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Taxonomic Categories of Predaceous &
Parasitic Arthropods Detailed text and references of
taxonomy are arranged according to key predatory or parasitic groups in
separate Master Text Files and Illustration files as follows: [e.g., <ANTHICID.TXT> = general
text emphasizing behavior of adults; <ANTHI1.ADU> = illustrations of
adults; & <ANTHI1.IMA> = text & illustrations of
immatures] (References stored in
Files <BIOLOGY.A> thru= <BIOLOGY.Z>) See <Taxnames> for texts
& images. Insect Parasitoids 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. 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. In some cases an individual coccinellid
larva was reported to have reached maturity by feeding on single large
specimens of scale insects. 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. Parasitoid
activity as a parameter in population dynamics resembles that of predators
rather than true parasites. 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. 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. Special Terms Hyperparasitism is parasitization of a parasitoid by another parasitoid. 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. Secondary parasitoids are generally
polyphagous and individual species tend to be geographically widely distributed
on continents. Technically,
phytophagous insects are primary plant parasites and their primary
parasitoids are "hyperparasites" of the host plant. Substituting the word
"parasitoid" avoids this difficulty. Autoparasitism is found in several species of Aphelinidae. Females develop as primary parasitoids,
but males are hyperparasitic on female larvae of their own species. Indirect Hyperparasitism is 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. It is the opposite of direct hyperparasitism. 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. Facultative
Hyperparasitoids are hyperparasitoids which may also develop
as primary parasitoids. It is the
opposite of "obligate hyperparasitoid." Superparasitism is parasitization of an individual host by more larvae of a
single parasitoid species than can mature in or upon that host
individual. 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. 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. Many parasitoid species
are thought to exhibit superparasitism in nature, particularly when
ovipositional pressures are great and hosts are scarce. Multiple Parasitism is the
simultaneous parasitization of a host individual by two or more species of
primary parasitoids. The Imago or Adult Parasitoid This
is 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. 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 as follows: 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. Mating
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. 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. 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. Ovigenesis
Female
parasitic Hymenoptera may be classified either as proovigenic
or synovigenic, with regard to the duration of
ovigenesis. 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. 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. Characteristics of Host-feeding Feeding
occurs directly on the blood that exudes from ovipositional wounds. 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. Host-feeding and
oviposition may occur on the same host individual. If the host is badly mutilated, oviposition may not occur. Ovisorption
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). 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. 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. 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. 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.. Exercises
Exercise 3.1--
How are true predators distinguished from parasitoids? Exercise 3.2--
Name and describe the several groups of parasitoids? Exercise 3.3--
Define autoparasitism, hyperparasitism, indirect hyperparasitism,
superparasitism,
facultative hyperparasitism, multiple parasitism. Exercise 3.4--
What are important attributes of an effective adult parasitoid? Exercise 3.5--
Discuss some of the effects of mating on the behavior of parasitoids. Exercise 3.6--
Discuss ovigenesis in parasitic insects. Exercise 3.7--
How is host-feeding important in parasitic insects? Exercise 3.8--
Briefly describe the ovisorption process in parasitoids. REFERENCES:
[Additional references may
be found at MELVYL
Library ] Bellows, T. S.,
Jr. & T. W. Fisher, (eds) 1999. Handbook of Biological Control: Principles and Applications. Academic Press, San Diego, CA. 1046 p. 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. 265. Gordh, G., E. F. Legner & L. E. Caltagirone. 1999.
Biology of parasitic Hymenoptera.
In: T. W. Fisher & T. S. Bellows, Jr.
(eds.), Chapter 15, p. 355-381, Handbook
of Biological Control: Principles and
Applications. Academic Press, San
Diego, CA 1046 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. 242. Legner, E. F. 1989. Wary genes and accretive inheritance in
Hymenoptera. Ann. Entomol. Soc. Amer.
82(3): 245-249. 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. |