Movement 3/9/2012
Dispersal
that results in an increase in the mean distance between individuals (Dingle, 1996)
Includes migratory and trivial flight (insects can disperse by either behavior, or by random locomotion)
Trivial Flight
Flight that has no migratory component Usually defined in crop pests as short flights between hosts
Example: soybean aphids moving between plants during the summer
Movements directed by resources and/or Home Range
Station keeping: like central place foraging:
Foraging Commuting (periodic movements) Territorial behavior (patrolling)
Ranging
Movement to explore an area Establish the boundary of a territory
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Life Stages: Remember Tinbergen's Questions
Not all life stages disperse; not all are mobile Only adults have wings; but not all adults have wings
Specialized Dispersal Stages
Eggs:
Inside parasitized host
Larvae:
As "crawlers"
Pupae:
Inside fruit, hosts
Adults
Triggers of Dispersal Behavior
Visual Cues
Color Search image
Olfactory Cues
Pheromones
Photoperiod Crowding
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What causes this transformation?
Why Disperse?
Limited resources
Competition
Between individuals Among family members
Food Mates Shelter
Weather
Season
Disaster
More reasons to disperse
Decline in conditions (reversible or not)
Pesticide application Everything already eaten Patch quality declines (marginal value theorem)
Change in territory Change in resource needs
small larvae eat different food from large larvae
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Natal Dispersal
Movement of a young animal from birthplace before mating Benefits
Avoid inbreeding Avoid competing with parents and siblings
Costs
Encountering the unknown
Disperal Categories
Active
Flying Swimming Walking
Passive
Ballooning Phoresy Rafting
An example of Phoresy
Lice and the Louse Fly Fly (Hippoboscidae)
Ectoparasite of birds Have lice attached to their abdomen
Lice
Only found on live birds
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Louse Fly
Louse
Benefits
Lice die if stranded on a dead bird they can't fly Transmission among live birds
Parent sitting on next to young Between males and females in pairs In nest materials
But, if the bird dies away from the nest, the lice are stranded
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Risks
Fall from the fly Flies are not as species specific in their host range as the lice, so:
What happens if the fly lands on a bird that is not an appropriate host?
Rafting
Unintentional Colonization
Bark beetles near Lake Tahoe
Bark beetles in Sacramento
How can rafting affect colonization?
Benefits
Inexpensive No need for dispersal capability
Risks
End up somewhere inappropriate No control
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Primary Dispersal Agent Commerce
Accidental Dispersal
Plants Seeds Soil Food Fruit Infected people or animals
Primary Dispersal Agent
On purpose
Honeybees Importing exotic butterflies, etc. Importing biological control agents
Dispersal Vs. Migration
What is the difference?
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What makes dispersal migration instead of trivial movement?
Activity of insect is persistent - carries the migrant beyond its original habitat to a new one A directionality to the dispersal and a minimum of turning and backtracking Migration is undistracted; insect ignores local cues
Cues to resources that promote growth and maintenance Cues for finding mates
Migration
Dynamic Migration Schistocerca gregaria The Desert Locust Phase Polymorphism
Homeostatic Danaus Migration plexippus The Monarch Butterfly
Dynamic migration - one-way single migration actively initiated by the insect but dependent for the most part on wind or tides with no navigation or directional control by the individual By far the most common in insects
aphids, leafhoppers, moths, locusts
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Schistocerca gregaria: the desert locust
THE "plague of locusts" of historical significance Still one of the most serious insect pests Found on Arabian Peninsula and Northern Africa Swarms number in the tens of millions of locusts Eat everything green
Desert Locust Biology
Hemimetabolous Feed on green plants as nymphs and adults Populations build quickly during the rainy season High-density results in gregarious forms of the hoppers via Phase Polymorphism
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Phase Polymorphism
First described by Boris Uvarov 1921 Described how S. gregaria was a single species of locust with two forms, not two separate species as was thought previously Many migratory insects exhibit this phenomenon. Most species have more subtle differences between the phases than S. gregaria
Solitary Form
Migratory Form
Morphological and Behavioral Differences Between Phases
Migratory
Brown Long wings Highly active Gregarious: forms into large cohesive groups Ignore local cues that lead to food or mates
Solitary
Green Short wings Fairly sedentary Avoids other individual locusts Respond to local cues that lead to food or mates
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What Stimulates the Phase Change?
When population density is high, solitary phase nymphs form "bands" of gregarious phase nymphs Experimentally manipulated by putting early instar nymphs in boxes with high density populations Also manipulated by putting one nymph into a box with metal projections
Tinbergen's questions
Cause Development Evolution Function
http://www.fao.org/ag/locusts/en/info/info/index.html
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2005 S. gregaria populations in Northern Africa
March 3, 2005
Swarms Bands
Adults
Hoppers
2006 S. gregaria populations in Northern Africa
March 2, 2006
March 2, 2007
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Adults
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Band Formation
Gregarious Hoppers
Desert Locusts can travel for hundreds of miles in a swarm
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Longest Migration for an Insect
Homeostatic migration - two-way migration which may take advantage of wind but is directed by the navigational ability of the insect and includes a return to the point of origin by the same individual or its progeny
Monarch Butterfly
D. plexippus Holometabolous Larvae feed on various milkweed species Common in North America Seasonal migration
Summer in US and Canada Winter in a few specific locations
Mexico California
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Differences between Dynamic and Homeostatic Migration
Dynamic
Not bidirectional Depends on wind, currents, etc.
Homeostatic
Both ways; there and back Insects navigate to a particular location
Evolutionary Basis of Migration
Costs
Energetically expensive Inherently unpredictable
Risk of predation Risk of not finding mates No food Not "making it" to where they are going
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Evolutionary Basis of Migration
Benefits
Escape intolerable environmental conditions In mass migrations, presence of mates assured Reduce competition (?) Increase genetic diversity via out-crossing Exploit distant resources
Radar Image of Heliothis sp. moths migrating near Beijing China
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