FILE: <ent129.6.htm> Comprehensive Account <Navigate to MAIN MENU>
I. ONTOGENY
A. Eggs
1. Size of eggs.
a. not correlated that large female parasitoids
deposit large eggs.
b. egg size is related to the number of
ovarioles and to the egg deposition rate.
(1). proovigenic females with large numbers of
ovarioles and a high deposition rate tend to
produce
small eggs.
(2). e.g., females of all known species of
Trigonalidae (Hymenoptera) lay up to 10,000 eggs at the
rate of
5,000 per day!
2. Chorion
a. the majority of endoparasitoid Hymenoptera
have semi-transparent and unsculptured chorions.
b. among ectoparasitoid Hymenoptera, chorions may
be adorned with tubercles, spines or ridges.
3. Egg Types
a. hymenopteriform
egg
(1). ovoid to spindle-shaped in outline and are
smoothly rounded at both ends.
(2). chorion is either smooth or variously
sculptured.
(3). deposited internally, externally or apart
from the host.
(4). of general occurrence in parasitoid
Hymenoptera, but also found in some families of parasitoid
Diptera.
b. acuminate egg
(1). elongate, tapering to a sharp point at one
or both ends.
(2). chorion is smooth.
(3). found largely among Ichneumonidae and
Braconidae, parasitoids possessing long ovipositors
for reaching hidden
hosts in galls, galleries, wood tunnels, etc.
c. stalked egg
(1). tube-like extensions at one end.
(2). generally found in parasitoid Hymenoptera
and a few Diptera.
(3).
may be deposited within, upon or
apart from the host.
d. encyrtiform egg
(1). dumbbell-shaped.
(2). deposited internally.
(3). one collapsed "bell" and a portion
of the stalk that connects the two, remain protruding from
the
ovipositional puncture.
(4). the projecting structures bear a
longitudinal rib along one side called the aeroscopic
plate that
functions in
larval respiration.
(5). found in many genera of Encyrtidae.
e. pediculate egg
(1). one end penetrates the host integument and
is variously twisted, expanded or knotted to serve
as an anchor
for the externally projecting egg body.
(2). found in Agriotypidae, Ichneumonidae and
Eulophidae.
f. macrotype egg
(1). large, oblong and ventrally flattened.
(2). deposited externally.
(3). found
only in Tachinidae.
g. microtype egg
(1). minute, oval, ventrally flattened.
(2). deposited on foliage apart from hosts and
hatch only upon being eaten by the host.
(3). common in Tachinidae and Trigonalidae.
h. membranous egg
(1). chorion is extremely delicate.
(2). deposited either internally or externally.
(3). found in Tachinidae and Sarcophagidae.
i. acroceriform egg
(1). pear-shaped and darkly pigmented.
(2). the smaller end bears a well-defined
circular cap which is forced off at eclosion.
(3). found in Cyrtidae (Diptera).
4. Polyembryony
Usually only a single
parasitoid is produced per egg in monoembryony. Sometimes the egg develops
polyembryonically.
a. has developed independently in four
hymenopterous families: Braconidae,
Encyrtidae,
Platygasteridae
and Dryinidae.
b. also present in a few species of
Strepsiptera.
c. the number of individuals arising from each
egg is extremely variable, ranging from two to 2,000
as in the
genus Litomastix
(Platygasteridae). The number is
apparently directly proportional
to the size
of the mature host larva.
d. host preference is shown, as, e.g., the
polyembryonic Braconidae and Encyrtidae only parasitize
Lepidoptera;
whereas polyembryonic Platygasteridae parasitize hosts in the dipterous family
Cecidomyiidae.
e. restricted parasitoid genera: only in the Encyrtidae is more than one
genus in a family known to
be
polyembryonic.
f. host stage attacked: all polyembryonic Encyrtidae and
Platygasteridae oviposit in the egg of their
hosts and
complete their development in the mature host larva or pupa. Thus, they are all
either
egg-larval or egg-pupal parasitoids.
g. Sex:
the parasitoid brood from a single host may be all of one sex or mixed.
h. Distinction from Gregariousness
(1). polyembryonic species oviposit in the egg or
very young host larva, with parasitoid maturity
occurring in
the mature host larva or pupa.
(2). exceptionally large numbers of progeny
usually develop in a single host.
(3). simultaneous development and emergence of
the brood.
(4). a portion of the broods consist of one sex
only, and the mixed broods show widely varying
sex ratios.
i. Polyembryonic development results in an
increased reproductive capacity, but does not
necessarily confer
a corresponding increased efficiency as a natural enemy. Polyembryony
may,
instead, be viewed as an effort on the part of the parasitoid to overcome
certain
unfavorable
factors in its environment.
j. Relatively few polyembryonic species have been
known to function effectively as biological
control
agents. However, as in the case of the
navel orangeworm, Amyleois transitella, they
may work in
concert with other parasitoids to produce effective biological control.
B. First-Instar Larvae
The most distinctive and
variable stage in the life cycle of many entomophagous parasitoids and
predators.
a. Planidium-type
larva
(1). Greek word meaning "diminutive
wanderer."
(2). all Eucharitidae and Perilampidae and males
of Aphelinidae; also dipterous Cyrtidae and many
Tachinidae.
(3). spindle-shaped, heavy sclerotized, possess
sensory organs and equipped for locomotion by
means of
thoracic or caudal ambulatory setae;
or by vigorous twisting, jumping or looping
movements.
(4). can survive weeks or more without feeding.
(5). they arise from eggs that are deposited
apart from their hosts.
(6). upon hatching, they search for or otherwise
contact their hosts. They are strongly
attracted
to any
moving object and attach themselves to passing hosts or to nonhost carriers
which
then carry them to their
hosts.
b. Triungulinid-type larva
(1). the counterpart of the planidium larva but
found in Strepsiptera, and coleopterous Meloidae
and
Rhipiphoridae.
(2). similar in all respects to planidium larvae,
with the exception that they possess segmented
legs
for
locomotion.
c. Sacciform-type larva
(1). body in bag-like form, lacking apparent
segmentation and lacking a tracheal system.
(2). develop only internally.
(3). found in certain Dryinidae,
Trichogrammatidae and Mymaridae.
d. Teleaform larva
(1). body segmentation also not apparent.
(2). cephalothorax and abdomen separated by a
deep constriction.
(3).
mandibles are very large.
(4). abdomen sub-spherical and bears a long,
blade-like process posterio-ventrally.
(5). internal larval forms found in Scelionidae.
e. Mymariform larva
(1). spindle-shaped and indistinctly segmented.
(2). head conical.
(3). body segments ringed with long spines.
(4). last abdominal segment greatly elongated and
tail-like.
(5). internal larval forms found in certain
Mymaridae and Trichogrammatidae.
f. Cyclopiform larva
(1). cephalothorax larger than abdomen.
(2). mandibles very large.
(3). abdomen tapers posteriorally and its last
apparent segment is usually forked.
(4). the majority of Platygasteridae; all internal.
g. Eucoiliform larva
(1). distinguished by the paired, fleshy ventral
processes on each thoracic segment.
Also, sharply
tapered,
often tail-like abdomen.
(2). internal; found in certain Cynipidae.
h. Mandibulate larva
(1). distinct segmentation and large, broad, somewhat
flattened, heavily sclerotized heads that are
armed with
large sickle-shaped mandibles.
(2). internal forms; found in many Ichneumonidae,
Braconidae, Serphidae and Diapriidae.
i. Microtype larva
(1). minute in size.
(2). integument delicate.
(3). each thoracic segment bears a series of
heavy spines or hooks.
(4). internal.
(5). hatch from microtype eggs of the
Trigonalidae and many species of Tachinidae.
j. Muscoidiform larva
(1). commonly called "maggots."
(2). found in the suborder Cuyclorrhapha of the
Diptera.
k. Hymenopteriform larva
(1). larvae spindle-shaped to spherical in
outline.
(2). usually 12-13 body segments distinguishable.
(3). integument bare or studded with sensory
setae and cuticular spines.
(4). includes both internal and external forms
and is of general occurrence in the Hymenoptera.
l. Agriotypiform larva
(1). bodies of these larvae bear a transverse row
of long, heavy spines dorsally on each segment.
(2). last abdominal segment bears two, long and
slender, sharply-pointed and heavy sclerotized
spines.
(3). external forms found only in Agriotypidae.
m. Vesiculate larva
(1). similar to hymenopteriform type, except that
the hindgut protrudes posteriorly as an enlarged,
spherical
sac.
(2). internal only.
(3). many Braconidae.
n. Caudate larva
(1). distinctly segmented, usually somewhat
elongate.
(2).
last abdominal segment is modified into
a fleshy, tail-like organ.
(3). internal only.
(4). found only in many Ichneumonidae and
Chalcidoidea.
C. The greatest variation
in larval form occurs in the first instar.
Development thereafter tends to
converge towards the
hymenopteriform larva in parasitoid Hymenoptera and towards the muscoidiform larva in the
cyclorrhaphous
Diptera.
The intermediate and last-instar larvae
of ectoparasitoid Hymenoptera generally do not undergo great
changes in form as they
progress towards maturity.
However, endoparasitoids, and those
species in which the eggs or larvae are deposited apart from their
hosts, usually undergo
conspicuous modifications during their larval development. These changes in
larval form may be completed by the second
instar or the transition may be more subtle, with progressively
more simplified larval
forms interposed between the first and last instar.
The intermediate stages of both dipterous and hymenopterous
parasitoids usually resemble the last instar
in form. The greatest change usually takes place at
the first molt among parasitoid species that possess
the most highly specialized primary larvae,
namely the planidium, cyclopiform, teleaform, agriotypiform
and mandibulate
types. Here, by the second instar the
larvae are either hymenopteriform or are very close
to the same.
D.
Special Larval Types
1. In certain Cynipoidea having eucoiliform
primary larvae, the 2nd instar is called polypodieform. This
unique intermediate stage larva has a
distinctly segmented body, several anterior abdominal segments
of which each bears a pair of
ventrally-directed, fleshy lobes.
2. Another distinctive 2nd instar larva is the histriobdellid type found among
Mymaridae egg parasitoids
that have sacciform primary larvae. This intermediate type has a cylindrical
body that is interrupted
by 6 annular constrictions. The head bears a pair of large, slender
curved mandibles; and both the head and
the last apparent body segment each bear
a pair of fleshy lobes.
D. An interesting phenomenon associated with
the larvae of parasitoid Hymenoptera is the fact that the
hindgut is not excretory in function
until the prepupal molt is about to occur.
Until this time the hindgut
ends blindly and may occupy much of the
body cavity of the larva, serving as both an organ of digestion
and storage. At the time of the prepupal molt, all fecal material accumulated
and stored in the hindgut
during larval feeding is released at one
time, forming what is called the meconium.
II. SEX DETERMINATION AND PARTHENOGENESIS
A. In Hymenoptera, sex determination follows
what is called Dzierzon's Law. Dzierzon was a
Silesian
priest who lived around 1845.
1. males are derived from haploid, unfertilized
eggs; females from diploid, fertilized eggs.
2.
diploidy
is brought about in either of two ways:
a.
as a modification of meiosis in the
ovary.
b. by fertilization of the haploid egg at the
moment of oviposition, which changes the sex of the egg from
male to female.
B. Genetics of Sex Determination
1.
History
a. originally thought to be like Drosophila (e.g., males = X;
females = XX)
b. Petrunkewitsch (1901) believed that gonads
were diploid even though the male body was
haploid.
c. Castle (1903) considered differential egg
maturation.
d. Nachtsheim (1913) proposed differential egg
maturation directed by the presence or absence of
a sperm
nucleus.
e. P. W. Whiting (1933) developed an early
theory of multiple alleles.
f. P. W. Whiting (1943) perfected the multiple allele theory
xa, xb, ..., xi -- any heterozygote
(diploid), xa/xb, xc/xd, etc. is female.
xa, xc, etc. -- any azygote (haploid) or
homozygote, xa/xa, xb/xb, etc. is male.
g. Cunha and Kerr (1957) developed the theory
of a series of male-determining genes in balance
with a series
of female-determining genes. The female-determining (FD) genes would be
additive in their effect, whereas
the male-determining (MD) would
not.
C. In most Hymenoptera, the spermatheca functions as a sex-changing
organ. There are two principal ways
in which this sex-changing process
operates.
1. In Braconidae, Ichneumonidae and aculeate
Hymenoptera (bees and wasps), the process begins when
stimuli from the oviposition site
activate the sperm stored in the spermatheca.
Prior to this necessary stimulation
by host contact, the stored sperm are
inactive (incapable of locomotion).
Once the sperm are activated, each
time an egg passes down the oviduct, it
stimulates several sperm to be emitted, which enter the egg through
the micropyle and fertilization results.
2. In Chalcidoidea, a secondary sex changing
mechanism is present following sperm activation. This is the
control of sperm emission from the
sperm duct of the spermatheca. The
passage of the egg down the oviduct
usually stimulates the emission of but
a single sperm. However, another
stimulation from the oviposition
site may secondarily stimulate a muscular
contraction that closes the aperture of the sperm duct, so the egg remains
unfertilized and male at deposition.
D. Three types of parthenogenetic reproduction
1. Thelytoky
a. obligatorily parthenogenetic.
b. each generation consists almost entirely of
females; males are rare.
c. the progeny of the virgin female are
necessarily uniparental.
2. Deuterotoky
a. both males and females are produced
parthenogenetically.
b. both males and females are uniparental.
c. the same as thelytoky except that there
are more males present in the population.
3. Arrhenotoky
a. the majority of parasitic Hymenoptera are
arrhenotokous.
b. females are derived from fertilized eggs
as a result of the spermatheca operating as a sex-
changing
mechanism.
c. in species exhibiting arrhenotoky, the
females, therefore, are usually biparental
and the males
uniparental.
III. HOST SELECTION
A. Analyses of the manner in which entomophagous
insects find their hosts and the bases for their host
preferences, as with phytophagous
insects, currently are subjects of active entomological inquiry.
B. Host parasitoids in nature attack several
host species, although a few monophagous
species are known.
C. No parasitoid appears to be completely
indiscriminate, however, in its choice of hosts. In nature only a
fraction of the species on which
development is actually possible are attacked by any one species.
D. Definite host preferences are expressed by
various groups of parasitoids.
1. most parasitoids of Scarabaeidae larvae are
in the hymenopteran families Scoliidae and Tiphiidae.
2. egg parasitoids are Trichogrammatidae,
Mymaridae and Scelionidae.
3. parasitoids of gall midges, Cecidomyiidae,
are Platygasteridae.
4. in the laboratory, however, spatial and
temporal barriers which separate parasitoids from their potential
hosts in nature can be removed. Parasitoids can be bred in numbers on
unnatural or factitious hosts. This
is actually practiced in the
mass-rearing of beneficial parasitoids for biological control.
Example: the oriental fruit moth parasitoid, Macrocentrus ancylivorous, can be mass-reared on potato
tuberworm larvae,
although this host/parasitoid relationship never occurs in nature. Similarly, synanthropic
fly parasitoids in the
genus Muscidifurax can be
reared on Drosophila in the
laboratory, which greatly
stunts the adults which
emerge. In nature Drosophila have never been found parasitized by this genus.
5. In the mid-1930's, the steps involved in
host selection were discovered by Laing, Salt and Flanders.
a. Salt: Step I = ecological selection, where the
parasitoid is brought into contact with its host; Step II =
psychological selection, where the
host is accepted once contact is made; Step III = physiological selection,
where the suitability of the host as a
food source is determined.
b. Laing: parasitoids find the environment of the host first, then the host itself.<