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I. Gordh &
Beardsley (1999) summarized the importance of various aspects of taxonomy relative to
biological control.
A. They defined taxonomy
as that branch of biology which involves the naming, identifying and
classifying of organisms.
B. Previous emphasis had
been placed on the importance of taxonomy to biological control by other
researchers (Clausen 1942, Sabrosky 1955, Schlinger & Doutt 1964,
Delucchi 1966, Compere 1969, Gordh 1977, 1982).
C. For applied biological
control workers, there is a need for names for the natural enemies and hosts
that are being deployed. Such names
provide an important mechanism for the dissemination of information.
D. In theory taxonomy is
important to biological control researchers because classification are
developed which are intended to reflect evolutionary relationships. Such classifications are helpful because
they are intended to predict details of biology and distribution.
II. History.
A. The urge to arrange,
organize, describe, name and classify is fundamental in human activity. Such an urge operates at all levels of
social organization. In ancient
civilizations names were applied to organisms, and the common names of many
organisms are in widespread usage today.
B. There are however
several problems inherent in common names.
1. Most serious is synonymy. Frequently more than one common name is
applied to a single organism (synonyms), or the same common name is used for
different organisms (homonyms).
Synonymy creates confusion and misunderstanding because the biological
characteristics and habits of similar organisms can differ greatly (Gordh
& Beardsley 1999).
2. In its earliest form, the scientific
name given to an organism was often impractical. Scientific names during the lifetime of John Ray (Wray)
(1628-1705) consisted of a series of Latin adjectives catenated in such a way
as to describe the animal. The system
was less ambiguous than the common name system, but it was cumbersome because
the name of an animal frequently was several lines or a paragraph long.
C. A major contribution
in the naming of organisms was made by the natural historian and physician
Carl Linnaeus (1707-1778), who is credited with developing the current
binomial system of naming organisms.
1. The start of zoological nomenclature
is taken as the 10th Edition of Linnaeus' monumental work, Systema Naturae.
2. The notable exception is the
nomenclature of spiders which originates with the work of Karl Alexander
Clerck (1710-1765), Aranei svecici. The accepted date of publication of these
contributions 1 Jan 1758, and this date is the official starting point of
zoological nomenclature.
D. During the following
century taxonomic zoologists followed the lead of Linnaeus and prepared
descriptions of species for publication but named the animal with a
binomen.
1. The binomen consists of two parts, the
generic name and the specific epithet.
With the accumulation of taxonomic descriptions, problems developed
with synonymy, homonymy, the inconsistent application of bionomens, and
related nomenclatural difficulties.
2. The first attempt to address these
problems was the "Strickland Code," prepared in 1846. This code was developed by a panel of
taxonomists, including Charles Darwin who was a noted taxonomist of barnacles.
3. Subsequently, an International Code of
Zoological Nomenclature was developed in 1906. This code has been altered slightly, but continues to represent
the basic guidelines for the formation and validation of zoological names for
taxa. The most recent revision was
published in 1985 (Gordh & Beardsley 1999). Complicated problems of nomenclature, or matters requiring the
fixation of names in the interest of stability, are referred to the
International Commission of Zoological Nomenclature which serves as a kind of
taxonomic supreme court.
III. Importance of Taxonomy to Biological Control
A. Danks (1988) reviewed
the importance of taxonomy to entomology.
Its importance for biological control was emphasized by Clausen (1942),
and subsequently by Sabrosky (1955), Schlinger & Doutt (1964), Gordh
(1977) and Knutson (1981).
B. The
Scientific
Name.
1. The scientific name of an organism is
of utmost importance (Gordh 1977). It
provides a key to the published literature regarding any zoological taxon and
without the correct name the researcher has no access to knowledge published
about an animal of interest (Gordh & Beardsley 1999).
2. The scientific name is a kind of shorthand
method for conveying an enormous amount of information about an organism
which is available in published literature.
All the information which has been developed about any organism
important in biological control is stored under the scientific name for that
organism. Because of this, the
correctness of the name needs to be emphasized.
C. Accurate Identification.
1. The need for identification is great in biological control,
but the importance of accurate identification is greater (Gordh &
Beardsley 1999). Two species which
are very similar morphologically are not always similar biologically. Subtle differences in morphology or
biology of closely related species can be profound. Distinguishing between variation in taxonomic characters within
a species and difference in character states between species (individual
versus interspecific variation) is frequently difficult. Understanding the functional significance
of the observed anatomical features which serve to distinguish between
species is an area of research which has lagged behind orthodox taxonomic
studies. Apparent slight anatomical
differences may reflect significant differences in the biology of two
organisms.
2. So called minor structural differences
can mean the difference between pest and nonpest status for species which are
potential threats to agriculture, or between establishment and failure to
establish in the case of natural enemies.
Some examples follow:
Pink Bollworm, Pectinophora
gossypiella (Saunders).--The gelechiid genus
Pectinophora contains three
described species: P. scutigera, P.
endema and P. gossypiella. Pectinophora scutigera occurs in Australia,
Papua New Guinea, Micronesia and Hawaii; P.
endema is restricted to eastern
Australia (Common 1958), while P.
gossypiella occurs only in
Western Australia and other world sites.
All species consume the flowers, seeds and seed capsules of
Malvaceae. Pectinophora endema
consumes only native Hibiscus
in Australia, and is not an agricultural pest. The remaining species consume other Malvaceae, including Gossypium spp. (cotton). Pectinophora
gossypiella is one of the
most serious cotton pests, and larvae of this species can diapause within the
seeds of the host plant, which accounts for its widespread distribution. By contrast, P. scutigera
does not diapause within seeds, is limited in distribution and is not
considered a major pest of cotton.
Holdaway
(1926) gave the name of P. scutigera based on larval
differences. Later Holdaway (1929)
described the structural characters of the adult genitalia to separate the
species. The validity of P. scutigera as a species was originally challenged, but is
now accepted (Zimmerman 1979).
The
importance of correct identification of the bollworms focuses on the pest
status of these insects and quarantine enforcement. In Australia P.
scutigera is not a
significant pest of cotton and its distribution is limited by intrinsic
biological characteristics. It does
not play a significant role in quarantine efforts. In contrast, P.
gossypiella is very
pestiferous in cotton. It occurs in
the Northern Territory and Western Australia but not in Queensland. Quarantine serves as an important barrier
restricting movement of this species.
Quarantine is expensive to the state and the commercial
enterprise.
Coffee Mealybug, Planococcus kenyae (LePelly).--This insect of
Kenya presents an interesting example of early failure and delayed success in
biological control caused by misidentification of the pest species. The pest first appeared during the 1930's
and caused serious losses to coffee in Kenya. First it was identified as the common, widespread, citrus
mealybug, Planococcus citri (Risso). Later it was determined as a related
Philippine species, P. lilacinus (Cockerell). Finally both of these identifications were
shown to be incorrect, but unfortunately, on the basis of these names, a
great amount of effort and expense was devoted to searching for and shipping
natural enemies of Planococcus
in the Asiatic tropics. Parasitoids
which appeared promising when collected could not be established in
Kenya. The problem was resolved when
the taxonomist LePelley examined specimens of the pest. He found relatively inconspicuous but
consistent morphological differences which indicated the that coffee mealybug
was an undescribed species, which he then named (LePelley 1935, 1943). It was then found that this mealybug also
occurred in Uganda and Tanzania where it was under natural biological
control. Parasitoids imported into
Kenya from those areas produced complete biological control.
California Red Scale, Aonidiella aurantii (Maskell).--The California red scale gives an excellent
example of the potential costs of incomplete taxonomic and biogeographic
knowledge of a pest species. This
scale is a member of a complex of species native to the tropics and
subtropics of the Old World (Africa through southeast Asia and the Orient)
(McKenzie 1937). It became a pest of
citrus when introduced into the New World without its associated natural
enemies (Compere 1961). Many
parasitoids associated with closely related Aonidiella species would not attack, or were not effective
against A. aurantii. The failure of early attempts at
biological control were due, at least in part, to the inability to
differentiate this species from such closely related species as A. citrina. Some
parasitoids in the Orient appeared promising to entomologists, but these
species failed when introduced into California because their preferred hosts
were other species of Aonidiella. This was apparent after Howard McKenzie
made a careful revision of the genus Aonidiella
and showed that the species could be separated on the basis of microscopic
differences.
IV. Natural Enemy Identification.
A. Gordh & Beardsley
(1999) emphasized that of equal importance to accurate determination of pest
species in biological control is the correct identification of the entomophagous
organisms which are found in association with target pests and which are
being considered for utilization in biological control.
1. Sometimes such natural enemies belong
to groups of small to minute insects, the species of which often resemble one
another. Taxonomic knowledge needed
to differentiate species level taxa in such groups has accumulated slowly and
with great effort. In many groups
knowledge remains incomplete.
2. Some examples of the problems
involving natural enemy taxa important to biological control are as follows:
Among the
Aphelinidae, an important family of entomophagous Chalcidoidea, the genera Aphytis and Marietta appear closely related
on the basis of morphology.
Superficially it is difficult to place some species in the correct
genus. Biologically the differences
between the genera are profound. Aphytis species are primary
parasitoids of armored scale insects while Marietta species are hyperparasitoids, usually associated
with armored scale insects or other Coccoidea. Since hyperparasitoids are viewed as deleterious to biological
control, importation or deliberate movement of Marietta could adversely affect biological control.
The family Encyrtidae,
another large group within the Chalcidoidea, contains a vast array of genera
whose species are primary parasitoids of phytophagous insects. However the same family also contains
genera whose species are mostly secondary parasitoids (e.g., Cheiloneurus, Quaylea). Recognition of these hyperparasitoids and
their elimination requires a taxonomic knowledge of the Encyrtidae. Failure to do so could result in the
introduction and establishment of undesirable species, which is thought to
have occurred in a few cases. A few
genera of encyrtids (e.g., Psyllaephagus)
contain both primary and secondary parasitoid species, which demands careful
biological and taxonomic study to separate the beneficial primary and
undesirable hyperparasitoids prior to releases.
In the case of the
California red scale, not only did difficulty in distinguishing the pest from
related species retard biological control, but this such was also encumbered
by a lack of knowledge about a very important group of armored scale
parasitoids, the genus Aphytis. DeBach et al. (1971) showed that this lack
of knowledge delayed achievement of biological control of California red
scale by 50 years. Early explorations
for natural enemies revealed the presence of Aphytis parasitoids at several localities in the
Orient. Specimens from these
collections were determined as Aphytis
chrysomphali Mercet, a
species already present in California which was not especially
effective. Therefore, no effort was
made to propagate and release new oriental Aphytis until after World War II (Compere 1961). The two most effective natural enemies of
red scale, Aphytis lingnanensis Compere and A. melinus DeBach, were not recognized as distinct species
until 1948 and 1956, respectively.
These species might have been introduced into California many years
earlier had a proper understanding of the taxonomy of Aphytis existed (Gordh & Beardsley 1999). Similarly, Aphytis holoxanthus
DeBach, the most effective parasitoid of Florida red scale, Chrysomphalus aonidum (L.), apparently was first
collected around 1900, but was ignored because it was confused with another
species. Aphytis holoxanthus
was made available for biological control in 1960 when DeBach recognized it
as a distinct species (DeBach et al. 1971).
Trichogramma is a cosmopolitan
genus of tiny parasitoids which occur as more than 120 species. All species for which the biology is known
develop as primary internal parasitoids of eggs. Trichogramma
has been used extensively against lepidopterous pests in classical biological
control or inundative release programs.
Some programs have produced contradictory results, with some workers
claiming success and others admitting failure. Poor taxonomic knowledge has contributed to conflicting
assessments. Early researchers rarely
deposited voucher specimens for their research and without material to
compare it was difficult or in some instances impossible to determine what
species of Trichogramma was
used in a release program. In one
example, most references to Trichogramma
minutum Riley, T. evanescens Westwood and T. semifumatum
(Perkins) made prior to 1980 probably are in error. It is now known that Trichogramma
contains many anatomically similar species which can be distinguished only by
microscopic differences on antennae and genitalia. Traditional reliance on body coloration is if limited utility
and has been shown to depend on environmentally induced variation. Many species display dark coloration at
the base of the forewings, and the name T.
semifumatum was often
applied to such forms. The latter
species is now recognized as endemic to the Hawaiian Islands based on one
collection (Pinto et al. 1978).
V. Biological Control Contributions to Taxonomy.
A. Gordh & Beardsley
(1999) stated that there exists an element of reciprocity between the
biological control worker and taxonomist which must be fully developed to
maximize the usefulness of taxonomy as an adjunct to biological control.
B. Biological control
workers can offer taxonomists important data necessary to complete taxonomic
identifications. The kinds of
important information include zoogeographical, biological, behavioral
ecological and hybridizational data.
1. Zoogeographical Data.--Biological
control researchers frequently engage in time consuming and expensive foreign
exploration. Often the results of
this work are not published and the imported material is not studied. Such material can provide potentially
important data for taxonomic studies in terms of understanding geographical
variation and expanding known limits of distribution.
2. Biological
Data.--Because
it is believed that there are trends toward habitat specialization and host
specificity in many groups of parasitic Hymenoptera, data on host range and host
preference can be obtained in the field and in the insectary. This information can be used by
taxonomists to refine their taxonomic analyses f groups. Also, information on pest species, such as
host plant preferences, can be shared with specialists.
3. Behavioral Data.--Subtle differences
in behavior between populations of what appears to be one species may point
to taxonomic differences between two or more closely related species. Behavioral differences between populations
cannot be easily obtained by the taxonomist who must rely on preserved
specimens, yet they must be made aware of such differences. Once behavioral differences are known, the
taxonomist may find encouragement to search more for minor anatomical
differences which can be used to distinguish between closely related taxa.
The kinds of important
behavioral differences are many. For
example, courtship behavior in Aphytis
appears to be controlled primarily by species specific sex pheromones
released by virgin females. Males are
attracted to the pheromone released by conspecific females. Also, males produce a pheromone which
appears to calm the virgin female and render her sexually receptive. Males and females do not normally respond
to members of the opposite sex belonging to other, even closely related
species (Rosen & DeBach 1979).
Additionally, other kinds of
behavior, such as host finding, may also be indicative of taxonomic
difference between populations which show no readily apparent anatomical
differences.
4.
Ecological Data.--Closely
related species often differ substantially in their ecological
requirements. Important data must be
kept on the ecological associations of entomophagous arthropods collected for
biological control purposes. Factors
such as elevations and season are important, but less apparent ecological
data, such as the type of plant community in which the species occurs, can
also provide valuable clues to the taxonomist who is attempting to
differentiate similar forms. Host
specificity among related species of parasitic Hymenoptera is often reflected
in their association with specific plants which harbor their insect
hosts. Thus, information on the plant
hosts on which parasitoids are collected may prove useful to taxonomists.
5. Hybridization Studies.--Most
classical taxonomists do not have access to insect rearing facilities, and as
a consequence these taxonomists are restricted in their ability to test
reproductive compatibility and to make judgements involving the biological
species concepts. While most museum
taxonomists would acknowledge reproductive compatibility as a viable approach
to the study of species limits, in reality they are limited to conceptual
acknowledgment only. Biological
control researchers with access to laboratory and insectary facilities are
able to provide detailed information regarding reproductive compatibility and
reproductive isolation. This kind of
information is important as is illustrated in such groups as Trichogramma (Pinto et al.
1986).
VI. Sources of Taxonomic Expertise
A. It is often difficult
to find specialists sufficiently expert in the taxonomy of pests and natural
enemies who are willing to provide biological control workers with the
unequivocal identifications required.
This has been especially true for groups of minute parasitoids that
are of major importance.
1. Dwindling public support for natural
history museums and for taxonomic research in general has intensified this
problem since the 1960's.
2. Many biological control specialists
have been required to undertake systematic research in an effort to solve
taxonomic problems associated with their own research. Thus, a considerable amount of basic
research, particularly with entomophagous forms, has been conducted by
scientists whose taxonomic interests originated with their involvement in
applied biological control. An
example is the detailed study of the aphelinid genus Aphytis by Rosen & DeBach (1979). As a result, Aphytis now is recognized as among the best understood
genera of Hymenoptera used in biological control. Similarly biosystematic studies by Dr. E. R. Oatman and
colleagues have elucidated Trichogramma
in the 1980's, and work with Muscidifurax
by E. F. Legner has shown great diversity in a group that was previously
regarded as monotypic.
B. Directories of
taxonomic specialists are published periodically (e.g., Blackwelder &
Blackwelder 1961), and although helpful, they are quickly outdated. An effective method of locating taxonomic
expertise is by consulting the most recent volumes of the Zoological
Record. Word of mouth approach is
very effective also.
Other details of
specimen preparation, techniques, etc. are given in Gordh & Beardsley
(1999).
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