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I. The
test for distinguishing parasitoids from predators is whether they reach maturity by consuming but a single
host
or several host individuals during the course of their development.
A.
Exceptions are found in some Scelionidae that develop individually in
spider egg masses, yet consume
several eggs. They can be called
either egg predators or egg-mass parasitoids.
B. In
some cases an individual coccinellid larva was reported to have reached
maturity by feeding on
single large specimens of scale insects.
II. Insect
parasitoids differ in several ways from true parasites:
A. A
parasitoid usually destroys its host as it develops to maturity.
B. The
host is usually of the same taxonomic class as the parasitoid (Insecta).
C. Insect
parasitoids are large relative to the size of their hosts.
D. The
adults are free-living; only the immature stages are parasitic.
E. There
is no heteroecism = passing through different stages on alternate hosts during
development.
F.
Parasitoid activity as a parameter in population dynamics resembles that
of predators rather than
true
parasites.
III. Groups
of Parasitoids
A.
Internal or endoparasitoid:
develops within the host's body cavity.
B.
External or ectoparasitoid:
feeds while immature from an external position.
C. Solitary: only one individual develops per host.
D.
Gregarious: several parasitoid
progeny of a single species habitually develop in or upon a single
host
individual.
E.
Various combinations of these categories can be used to distinguish,
e.g., solitary endoparasitoids,
or gregarious
ectoparasitoids.
IV.
Because various developmental stages of insects are parasitized, the
parasitoids involved may variously be called:
A. egg
parasitoids, larval or nymphal parasitoids, adult parasitoids, etc., depending
on the host stage attacked.
B.
intermediate categories are used to distinguish, e.g., those cases where
a parasitoid oviposits in a host
larva in which initial development occurs, but the parasitoid continues
to develop within and emerges from
the
host pupa (= a larval-pupal parasitoid).
Other examples are an egg-larval parasitoid, and a larval-adult
parasitoid, etc.
V. Other
Terms
A. Hyperparasitism
1. parasitization
of a parasitoid by another parasitoid.
2.
various degrees are primary, secondary, and tertiary. As an example, if a parasitoid attacks a
phytophagous insect it is called primary; a parasitoid of the primary
would be the secondary. Degrees of
parasitism beyond secondary are uncommon.
3.
secondary parasitoids are generally polyphagous and individual species
tend to be geographically
widely distributed on continents.
4.
technically, phytophagous insects are primary plant parasites and their
primary parasitoids are "hyperparasites"
of
the host plant. Substituting the word
"parasitoid" avoids this difficulty.
B. Autoparasitism
1. found
in several species of Aphelinidae.
2.
females develop as primary parasitoids, but males are hyperparasitic on
female larvae of their own species.
C.
Indirect Hyperparasitism
1. that
type of hyperparasitism in which a parasitoid attacks a host insect upon which
it itself is incapable of
developing
with the purpose of encountering the primary parasitoid which the secondary
host may contain.
2.
opposite of direct hyperparasitism.
3. this
classification depends on whether or not the hyperparasitoid can discriminate
between parasitized and
unparasitized secondary hosts. A
direct hyperparasitoid will recognize parasitized secondary hosts and restrict
its
oviposition to these; whereas, an indirect hyperparasitoid will attack all
secondary hosts it encounters,
whether parasitized or unparasitized.
D.
Facultative Hyperparasitoids
1.
hyperparasitoids which may also develop as primary parasitoids.
2.
opposite of "obligate hyperparasitoid."
E.
Superparasitism
1.
parasitization of an individual host by more larvae of a single
parasitoid species than can mature in or upon
that
host individual.
2. it
results when a parasitoid female or a succession of females of the same
species, lay a super-abundance
of
eggs in or upon a single host individual.
3.
superparasitism results in a waste of progeny through mortality
generated by intraspecific competition,
or
it results in stunted or weakened progeny, also as a result of such
competition.
4. many
parasitoid species are through to exhibit superparasitism in nature,
particularly when ovipositional
pressures are great and hosts are scarce.
F.
Multiple Parasitism
The simultaneous parasitization of a host
individual by two or more species of primary parasitoids.
VI. The
Imago or Adult Parasitoid
A. A
critically important stage in the maintenance of any host-parasitoid
relationship, and of especially
importance to biological control because the female parasitoid finds and
selects the host of her progeny.
B. If an
entomophagous insects is to act usefully as a regulatory factor, the females of
the species will display
certain characteristics of an effective natural enemy.
1.
demonstrate a high searching capacity = ability to find the host a low
host densities.
2.
reasonably host-specific, not polyphagous.
3.
possess a high potential for increase, largely as a result of a high
fecundity and a short period of
development relative to that of its host.
4.
demonstrates the ability to occupy and survive well in all ecological
niches occupied by its host.
5.
relative to biological control practices, some workers feel that a good
natural enemy should also be easily
cultured in the insectary, so that
adequate numbers can be reared to facilitate colonization and
distribution.
However, C. P. Clausen has stated that a truly effective parasitoid
could be established with the release of
a
single mated female.
6. the
female should be able to restrict oviposition to hosts suitable for the
development of her progeny; i.e.,
to
recognize healthy hosts and to avoid ovipositing in already parasitized hosts,
thus avoiding superparasitism
and
multiple parasitism.
VII. Mating
A. A premating
period following adult emergence is generally not characteristic of
parasitoids. If the opposite
sex
is present upon emergence, then mating usually proceeds immediately in most
parasitic Hymenoptera.
There are a few cases of a premating period of a few days to three weeks
duration.
Predators, on the other hand, generally exhibit a premating period (few
days to several months), particularly if
a period of reproductive diapause, hibernation,
or aestivation is interposed between adult emergence and mating.
B. A single
mating is often sufficient to insure that a short-lived female can produce
female offspring
throughout her reproductive life.
Females with sperm in their spermatheca
(sperm-storage organ) will usually resist the further attention of males.
Males, on the other hand, generally are prone to mate repeatedly;
however, females with sperm may not stimulate
mating
behavior in males. Some pteromalid
parasitoids that attack synanthropic Diptera go into a short dispersal
phase
prior to and after mating. Mating
occurs at the site of female eclosion.
C. Mating
may influence the behavior of the female parasitoid. In the Aphelinidae, unmated autoparasitoids
oviposit only in coccid hosts already parasitized by the same or a
closely related species, and thus function
as
hyperparasitoids. Mated females,
however, function both as hyperparasitoids and as primary parasitoids,
ovipositing in coccid hosts whether these are parasitized or not. If at the insertion of the ovipositor a
primary
parasitoid is located, she deposits an unfertilized, haploid male
egg. But if the coccid host is not
parasitized,
she
lays a fertilized, diploid female egg.
In
Pteromalidae, mating may change the rate of oviposition, longevity and
gregarious behavior according to
the
particular male's genetic make-up.
Males are able to change a female's oviposition phenotype upon mating,
by transferring an unknown substance with the
seminal fluid (Legner 1989). This
subject will be treated in greater
detail on
the succeeding section on polygenes.
VIII. Ovigenesis
Female parasitic Hymenoptera may be classified either as proovigenic or synovigenic,
with regard to the duration
of ovigenesis.
A.
Proovigenic females reach the adult stage already having elaborated a
complete or nearly complete complement
of
mature eggs which they usually oviposit in short order if hosts are
available. They develop no further
eggs,
however, once oviposition begins.
Only the store of nutrients carried over from the larva is drawn upon
during
ovigenesis.
All
proovigenic Hymenoptera are endoparasitoids.
This is because their eggs are alithal, or "yolk-free" and
must
be placed in the host's body fluids in order to
obtain nutrients through absorption.
B.
Synovigenic Hymenoptera continue to produce eggs throughout their
oviposition period and include the
greater number of parasitic species.
Feeding by the adult female provides the nutrients necessary for the
continuous elaboration of eggs. Protein
requirements for ovigenesis are satisfied in
nature either by storage during larval development or by feeding as
adults on the blood of their hosts (host-feeding).
The
adults also may feed on honeydew, plant exudates or tender plant tissues to
obtain carbohydrates. Thus,
the
source of food available to parasitoid adults is important to biological
control since it affects parasitoid
distribution and effectiveness.
Host-feeding and the accompanying host mutilation by adult females are
also important to biological control
in that
they constitute forms of predation.
IX. Host-feeding
A.
Feeding occurs directly on the blood that exudes from ovipositional
wounds.
B. When
hosts are found in cells, cocoons or puparia, the parasitoid female may
construct a kind of feeding tube
to
obtain a blood meal. The ovipositor is
inserted into the "hidden" host and a waxy secretion flows around the
ovipositor, which hardens in the form of a tube or
"straw>" Once the
ovipositor is withdrawn, this feeding tube
serves to connect the puncture in the host's body with the outside. The blood rises to the top through capillary
action, internal pressure and possibly by
suction from the parasitoid's mouthparts.
C.
Host-feeding and oviposition may occur on the same host individual. If the host is badly mutilated,
oviposition may not occur.
X. Ovisorption
A. If
there are no sites available to stimulate the deposition of eggs, the ovarian
eggs of a synovigenic female
that has commenced oviposition are absorbed into her blood stream. This phenomenon is called ovisorption
or egg
resorption. The process was apparently
originally described by Weyer (1927) working on ants.
Biological control workers related ovisorption to the effectiveness of
parasitoids in regulating their hosts
(Flanders 1935). Insect
physiologists also noted the phenomenon almost simultaneously in other orders
of
insects (Pfeiffer 1939, Wigglesworth 1936).
B. The
cyclic process of ovigenesis - ovisorption - ovigenesis, permits the retention
of metabolites and this
is
physiologically economical in that it conserves materials used in ovigenesis.
C. While
ovigenesis may require several days, the egg resorptive process may occur in a
few hours. This
egg
degeneration apparently occurs only in the ovarioles, not in the oviduct.
D. The
phenomenon of ovisorption seems to be correlated with a high searching capacity
in parasitic
Hymenoptera. Those species
possessing facultative oviposition generally are the most effective biological
control agents at low host densities.
This effectiveness may result from the conservation of egg-forming
material and the resulting long reproductive life of the female.
E.
Proovigenic parasitoids are generally more effective initially in
reducing host population densities.
This
is
because they have a greater number of eggs stored and ready for deposition and
can thus respond
immediately to high host densities. Synovigenic parasitoids, however, are potentially more effective
at the
lower
host densities because they are able to spend more time in host-searching,
during which time ovisorption
conserves
nutrients..
REFERENCES:
Clausen, C. P. 1940.
Entomophagous Insects,
McGraw-Hill Book Co., Inc. (reprinted by Hafner Publ., Co., Inc., New
York, 1962). 433 p.
DeBach, P. (ed.). 1964.
Biological Control of Insect Pests and Weeds. Reinhold Publ. Co., New York.
844 p.
Hopkins, C. R. & P. E.
King. 1964. Egg resorption in Nasonia vitripennis (Walker)
(Hymenoptera, Pteromalidae). Proc.
Roy. Ent. Soc. London (A) 39:
101-07.
Hopkins, C. R. & P. E.
King. 1966. An electron-microscopical and histochemical study of the oocyte
periphery in Bombus
terrestris during vitellogenesis. J. Cell Sci. 1: 201-16.
King, P. E. & J. G.
Richards. 1968. Oosorption in Nasonia vitripennis
(Hymenoptera: Pteromalidae). J. Zool.
London
154: 495-516.
Legner, E. F. 1989.
Wary genes and accretive inheritance in Hymenoptera. Ann. Ent. Soc. Amer. 82: 245-49.
Telfer, W. E. 1965.
The mechanism and control of yolk formation. Ann. Rev. Ent. 10:
161-84.
Waage, J. & D. Greathead (eds.). 1986.
Insect Parasitoids. 13th Symp.
Roy. Ent. Soc., London. Academic Press,
San Diego. 389 p.