Work Group on Bemisia tabaci
Newsletter No. 13. Summer 2000
All available newsletters > | 4
| 5 | 6
| 7 | 8
| 9 | 10
| 11 | 12
| 13 |
Index >
Introduction
by D. Gerling (Tel Aviv University, Israel)
The idea of establishing a newsletter to serve those interested in Bemisia
research took shape during the 1984 symposium on Bemisia at the
International Congress of Entomology in Hamburg, Germany. At that time,
the pest was spreading and all agreed on the dearth of knowledge regarding
the people working in the field, the limited availability of relevant
literature as well as the extent to which the pest was causing damage.
The appearance of the first newsletter shortly thereafter contributed
to forming a loosely knit workgroup of those who were interested in various
aspects of the topic.
Since then, our knowledge of the Bemisia complex and its agricultural,
physiological and evolutionary ramifications has increased greatly [from
fewer than 800 publications produced between 1889 and 1986 (when Bemisia
tabaci was described by Gennadius) to ca. 3500 produced since]. Bemisia
has spread to most of the known tropical, subtropical and temperate agricultural
regions, becoming a pest of both greenhouse and outdoor crops. New virulent
types of species and strains have been discovered, and it is currently
the focus of worldwide, state-of-the-art activity concerning the determination
of its taxonomic status.
The means of disseminating knowledge have also changed, and information
is available on numerous topics through e-mail and Internet connections.
Thus the functions a newsletter such as ours have also changed and we
believe its main present importance lies in attempting to provide those
interested in Bemisia with clear and concise summaries of what
is known and what is being done in Bemisia research.
In the present issue, we decided to concentrate upon the taxonomic status
of Bemisia, its potential as a viral vector and the state of knowledge
regarding some of its natural enemies. We believe that clear and concise
information on these topics, which stand at the forefront of Bemisia
research, need to be shared by all those in the field, who will then be
able to utilize this information in their own work.
Bemisia tabaci - How Many Biotypes Are There?
by G.K. Banks (School of Biological Science, University of Wales)
and P.G. Markham (Department of Virus Research, John Innes Centre, UK)
Since the mid-1980's when B. tabaci emerged as a major pest in
the USA, causing agricultural losses of millions of dollars (Perring et
al., 1993), there has been an urgent need to resolve the taxonomy and
to understand more about the biology of B. tabaci worldwide. Biological
differences amongst B. tabaci populations were already described
based on virus transmission, host plant preference and fecundity (Costa
and Brown 1991; Brown and Bird 1992; Bedford et al., 1992; 1994). One
of the initial biochemical studies on samples of B. tabaci representing
a worldwide geographic range involved isoenzyme analysis, and 12 distinct
non-specific esterase profiles were obtained (Brown et al., 1995). Currently
there are twenty distinct esterase patterns published for different populations
of B. tabaci over the world (Brown et al., 1995; Rosell et al.,
1997 and Banks et al., 1999).
Until recently the naming of biotypes has been based on these esterase
profiles, and their unique patterns have been the main criteria for the
differentiation of and reference to populations of B. tabaci. However,
molecular analyses have now provided data that allow the genetic relationships
among biotypes to be studied and compared. Dendrograms produced from genetic
similarities based on randomly amplified polymorphic DNA (RAPD)-PCR and
amplified fragment length polymorphism (AFLP) analyses (Guirao et al.,
1997; Cervera et al. in preparation) along with molecular phylogenies,
constructed from sequences of mitochondrial and nuclear genes (Frohlich
et al.,1995; 1999; De Barro et al., 2000; Banks et al., in preparation)
all show remarkable similarity in their delineation of relatedness of
different populations. These studies, however, group together some populations
that have unique esterase patterns and this raises the question of what
constitutes a biotype for nomenclature and diagnostic purposes and what
really defines a biotype in genetic and biological terms. If identification
is required for quarantine purposes then this information can be best
obtained by methods such as esterase analysis or RAPD PCR, although these
techniques are not foolproof, especially in highly polymorphic populations,
e.g. within populations associated with cassava in Africa (Legg et al.,
1994). Phylogenetic studies have shown that there are there are distinct
groups of related whiteflies that can be separated using molecular markers
and that there are genetically similar populations present over a wide
geographic range. The number of clades varies according to the size, composition,
and geographic origin of the data set; therefore, there cannot be a fixed
number of groups using this method. The most comprehensive studies to
date show that there may be as few as a five to seven clades worldwide.
Molecular phylogenetics supports the evidence that a population of silverleafing
whitefly, formerly called the B biotype, has been transported round the
world. It also shows that there is genetic diversity within a B biotype
clade in contrast to what was first assumed (De Barro et al., 2000; Banks
et al., in preparation).
The classification of B. tabaci is still unresolved and a subject
of scientific debate, and the naming of the B biotype or silverleafing
whitefly as a new species, B. argentifolii is still contentious.
The combined evidence from detailed morphological studies (Rosell et al.,
1997), molecular phylogenies (Frohlich et al., 1995; 1999; De Barro et
al., 2000; Banks et al., in preparation) and interbreeding experiments
(Rondo et al., 1999 in preparation) strongly tends to support a species
complex theory and not the present systematics of B. argentifolii
as a discrete species from other populations of B. tabaci. No doubt
the debate will continue until enough data is collated to maintain or
reverse the present classification.
Bemisia tabaci - What’s in a Name?
by P. De Barro, P.J. Driver, F. Schmidt, S. Naumann, I. McKenzie,
J. Curran (Whitefly Research, CSIRO Entomology, AU)
The question over the nomenclature of Bemisia tabaci came to the
fore with the renaming of the B biotype to Bemisia argentifolii.
There is no doubt that B. tabaci is a highly variable species.
There are numerous papers that describe differences across a wide range
of biological and genetic parameters. Together these differences have
been used to characterise numerous biotypes and were the basis for the
renaming of the B biotype. However, many of the differences, especially
the morphological and biological ones, exhibit remarkable plasticity.
One of the key supports for the argument was the sexually incompatible
between the A and B biotypes. Unfortunately, this alone though is not
enough. Sexual incompatibility can be due to factors such as cytoplasmic
incompatibility and behavioural isolation, and while they may be driving
forces that lead to speciation they are not in themselves proof of species
differences. Further, sexual incompatibility is not unique to the B/A
interaction, as it is known to occur between several other biotypes.
Two recent papers, Frohlich et al. (1999) and De Barro et al. (2000)
shed further light on the problem. By comparing a number of nuclear and
mitochondrial gene regions from individuals taken from a large number
of geographic areas the authors have been able to examine the phylogenetic
structure of the B. tabaci complex. The results show that there
is a strong geographic structure to phylogenetic trees generated. This
structure raises the following issue. The sister clade to the B biotype
are the non-silverleafing populations from Egypt, Spain, Sudan and Nigeria.
The next closest relative is the cluster of populations from the Americas
which includes the A biotype. If the A biotype is B. tabaci and
the B biotype a different species then we have to rename the Egypt, Spain,
Sudan and Nigeria population. This then raises the issue of the correct
taxonomic designations for the remaining four clades from Asia, Australia
and West Africa. Taken to its logical conclusion, the monophyly of B.
tabaci becomes open to serious doubt..
Perhaps the best way to view B. tabaci is as a complex belonging
to the one species with distinct geographically based populations that
exhibit variation across a number of traits. There is certainly insufficient
data to support the raising of any biotype to new species status and therefore
the use of B. argentifolii should be discontinued.
An Overview of the European Whitefly-Transmitted Virus
Problems
by I. Bedford (John Innes Centre, UK)
Presently, over a thousand different whitefly species have been identified.
Most of these exist in the tropical and subtropical regions of the world,
although some are common throughout the more temperate regions and can
survive the cold winters of northern Europe.
Over the recent years a few whitefly species have however, become serious
pests of certain crops within important agricultural regions and many
of these pest species are now present within the European boundaries.
Aleurothrixus floccossus, the citrus whitefly, for example is currently
a great problem to citrus growers in southern Europe where large infestations
distort leaves, soil fruits and reduce yields, and spiralling whiteflies,
such as Aleurodicus dispersus and Lecanoideus floccissimus
are now causing phenomenal damage to tropical fruit and ornamentals in
the Canary Islands, where crops such as banana are having their leaves
sucked dry by massive infestations of these pests.
Despite the severe damage these whiteflies are inflicting, by direct
feeding, they do not compare with the problems caused by the few whitefly
species that can also acquire and transmit plant viruses. At present,
only three whitefly species are known to vector plant viruses. One of
these, the banded whitefly, Trialeurodes abutilonea, is only found
in North America, yet the other two, the glasshouse whitefly, T. vaporariorum
and the tobacco whitefly, Bemisia tabaci are currently found on
every continent and are both well established within mainland Europe.
As vectors of plant viruses, the Trialeurodes species only appear
capable of transmitting a few viruses within the closterovirus group,
whereas B. tabaci can acquire and transmit over 60 different viruses,
which include Clostero-, Luteo-, Poty-, Carla- and Nepoviruses along with
a large number of geminiviruses. The vast majority of these viruses are
found within the warmer regions of the world, particularly Asia, Africa
and South America, and are often associated with indigenous weed species.
A long- established co-existence between the virus and the host plant
has usually resulted in dramatic symptoms, often a vivid yellow-veining
which appears to have little effect on plant development, enabling them
to grow, flower and set seed. However, as agriculture has spread into
new areas, some of these viruses have also infected important crop plants,
such as tomato and cucurbits, often with decimating effects.
The potential for these viruses and vectors to spread into new locations
and infecting new hosts, has also become greater as temperate regions
of the world become hotter and the worldwide trade in ornamental plants
and exotic produce continues to expand. In fact, the past decade has already
seen an alarming increase in whitefly-transmitted virus problems within
Europe.
Initially, a closterovirus similar to beet pseudo yellows (BPYV), and
transmitted only by T. vaporariorum to cucurbit crops, was the
only whitefly virus problem within areas of southern Europe but, in the
mid 1980's. However, as B. tabaci became more of an agricultural
pest, a geminivirus it acquired and transmitted, began infecting tomato
crops in Italy and Sicily. This was named Tomato yellow leaf curl virus
- 'Sardinian isolate' (TYLCV-Sar), and identified by a distinctive inter-veinal
yellowing and often a severe leaf curling. It also caused stunting of
infected plants and a dramatic loss in yield.
During the early 1990's, TYLCV-Sar spread into most of the major field
and protected tomato crops along the south and south eastern regions of
Spain including the intensive horticultural region around Almeria. The
scale of the resulting epidemic and associated yield losses generated
considerable concern for the future of an industry that supplies over
750,000 tonnes of tomatoes to northern Europe each year; It also triggered
many related research projects.
Concern that this virus could ultimately spread into the glasshouse tomato
crops of northern Europe and even threaten the UK’s tomato industry,
led to the establishment of quarantine legislation, plant health passports
and a notifiable pest status for B. tabaci. There is also legislation
to prevent non-European B. tabaci from entering Europe, which is
aimed at reducing the risk of introducing new viruses to the continent.
The original source of TYLCV-Sar has never been established, but it does
appear to be unique to the Mediterranean basin. However, UK and Spanish
whitefly scientists, who formed a small collaborative network in 1996,
undertook a number of epidemiological studies on this virus. One of the
studies identified the very common European weed, black nightshade, Solanum
nigrum, as an excellent reservoir for both the virus and the whitefly
vector. It is possible that TYLCV-Sar first spread from this weed into
tomato for the first time in the 1980's, but it almost certainly now plays
an important role in the continuation and spread of the virus, particularly
where it grows within field crops.
As B. tabaci - related problems have continued, growers have understandably
increased the use of insecticides. This has not always solved the problems,
and many have dramatically escalated, particularly where viruses are concerned.
However, Trialeurodes problems appear to have lessened, where B.
tabaci have increased probably due to their inability to compete,
particularly in the hotter months of the year. This has had a dramatic
effect on the Trialeurodes-transmitted cucurbit virus, which seems
to have been totally displaced by a similar closterovirus that is transmitted
by B. tabaci. - Cucurbit yellow stunting disorder virus
(CYSDV). This new virus presently infects almost every plant within every
cucurbit crop within the Iberian peninsula, producing symptoms that begin
as a leaf mosaic then develop into a full yellowing as the leaf matures.
In 1995, TYLCV appeared for the first time in Portugal, on tomato plants
within the Algarve. The outbreak was very severe and had affected very
young tomato plants. Since the symptoms of whitefly-transmitted viruses
appear about two weeks after inoculation, it seemed probable that the
Portuguese tomatoes were first infected soon after germination and maybe
within a nursery. Molecular studies using DNA probes subsequently found
this virus to be different to the one that was already in Spain, although
it was identical to a tomato virus that had been endemic in the Middle
East since the 1960's. This had been named TYLCV - Is (the Israeli strain).
Although TYLCV-Is is transmitted by B. tabaci and produces similar
symptoms to those of TYLCV-Sar, it has a different alternative host range.
TYLCV-Is does not infect S. nigrum, yet readily infects other common
weeds such as, Datura stramonium, and various Nicotiana
species. It has also been found to infect an exported ornamental plant,
Lisianthus, offering a possible explanation for the source of the
first outbreak, probably within a nursery where infected ornamental plants
and tomato seedlings were being grown in close proximity.
Whatever the cause, this new virus, spread through tomato crops in the
Algarve very rapidly, and by 1997, had reached the tomato crops within
southern Spain, co-infecting plants with the Sardinian strain. Due to
the severity of symptoms, many tomato growers have been forced to replant
greenhouse crops up to 3 times during the summer months and, around the
Malaga region, growers have recently had to abandon the growing of outdoor
tomato crops.
On top of these increased problems to the tomato industry, the appearance
of the Israeli virus has brought additional fears to growers within southern
Spain as this TYLCV severely infects green beans (Phaseolus vulgaris)
and more recently sweet pepper plants.
The speed at which this new and decimating virus has become established
within southern Europe, where its insect vector and susceptible hosts
are present, has strongly underlined the importance of monitoring plant
movements and checking for pests and diseases on all imported plant material
in the future. It has also emphasized the necessity to uncover and risk
assess any other whitefly-transmitted viruses that may already exist in
Europe, as demonstrated by the recent discovery of a geminivirus infecting
Ipomea Indica near Malaga. However, laboratory experiments have subsequently
found that it could not be transmitted to other plant or crop species.
As a direct result of the UK-Spanish collaborative successes and the
urgent need to address the continued escalation and spread of whitefly
problems in southern Europe, the European Whitefly Studies Network (EWSN),
was established. Funded by the EC under the FAIR 6 programme (CT98 4303)
this Concerted Action initially involved 27 scientists within 13 different
European countries. However, with the support of Novartis AG and Koppert
Biological Systems (who are both actively involved in EWSN), a further
18 European scientists are able to participate in the network.
EWSN began with the following three objectives:
- To establish and formalise links between whitefly researchers in Europe.
- To collate information on European whitefly related problems and current
research.
- To improve the exchange of information between researcher
An initial workshop was held at the John Innes Centre, Norwich, UK (May
3rd - 7th 1999), that enabled delegates to present and discuss all current
whitefly research topics within Europe as well as review the present agricultural
problems. Five separate discipline groups were also established, covering
virology, epidemiology, systematics, natural enemies and plant protection.
Three specialised working groups were then planned for 2000, to enable
participants within any of the discipline groups to review and standardise
associated methodologies and techniques.
The first of these working group meetings was held at Stuttgart University,
Germany, in March 2000 which has enabled 14 molecular virologists within
EWSN to standardise reliable and reproducible protocols for identifying
and characterising whitefly-transmitted viruses in Europe.
The second meeting is being held at the John Innes Centre, from 17th
- 20th May 2000, where delegates are going to review and determine the
most effective methods, for identifying and characterising whitefly species,
biotypes and natural enemies. This will cover taxonomic, biochemical and
molecular techniques.
During December 7th - 9th 2000, the third working group meeting will
be held in Spain at the new Koppert facility in Aguilas. This meeting
will enable EWSN members involved in whitefly pest management and IPM,
the opportunity to examine and compare successes and failures of chemical,
biological and physical control systems throughout Europe. It is hoped
that this meeting will also involve representatives from countries outside
of Europe that have been managing whitefly problems successfully for many
years.
At the end of February 2001, the EC funding for EWSN finishes, and the
project will conclude with a workshop to review the achievements that
have been made over the previous two years. This will then be followed
by an international whitefly symposium that is being organised by EWSN
at Ragusa, Sicily within the University’s, faculty of Agriculture.
EWSN has, during its first year grown from strength to strength and demonstrated
beyond doubt its important role within European crop protection. It is
hoped that after the EC funding finishes, EWSN will continue to operate
for as long as possible, although this will obviously depend on securing
the necessary financial support which will be sought over the next few
months.
EWSN presently has over 60 members, creating an infrastructure that links
research laboratories with related industry, field support services and
ultimately the growers that are affected by whitefly problems within Europe.
It also provides a unique forum for disseminating information on European
whitefly problems and research activities to academia and industry worldwide.
With thanks to our sponsors, we have been able to increase the quality
frequency and volume of EWSN’s outputs. Our regular newsletters for
example, are now sent to almost one and a half thousand addresses worldwide
every 2 months and an interactive website is presently being developed
(www.jic.bbsrc.ac.uk/hosting/eu/ewsn).
EWSN also provides the coordinators with the ability to assemble a team
of scientists whenever necessary, to travel and assess new whitefly related
problems within Europe, and where possible provide advice on their control.
For example, during November 1999, with the support of the BBSRC and Bayer,
18 members of EWSN carried out a comprehensive survey of agricultural
regions within Tenerife and Gran Canaria, where growers were reporting
the appearance of viruses in their crops for the first time.
Through a series of meetings throughout the islands, participants were
able to meet growers and cooperative staff, and discuss the new agricultural
problems. They were then able to visit affected crops and collect samples
of whiteflies and viruses which were then analysed on return to the delegate’s
respective laboratories. The findings, which included first reports of
new viruses on the islands, were presented in a report that has recently
been distributed within EWSN and to the growers and cooperatives on the
islands. This included confirmation of CYSDV on Tenerife and a new tomato
virus, Tomato chlorosis closterovirus (ToCV), which is transmitted
by both B. tabaci and T. vaporariorum, on Tenerife and Gran
Canaria. ToCV has also recently been found in southern Spain as well.
The Canary Island trip along with others that have been undertaken within
the agricultural regions of southern Europe identified a number of very
important facts regarding the present whitefly problems:
Firstly, it is essential that if whiteflies and their associated problems
are to be controlled successfully, then growers must be provided with
information and accurate advice on minimising whitefly ingressions and
managing insecticide usage. Secondly, a failure to understand the following
facts about whitefly-transmitted viruses can often lead to further problems:
- Viruses are often transmitted rapidly and very efficiently where,
in the case of Tomato yellow leaf curl viruses, a single infected
whitefly has a 60% chance of transmitting the virus to a healthy plant.
Under these situations, serious limitations are put on the use of biological
control, since whitefly thresholds would certainly remain too high to
prevent further transmission.
- Insecticides may reduce virus spread within a crop, but rarely prevent
infected whiteflies from bringing a pathogen into a crop. Studies have
shown that before infected whiteflies are disabled by prophylactic treatments,
they often feed and successfully inoculate the virus. Also, these viruses
are acquired by whitefly larval stages, enabling already infectious
adults to emerge from the protected environment of the puparium stage.
- Virus symptoms usually appear on infected plants 2 weeks after they
have been inoculated. These symptoms are often regarded as a sign of
whitefly infestation and prompt additional spraying, even when whiteflies
themselves, are no longer a problem to the crop. Tomato rarely hosts
a large population of B. tabaci, so without virus problems, minimal
control measures would be needed.
- An overuse of insecticide can lead to resistance. The process of spraying
insecticides can also serve as a means to disturb and disperse whiteflies
and the viruses onto other plants. Unless managed carefully, whitefly-transmitted
viruses on one crop can rapidly increase the potential for serious control
problems on others.
In summary, a major factor for reducing the present virus problems, centres
on halting the movement of whiteflies within and between crops and addressing
the complexity of reasons behind the present epidemics. It is possible
that by providing growers with the necessary information to recognise
and deal with developing problems at an early stage, that the present
whitefly situation in southern Spain could be dramatically improved. Training
courses and information packs for growers could, with financial backing,
be organised through the European Whitefly Studies Network These would
provide guidelines for the early recognition of virus infected plants
and weed reservoirs, as well as advice on reducing whitefly movements
within and between crops, and managing an insecticide program that reduces
the risks of insecticide resistance problems. The possibility also exists
to establish ‘model’ greenhouses within problem areas that demonstrate
the effectiveness of nettings and trap plants in reducing whitefly problems.
This information would also be made available to growers within other
parts of Europe where whiteflies are increasingly becoming more difficult
to control within traditional growing systems.
Further information on the activities of the European Whitefly Studies
Network can be obtained from the EWSN office, John Innes Centre, Colney
Lane, Norwich, UK.
Whitefly Status in Latin America
by Dr. Luko Hilje (Unidad de Fitoprotección, CATIE. Turrialba,
Costa Rica)
Since the mid 1980's, some 18 crops in Latin America have been affected
by Bemisia tabaci (and B. argentifolii). Even though there
have been cases of direct damage in crops like cotton, melons and soybean,
the bulk of situations refer to transmission of geminiviruses, especially
in food crops such as beans, tomato and bell pepper. The number of whitefly
biotypes (at least three), wild host species (some 54) and types of viruses
(at least 17 in tomatoes alone), as well as the kinds of interactions
between them (which are favored by suitable year-round temperature and
humidity regimes), make it difficult to deal with this problem.
Historically, this problem followed an uneven pattern, in geographical
terms. It started in Central America and the Caribbean, appearing in an
almost simultaneous pattern from 1986 to 1991. Later on, it affected Mexico,
Venezuela, Ecuador, Colombia and Argentina, and in the last two years
it has extended to Peru and Brasil. Economic losses, although not properly
assessed so far, have been really high, causing a serious crisis in many
countries. In response to this, in the early 1990´s, task forces
at the local, regional or national levels were appointed. They drafted
work plans and set up priorities on: whitefly bioecology and geminivirus
epidemiology; biotype and geminivirus diagnosis; management tactics (cultural
practices, host-plant resistance, biological control, chemical control
and insecticide resistance management); training of extension agents and
growers; and on-farm validation and transfer of IPM tactics.
In addition, an international network (Action Plan for Whitefly and Geminivirus
Management in Latin America and the Caribbean) was created in 1992, in
order to set research and extension agendas aimed at the development and
implementation of integrated pest management (IPM) approaches. This network,
which currently involves 18 countries, is coordinated by CATIE (a regional
agricultural institution). Their members meet once a year to exchange
information and review the status of the Action Plan. In addition, a quarterly
newsletter (Whitefly Update), which now is available by Internet, is widely
distributed. (See section at printed and on-line magazine Revista
Manejo Integrado de Plagas).
So far, accomplishments between countries have been rather uneven. Nonetheless,
growers are now better aware of the implications of the problem in economic,
agricultural and environmental terms, and thus are more prone to adopt
and implement IPM programs. They understand the need to look for multi-tactic
approaches instead of a single-tactic, and in some cases are willing to
participate in area-wide preventative and curative approaches, which involve
quarantine regulations and cultural practices such as planting dates and
host-free periods. In spite of these advances, there is still a pressing
need to increase coverage of successful IPM programs, especially by involving
growers through participatory research, in order to strengthen adoption
and implementation of such programs.
Morphological, Molecular and Taxonomic Perspectives
on Encarsia (Hymenoptera: Aphelinidae)
J. Heraty (Department of Entomology, University of California)
The genus Encarsia (Hymenoptera: Aphelinidae) is a diverse and
cosmopolitan group of species usually parasitic on whiteflies, armored
scales, or themselves (autoparasitoids). At present there are more than
200 described species(Woolley & Heraty 1999). Encarsia are
one of the most important parasitic groups being exploited in biological
control, and various species are currently being collected as part of
foreign exploration efforts to search for biological control agents. Several
species have demonstrated their importance for control of San Jose Scale
(E. perniciosi), Greenhouse whitefly (E. formosa), ash whitefly
(E. inaron), and citrus whitefly (E. lahorensis). New programs
are focusing on the control of Bemisia with E. protransvena
and E. sophia (=transvena), and on citrus whitefly in California
with E. variegata. Biological and taxonomic characteristics remain
poorly known even for common species of Encarsia. In this brief
article, I will focus on taxonomic studies of Encarsia over the
last five years, and hint at the implications of these programs for biological
control.
Many species of Encarsia are undescribed. However, we must be
able to accurately recognize species with the greatest potential for control.
A common assumption is that closely related species may share similar
habits and host preferences to known species and are therefore desirable
candidates for biological control. These relationships are most commonly
determined by the presence of shared derived morphological characters.
Unfortunately, species groups of Encarsia, which are our first
approximation of related species, are often defined by combinations of
characters, many of which are characteristic of one or more species placed
in other species groups. Even obvious group characteristics are found
in unrelated groups of species; for example, the close placement of scutellar
sensilla, which were considered diagnostic of the strenua -group, are
now known to be convergent and found in several very unrelated groups
of species (Heraty & Polaszek 2000).
Understanding the species groups of Encarsia is of primary importance,
Currently, species are grouped arbitrarily on the basis of overall similarity.
This can lead to misconceptions about behavior and host associations that
are crucial for biological control programs. Analysis of morphological
characters has led to differing opinions as to the relationships, composition
and placement of species into groups of Encarsia (Hayat 1998; Huang
& Polaszek 1999).
Beyond morphology, molecular systematics (comparing species based on
their genetic similarities) offers a new character system. For separating
closely related species, restriction site analysis can be used to accurately
separate species of E. formosa and E. luteola, which are
otherwise difficult to separate using morphological characters alone (Babcock
& Heraty 2000). This provides novel ways to separating species in
lab and field situations, and also a means of separating different life
stages for biological studies. Although severely limited by the number
of taxa that can be sampled, the analysis of nucleotide sequences can
be used to test the relationships of existing groups and, perhaps more
importantly, evaluate the morphological characters used to define those
groups (Babcock et al. submitted). This latter point is probably the most
relevant for sorting field collected material in biological control programs.
In a preliminary study of relationships in Encarsia, nuclear sequences
of the D2 expansion region of 28S rDNA were determined from 67 strains
of 24 species representing 10 species groups of Encarsia, two strains
of Encarsiella noyesi Hayat, and one strain of Coccophagoides
fuscipennis Girault (Babcock et al. submitted). Separate and combined
analyses of molecular and morphological data provide support for many
nodes not resolved by morphology alone, and offer insights into which
morphological characters are useful for supporting group relationships.
A single preferred hypothesis of relationships was obtained from the analysis
of combined data, but Encarsia had to be constrained as monophyletic
(cf. http://cache.ucr.edu/~heraty/Aphelinidae.
html). The inaron, luteola and strenua species
groups are supported as monophyletic, whereas the aurantii and
parvella species groups are not. The four-segmented midtarsus,
often considered to be a convergent character (Hayat 1998), was not and
supported a single lineage of diverse species (cubensis + luteola
groups). Another trivial character, presence of a specialized seta at
the apex of the costal cell, defines a single lineage, the strenua group.
In general, traditional species recognized by morphological characters
were supported by the molecular data. Shifts to parasitism of Diaspididae
and transformation to parthenogenesis appear to have occurred multiple
times.
Basic taxonomic studies are by no means passe. Encarsia is one
of the most diverse and economically important group of parasitoids. New
identification keys are paramount in our ability to recognize species
(Schauff et al. 1996, Hayat 1998, Huang & Polaszek 1998). New species
continue to be described (Evans 1997, Evans & Polaszek 1997, Evans
& Castillo 1998, Hayat 1998, Huang & Polaszek 1999, Heraty &
Polaszek 2000). Several recent taxonomic changes affect the species attacking
Bemisia in the southern U.S.: E. bimaculata is a new species
specific to Bemisia (originally imported from India), E. transvena
is now E. sophia (a species described first by Girault in 1913
in Australia), and what was regarded as E. strenua in the U.S.
and Caribbean is now E. protransvena (a species described by Viggiani
from Florida; E. strenua exists but occurs almost exclusively in
the Orient on different genera of whiteflies). Although sometimes frustrating,
these name changes will provide stability in our interpretation of species.
It also emphasizes the importance of depositng voucher material; what
is a relevant species today may change tommorrow and vouchers provide
the necessary reference for earlier biological or ecologial studies.
Morphological studies of Encarsia have been provided from a number
of sources, but exemplary awards within the United States have been provided
by the USDA, National Biological Control Institute (postdoctoral fellowships
to Schauff, Evans and Heraty in three separate awards, and a facilities
grant for cataloging to Woolley and Heraty), California Department of
Food and Agriculture for work on the strenua group, and the California
Citrus Growers for studies in molecular systematics. These funds promote
productivity and thus advance our knowledge of this group. Unfortunately
we are still only dealing with about 243 species in a group that is likely
to be megadiverse. Noyes (1990), in a single canopy insecticide-fogging
sample in Sulawesi recorded more than 156 morphospecies of Encarsia!
How many of these or other undescribed Encarsia have potential
as biological control agents? To address these problems, we need, of course,
continued funding, but to complete the circle of information we also need
continued support by the field biologists who continue to supply material
and the biological information we need to understand this complex group.
References (emphasizing studies of Encarsia over last five
years)
- Babcock, C. S., and Heraty, J. M. (2000). Molecular markers
distinguishing Encarsia formosa Gahan and Encarsia luteola
Howard (Hymenoptera: Aphelinidae). Ann. Entomol. Soc. Am. 93: (in press).
- Babcock, C. S. , Heraty, J. M., De Barro, P. J., Driver, F. and
Schmidt, S. (submitted) Preliminary phylogeny of Encarsia Förster
(Hymenoptera: Aphelinidae) based on morphology and 28S rDNA. Molecul.
Phylo. Evolut.
- Evans, G. E. (1997). A new Encarsia (Hymenoptera: Aphelinidae)
species reared from the Bemisia tabaci complex (Homoptera: Aleyrodidae).
Florida Entomol. 80: 24-27.
- Evans, G. A., and Castillo, J. A. (1998). Parasites of Aleurotrachelus
socialis (Homoptera: Aleyrodidae) from Colombia including descriptions
of two new species (Hymenoptera: Aphelinidae: Platygasteridae). Florida
Entomol. 81: 171-178.
- Evans, G. A., and Polaszek, A. (1997). Additions to the Encarsia
parasitoids (Hym.: Aphelinidae) from Costa Rica. Florida Entomol. 79:
582-586.
- Evans, G. E., and Polaszek, A. P. (1998). The Encarsia cubensis
species-group (Hymenoptera: Aphelinidae). Proc. Entomol. Soc. Wash.
100: 222-233.
- Hayat, M. (1998). Aphelinidae of India (Hymenoptera: Chalcidoidea):
A taxonomic revision. Mem. Entomol. Intern. 13: 416 pp.
- Heraty, J. M. and Polaszek, A. (2000). Morphometric analysis
and descriptions of selected species in the Encarsia strenua
group (Hymenoptera: Aphelinidae). J. Hymen. Res. 9: 142-169.
- Huang, J., and Polaszek, A. (1998). A revision of the Chinese
species of Encarsia Förster (Hymenoptera: Aphelinidae):
parasitoids of whiteflies, scale insects and aphids (Hemiptera: Aleyrodidae,
Diaspididae, Aphidoidea). J. Nat. Hist. 32: 1825-1966.
- Noyes, J.S. (1989). The diversity of Hymenoptera in the tropics
with special reference to Parasitica in Sulawesi. Ecol. Entomol. 14:
197-207.
- Schauff, M. E., Evans, G. A., and Heraty, J. M. (1996). A pictorial
guide to the species of Encarsia (Hymenoptera: Aphelinidae) parasitic
on whiteflies (Homoptera: Aleyrodidae) in North America. Proc. Entomol.
Soc. Wash. 98: 1-35.
- Woolley, J.M. And J.M. Heraty. 1998. Encarsia species
of the world: A
searchable database. [a catalogue of about 313 species of Encarsia
with information on types, distribution and hosts]
- World Distribution of Parasitoids of Bemisia tabaci Group.
D. Gerling is developing a listing of known parasitoids of the B.
tabaci group, and their distributions. For comments, please contact
Dan.
Meetings
Action Plan for the Management of Whiteflies and Geminivirus in Latin
America and the Caribbean
This includes the following countries: Mexico, Guatemala, Belize,
El Salvador, Honduras, Nicaragua, Costa Rica, Panama, Dominican Republic,
Cuba, Puerto Rico, Haiti, Venezuela, Colombia, Ecuador, Brazil, Peru and
Argentina. The advances of the Action Plan, between 1998-1999, were presented,
in the VIII Workshop, which took place in October of 1999 in Recife, Brazil.
Three hundred and fifteen people were present form 13 countries (including
Spain, U.S.A. and Israel) and 15 Brazilian states, including researchers,
lecturers, extensionists, students, agricultural producers, and technicians
from the private sector (agrochemical companies and banks). Furthermore,
as well as a large number of poster presentations (89), there were magisterial
talks by researchers of world significance, as well as panels with different
focuses (countries, crops and current topics). The 2000 workshop will
take place in Panama and that of 2001, in Cuba. Those interested should
contact Luko Hilje.
The XXI Congress of Entomology
A full day symposium on Bemisia will be held in the XXI Congress of Entomology,
August 2000, in Brazil. The program will be as follows:
- Challenges and Opportunities for Pest Management of Bemisia
in the New Century
Organizers: Steven E. Naranjo, Maria R. V. Oliveira, Peter C. Ellsworth,
Odair A. Fernandes
- MORNING SESSION (Sponsored by Session 14: IPM)
- Introduction to Morning Session of Symposium, Odair A. Fernandes
- History and Current Status of Bemisia, Maria Regina Vilarinho
Oliveira, Thomas Henneberry & Raul Leon-Lopez
- The Bemisia Species Complex: A Challenging Systematic Issue,
Thomas Perring
- Overview of Insecticidal Control and Resistance Management, John
Palumbo, Rami Horowitz & Nilima Prabhaker
- Biological Control with Predators and Parasitoids, Dan Gerling
& Oscar Alomar
- Biological Control with Fungi, Marcos Faria, Lance Osborne, Zdenek
Landa & Ceske Budejovic
- Cultural Practices for Managing Whiteflies, Phil A. Stansly, Luko
Hilje & Heather Costa
- AFTERNOON SESSION (Sponsored by Session 2: Agricultural Entomology)
- Introduction to Afternoon Session, Maria Regina Vilarinho Oliveira
- Host Plant Resistance for Bemisia tabaci other Whitefly
Species, and Associated Viruses, Anthony Bellotti & Francisco
Morales
- Ecological Considerations for Management of Multiple-Crop Pests,
Peter Ellsworth, John C. Palumbo, Steven E. Naranjo & Steven
J. Castle
- Conservation and Evaluation of Natural Enemies in IPM Systems,
Steve Naranjo & Walker Jones
- International and National Research Programs for the Development
of IPM Systems, Pamela Anderson, Tom Henneberry & Maria Regina
Vilarinho Oliveira
- IPM of Bemisia tabaci in Australasia, Paul De Barro, Felice
Driver, Ian Naumann, Stefan Schmidt, John Trueman & John Curran
- Implementation and Adoption of IPM Systems, Jose L. Martinez-Carrillo,
Reuben Ausher, Peter Ellsworth, Luko Hilje, & Reuben Ausher.
The Third International Workshop on Bemisia
The Third International Bemisia Workshop was scheduled to take
place in July 2001 at the John Innes Institute, in conjunction with the
meeting of the Geminivirus Workgroup. However, due to the forthcoming
symposium sponsored by EWSN (see below), it was decided to postpone the
workshop. No date has been set as yet, and members of the International
Bemisia Workgroup and colleagues are invited to offer their suggestions
as to the date and place best suitable. Meanwhile, although the EWSN meeting
will cover all whiteflies and all aspects of whitefly in Europe, the major
thrust will concern Bemisia. Thus, we are confident that the exchange
of facts that will take place in that meeting will contribute measurably
to the exchange of information and fostering of ideas concerning Bemisia
research.
European Whitefly Symposium
The European Whitefly Studies Network (EWSN: a Concerted Action funded
by the European Union FAIR 6 CT98-4303), is organising an international
whitefly symposium at the University of Catania, Faculty of Agriculture,
Ragusa, Sicily (Italy) from February 28th to March 3rd, 2001. This symposium
will address the following whitefly-related disciplines:
- Virology
- Pest and Disease Epidemiology
- Faunistics and Systematics
- Natural Enemies
- Plant Protection/Integrated Pest Management
The majority of delegates will comprise EWSN members and local personnel
active in whitefly research and management. Presentations will be given
by these delegates and also a small number of invited international speakers.
The symposium is already attracting a high level of interest and persons
wishing to attend are advised that the number of delegates is likely to
be strictly limited. Registrations should be made as soon as possible
by contacting the EWSN research facilitator: Mr. David Oliver at
John Innes Centre.
Whitefly E-Mail Exchange Group
If you have e-mail access, you can send and receive information among
a large group of whitefly workers by joining the Whitefly Listserver.
To automatically join, send a message with no Subject to:
listserv@listserv.tamu.edu
In the body, type subscribe. To get off the network, type the message
unsubscribe to the same address above.
After getting subscribed, messages to the group must be sent to a different
address:
whitefly-l@tamu.edu
For friendly help, contact Reyes Garcia III, Whitefly Net Administrator
|
