (Oncopeltus fasciatus). From their extensive range associated with North American host milkweed plants (Asclepiadaceae), individuals of both species migrate south. At least in this migrant generation of monarchs, a magnetic compass complements solar navigation in deriving the bearings towards the overwintering site. Shortening day length induces a reproductive diapause in which flight inhibition is removed and energy is transferred to flight instead of reproduction. The overwintering generation of both species (monarch butterflies at their winter roost are shown in Plate 3.5) is in diapause, which ends with a two- (or more) stage migration from south to north that essentially tracks the sequential development of subtropical to temperate annual milkweeds as far as southern Canada.
The first flight in early spring from the overwintering area is short, with both reproduction and flight effort occur- ring during days of short length, but the next generation extends far northwards in longer days, either as individuals or by consecutive generations. Few if any of the returning individuals are the original outward migrants. In the milkweed bugs there is a circadian rhythm (Box 4.4) with oviposition and migration temporally segregated in the middle of the day, and mating and feeding concentrated at the end of the daylight period. As both milkweed bugs and monarch butterflies have non-migratory multivoltine relatives that remain in the tropics, it seems that the ability to diapause and thus escape in the fall has allowed just these two species to invade summer milkweed stands of the temperate region. In contrast, amongst noctuid moths of the genus Spodoptera (armyworms) a number of species show a diapause-related migration and others a variable pre-reproductive period.
It is a common observation that insects living in “temporary” habitats of limited duration have a higher proportion of flighted species, and within polymorphic taxa, a higher proportion of flighted individuals. In longer-lasting habitats loss of flightedness, either permanently or temporarily, is more common. Thus, amongst European water-striders (Hemiptera: Gerridae) species associated with small ephemeral water bodies are winged and regularly migrate to seek new water bodies; those associated with large lakes tend to wing-lessness and sedentary life histories. Evidently, flightedness relates to the tendency (and ability) to migrate in locusts, as exemplified in Chortoicetes terminifera (the Australian migratory locust) and Locusta migratoria which demonstrate adaptive migration to exploit ephemerally available favorable conditions in arid regions (see section 6.10.5 for L. migratoria behavior).
Although the massed movements described above are very conspicuous, even the “passive dispersal” of small and lightweight insects can fulfill many of the criteria of migration. Thus, even reliance upon wind (or water) currents for movement may involve the insect being capable of any or all of the following:
- changing behavior to embark, such as young scale insects crawling to a leaf apex and adopting a posture there to enhance the chances of extended aerial movement;
- being in appropriate physiological and developmental condition for the journey, as in the flighted stage of otherwise apterous aphids;
- sensing appropriate environmental cues to depart, such as seasonal failure of the host plant of many aphids;
- recognizing environmental cues on arrival, such as odors or colors of a new host plant, and making controlled departure from the current.
Naturally, embarkation on such journeys does not always bring success and there are many strandings of migratory insects in unsuitable habitat, such as ice-fields and in open oceans. Nonetheless, it is clear that some fecund insects that can make use of predictable meteorological conditions can make long journeys in a consistent direction, depart from the air current and establish in a suitable, novel habitat. Aphids are a prime example, but certain thrips and scale insects and other agriculturally damaging pests are capable of locating new host plants by this means.