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Social Insects

Insect Ecology

Social Insects

•  Sociality evolved multiple times in insects

•  Much of Earth’s fauna consists of social insects

•  They play major roles in entire ecosystems

•  Proliferation of ants and termites associated with change from solitary to social lifestyle

Social Insects

•  Consistent trend: •  Numerous forms of subsocial

in numerous lineages –  Aggregations TO –  Parental care TO –  Parental care & nesting TO –  Communal nesting TO

•  Increasing division of labor and reproduction in fewer lineages TO

•  Primitively eusocial in fewer lineages TO

•  Advanced eusocial fewer lineages

Subsocial •  More widespread

–  13 orders of insects –  no reproductive

division of labor

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Eusocial

•  Three traits –  Some individuals in

colony reproduce, others sacrifice reproduction

–  Cooperation in tending young

–  Overlap of adult generations within colonies

•  Ants, termites, some bees and wasps

Aggregations as subsocial behavior

•  Food exploitation –  bark beetles

•  Roosting

Aggregations as subsocial behavior

•  Aposematism –  monarch butterflies –  ladybird beetles

•  Chemical defenses –  sawflies

•  Parental care

Parental care •  What parental care do

all insects provide? •  Post hatching attention,

provision and protection of food –  Without nesting

•  Tending eggs and young –  Care of nest –  Can involve feeding –  Blattodea, Orthoptera,

Dermaptera, Hemiptera, Coleoptera and Hymenoptera

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Membracid nannies

•  Parental care by ants.

•  Ants obtain honeydew from treehoppers

Nesting as social behavior

•  Eggs laid in structure and parents tend young –  Types of nests

•  New construction •  Pre-existing

–  Types of care •  Vigilence •  Nutrition

Nesting as social behavior

•  Solitary nesting –  No reproductive

division of labor –  Examples:

•  ground nesting crickets, •  earwigs, angel insects •  some beetles, •  Hymenoptera

–  What orders? –  Nesting is not known in

other orders.

Nesting as social behavior

Most nesting subsocial

Hymenoptera are parasitoid wasps.

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Communal nesting

•  Nests shared among individuals

•  Parents clean, provision, defend nests

•  Conflicts among nest mates common

•  Halictinae, Megachilinae, Andreninae

Social aphids & thrips

•  Aphids –  Behavioral and

morphological differences

1.  Soldiers •  All first instar •  molt into normal

feeding individuals •  Reproduction delayed

if molt is delayed 2.  Special caste of

soldiers

Social aphids

Adults

Soldier nymphs Remain at 2nd instar without molting and growing.

Sclerotized cuticle

Sterile

Attack natural enemies & competitors

Perform housekeeping.

Reproductive (normal) nymphs

Molt and grow.

Soft cuticle.

Contribute to reproduction.

Perform neither defense nor house-keeping.

Genetically identical clones by parthenogenesis

Differences likely attributable to

differences in gene expression between

the castes POLYPHENISM

Social aphids

Soldiers injecting venom into predatory lacewing larva

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Social aphids & thrips

•  Thrips –  Behavioral and

morphological differences

–  Dispersers & soldiers in colony in gall

–  Soldiers defend gall against other thrips species and usually reproduce less than dispersers

Social thrips

Dispersers are highly fecund, participate in extensive brood care, and are adept at initiating and growing new galls.

Soldiers have low fecundity and are involved primarily in defense of the gall from kleptoparasites.

They cannot initiate or grow galls.

Quasisociality & Semisociality

•  Communal nest- adults cohabit a single nest, but each care for their own young.

•  Quasisociality- adults cohabit a single nest, and cooperate in brood care.

•  Semisociality –  Division of reproductive labor –  Workers are sisters to queens

•  =caste system •  No morphological difference between queen and workers

–  No overlap of adult generations

Eusociality

•  Multigenerational colonies •  Polyphenism:

morphological differences between queen and workers

•  Polyethism: behavioral differences

•  Caste system –  Reproductives –  Workers –  Soldiers –  Subcastes

•  Found only in Hymenoptera and ALL Isoptera

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Natural selection & Eusociality

•  Why would some individuals NOT be involved in reproduction?

Natural selection & Eusociality

•  W.D. Hamilton gave us the tools to solve one of Darwin’s greatest challenges.

•  The evolution of altruism.

Eusocial Hymenoptera

•  Why are there so many origins of Eusociality in the Hymenoptera?

•  How are ‘r’ and ‘B’ maximized with respect to ‘C’?

Super-relatedness in Hymenoptera

•  What sex-determination system do all Hymenoptera have?

•  So, what is the genetic relatedness between sisters?

•  How could this result in a maximization of ‘r’?

•  What benefits might there be of this extensive social behavior?

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Eusocial Hymenoptera

•  Which is/are the male(s) in the above figure? •  Which is/are the female(s) in the above figure? •  Feeding quality + substance yields polyphenism

in diploids, which are all ____________.

Hymenoptera •  Numerous origins of

eusociality from subsociality.

•  Primitively eusocial –  Females morphologically

similar –  Colonies usually annual

•  Advanced eusocial –  Ants, some wasps, many

bees –  Behavioral and

morphological differentiation

–  Workers different than queens

Primitive eusocial Hymenoptera

•  Numerous transitional lineages, e.g.:

•  Polistine Wasps, a few others –  >1 female forms colony –  Colony lasts 1 yr –  Variable # reproductives

•  Bumblebees, Halictinae –  >1 female forms colony –  ‘winning’ female

reproduces and is aggressive

–  Sexual retardation reversed if queen dies

Bumblebees

•  Colony foundation by one or more females •  Pheromones used to modify worker behavior •  Ovarian development in late season workers •  Queen may be driven from nest •  Workers produce male offspring parthenogenetically

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Advanced eusocial Hymenoptera

•  Many bees and some wasps, ants

•  Females dimorphic •  Specialization of

workers •  Wasps

–  Queen founds first brood of workers

–  Subsequent generations include males, then reproductive females

Eusocial honeybees (Apidae)

•  Colony develops and may found new colonies during summer

•  Nest made of wax secreted by bees •  Castes: queen (larger), worker (smaller), drone •  Old workers: hive; Young workers: field

Extreme eusociality: ants (Formicidae)

•  All ants are eusocial •  Workers may be

polyphenic –  Trophogenic –  Feeding

•  Predatory •  Seed and grain

harvesters

Amblyopone are specialist predators of centipedes

Extreme eusociality: ants (Formicidae)

•  All ants are eusocial •  Workers may be

polyphenic –  Trophogenic –  Feeding

•  Mutualists with plants •  Honey dew specialists

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Extreme eusociality: ants (Formicidae)

•  All ants are eusocial •  Workers may be

polyphenic –  Trophogenic –  Feeding

•  Fungus harvesters

Extreme eusociality: ants (Formicidae)

•  All ants are eusocial •  Workers may be

polyphenic –  Trophogenic –  Feeding

•  Parasites (slave-makers & usurpers)

Polyerges Slave-maker

Formica Slave

A queen Polyergus slave-raiding ant attended to by a Formica worker. Parasitic queens such as Polyergus emit pheromones that are attractive to host ant workers

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Extreme eusociality: ants (Formicidae)

•  All ants are eusocial •  Workers may be

polyphenic –  Trophogenic –  Feeding

•  Predatory •  Seed and grain

harvesters •  Mutualists with plants •  Honey dew specialists •  Fungus harvesters •  Parasites (slave-makers

& usurpers)

Polyerges Slave-maker

Formica Slave

A queen Polyergus slave-raiding ant attended to by a Formica worker. Parasitic queens such as Polyergus emit pheromones that are attractive to host ant workers

Eusocial Isoptera

•  What sex determination system do termites have?

•  Uh-oh, ‘zup? •  Perhaps something

about maximizing ‘B’ when feeding on wood?

•  What else happens to wood-feeding insects in their population structure (remember the Normark paper)?

Eusocial Isoptera

•  In what way does this caste determination structure differ from Hymenoptera?

Termites

•  Lower termites –  Wood-digesting

endosymbionts –  Loose castes, all

workers immature –  Morphology and caste

may change after molting

–  Queen similar to workers

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Termites

•  Higher termites: Termitidae –  No symbiotic

flagellates •  How do they digest

wood? –  Rigid caste system –  Queen differs from

workers: Physogastry

Termite mounds