dispersal & migration

Lepidoptera & Orthoptera: Migration & Dispersal

Introduction

  • Lecture 9 of Insect Biology 11:370:381

  • Presented by theycantalk.com

Dispersal

  • Definition: Movement of individuals from one place to another.

  • Purpose of Dispersal:

    • Access resources over a greater geographic range.

    • Reduce competition.

    • Escape from predation and unfavorable conditions.

Types of Dispersal

  • Active Dispersal:

    • Involves moving from one location to another without external assistance.

    • This method is the normal mode for most insects and increases the probability of finding suitable habitats.

  • Passive Dispersal:

    • Involves assistance from external sources or human-made structures.

    • Examples include utilizing wind currents for movement or anthropogenic factors.

Benefits and Limitations of Passive Dispersal

  • Benefits:

    • Allows insects to move long distances with minimal effort.

    • If occurring during the immature stage, it increases the window of opportunity for dispersal.

  • Limitations:

    • Little control over the destination and survivability of the individuals during passive dispersal.

    • Balance required with high reproductive rates to optimize survival and dispersal efficacy.

Wind Dispersal: Ballooning

  • Adaptations: Insects develop adaptations that allow them to capture wind currents, such as long hairs or silken threads.

  • Behavior:

    • Insects exhibit ballooning behavior by leaving their egg mass, climbing to the end of a branch, and then dropping down on a silk strand.

    • They wait to be blown by the wind until landing on a suitable host.

Passive Dispersal: Travel & Facilitation

  • Trade and tourism can facilitate the spread of insects, particularly invasive species.

  • Implications of Invasive Species:

    • Can pose threats to local ecosystems, biodiversity, and public health.

Adaptation for Locomotion

  • Effective locomotion in insects requires:

    • Efficient locomotory appendages.

    • Adequate energy resources.

    • Specialized sensory systems and neuromuscular systems.

Migration

  • Definition of Migration:

    • Involves the movement from one area to another. Unlike dispersal, migration is seen as a specialized behavior.

    • It encompasses long distances, sometimes thousands of miles.

Characteristics of Migration

  • Migration as an extension of movement to find new habitats.

  • Migration involves:

    • Persistent movement.

    • Straight movement toward a specific destination.

    • Unresponsiveness to most stimuli that typically incite normal behaviors, such as foraging or seeking mates.

    • Observable pre- and post-migratory behaviors, including physiological changes and resource reallocation.

Multigenerational Migration

  • Example: Green darner dragonfly, a migratory species in North America.

  • Life-history consists of both resident and migratory populations, considered an adaptive strategy.

  • Reproductive efforts spread across multiple, widely separated water bodies to lower risks from predation and competition.

Phases of Multigenerational Migration

  1. First Generation:

    • Emerges between February and May, migratory, dies in the North.

  2. Second Generation:

    • Emerges in the North, migrates South, dies after migration.

  3. Third Generation:

    • Offspring of migratory individuals emerge in the South in November, this generation is non-migratory.

Migratory Syndrome

  • Refers to a suite of pre-migratory behaviors that prepare insects for long-distance travel.

  • Often occurs before the exhaustion of resources in the environment, indicating an anticipatory component.

  • There is a trade-off between migration and reproduction; where internal resource allocations shift from reproduction to fuel and muscle protein, decreasing adult reproductive capability.

Monarch Butterfly Migration

  • Known as one of the world’s most famous species due to its remarkable migration.

  • Annual migration involves 1 to 3 billion individuals over multiple generations from Southern Canada to overwintering sites in Mexico and California.

Triggers for Migration

  • Triggered by external factors such as photoperiod and temperature rather than population density.

  • This migration allows avoidance of lethal winter conditions while also tracing the distribution of host plants like milkweed for caterpillars.

Behavior During Migration

  • Monarch butterflies cluster together to create microclimates, remaining largely inactive during the winter months (reproductive diapause).

  • Females delay egg production until after migration, laying eggs on milkweed plants during their return journey.

Navigational Mechanisms

  • Monarchs can locate ancestral sites through:

    • Genetic memory which provides navigational cues from previous generations.

    • A sun-based compass system to orient themselves using circadian rhythms relative to the position of the sun.

  • Also likely utilize geographical features and landmarks, as well as chemical markers left on plants by prior generations.

  • This area is a current research focus aimed at understanding complex navigation further.

Conservation Status

  • Monarchs serve as a symbol of conservation in America, classified as a keystone species.

  • Evidence shows overall population numbers declining due to various threats impacting migratory success, rather than sheer population size.

Environmental Constraints on Migration

  • Habitat Loss:

    • Destruction of overwintering sites and reduction of host plants along migratory routes limits resilience against environmental changes.

    • The use of insecticides and herbicides has a negative impact on survival and migration success.

Climate Change

  • Significant in assessing migratory success and understanding its impacts on migratory insects is critical for conservation and pest management purposes.

  • Large-scale movements of certain insects can lead to serious agricultural damage.

Example: Migratory Locust

  • Characteristics:

    • Generalist herbivores, polyphagous, preference for crops like cereals and grasses.

    • Considered significant pests in agriculture due to their destructive potential.

Developmental Changes in Locusts

  • Overcrowding conditions cause developmental changes, influencing juvenile locusts to develop migratory phenotypes.

  • Phenotypic Plasticity:

    • Refers to the ability of locusts to alter behavior, morphology, and physiology in response to environmental stressors.

Locust Swarming Behavior

  • Swarming is linked with the end of the dry season and fast vegetation growth, leading to increased grasshopper populations.

  • Overcrowded conditions inspire a transition into gregarious (swarming) phases, resulting in egg deposition in tremendous quantities during swarm outbreaks.

Impact of Climate on Swarms

  • Large locust swarms can travel hundreds of miles and their behaviors are significantly impacted by climate changes and rainfall patterns, which can lead to recurring outbreaks even in previously dry areas.

  • Management challenges arise as responses must be immediate to mitigate damage.

Pest Control Challenges

  • Pesticide Application Issues:

    • The use of pesticides to control locust populations can render insects unsuitable for human consumption through bioaccumulation.

    • Spraying during locust outbreaks complicates food availability as edible plants are simultaneously consumed by the locusts.

Conclusion

  • Comprehensive understanding of these topics is fundamental for maintaining ecological balance and for effective pest management and conservation efforts.

Questions?

  • Open to discussions regarding any points covered in the lecture.