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
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
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
V. Other Terms
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.
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."
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.
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.
Female parasitic Hymenoptera may be classified either as proovigenic or synovigenic, with regard to the duration
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
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.
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.
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
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
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.