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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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