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The present issue of "Bemisia" will deal mainly with two subjects: The biotype or strain problem, and an update of work on parasitoids. We shall also devote some space to reports, meetings and references.
The possibility that recent outbreaks of Bemisia tabaci are connected with the proliferation of different biotypes was already considered in 1977. However, studies along these lines began only in the 1980's (e.g. work by V. Dittrich in CIBA GEIGY, by R. Sawicki and coworkers in Rothamstead experiment station, by D. Wool and D. Gerling in Tel Aviv, and by L. Fishpool and C. Burban in Cote d'Ivoire). Most work comprised the running of electrophoretic profiles of the specimens, using esterases. It gained strong impetus following outbreaks of Bemisia tabaci in Florida from 1986 on, because they were of unusual severity and were accompanied by unfamiliar plant disorders. The studies that were conducted revealed that at least two biotypes (A and B) of the insect presently occur in the U.S. Since then, a number of groups have been actively engaged in studies of various aspects associated with the following questions:
We approached a number of workers engaged in the study of these questions and present their responses. We realize that the list is partial, both because not all have responded and because we might have missed some workers who have also contributed to this subject. We take the opportunity to encourage researchers to send us their contribution.
A. David Wool and Dan Gerling, Department of Zoology, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Israel. A.C. Bellotti, F.J. Morales and B. Nolt. CIAT, Cali, Columbia.
In the last few years, considerable work has been carried out in Israel and Colombia (South America) on genetic variation (measured electrophoretically), Esterase activity and insecticide resistance in Bemisia tabaci populations (Wool et al., 1991; Wool & Greenberg, 1990; Wool et al., 1992a,b; papers in press).
I. Electrophoretic studies. (1) Bemisia tabaci populations in Israel appeared to be electrophoretically very similar at the major EST locus apart from variations related to insecticide application (the frequencies of the slow (s) EST isozyme were higher in samples from agricultural fields). No host races or geographical races were detected in large samples from many crop localities. (2) In contrast, samples from Colombia were highly differentiated into at least four electrophoretically-distinct "biotypes". These "biotypes" were geographically separated, apparently regardless of host plant. (3) Small samples from Kenya, Florida, Arizona and the Sudan seemed to be electrophoretically different from the Israeli Bemisia tabaci but this result is tentative.
II. The activity of EST in Israeli Bemisia tabaci was lower in samples from agricultural fields (which were 2-4 times more resistant to an organophosphorus insecticide) than were control populations. This result could be also due to direct effects of exposure to pesticides, although we found no inhibitory effects of the particular pesticide on EST activity in the laboratory. Selection for insecticide resistance resulted in increased EST activity, but the use of EST as a marker for resistant individuals in the field is questionable, since EST activity variation in both resistant and control samples is very large and affected by a variety of non-genetic factors unrelated to resistance.
Wool, D. & S. Greenberg. 1990. Esterase activity in whiteflies (Bemisia tabaci in Israel in relation to insecticide resistance. Entomol. Exp. Appl. 57: 251-258.
Wool, D., D. Gerling, A.C. Bellotti, F.J. Morales and B. Nolt. 1991. Spatial and temporal genetic variation in populations of the whitefly Bemisia tabaci (Genn.) in Israel and Colombia: an interim report. /Insect. Sci. Appl. 12: 225-230.
Wool D.,D. Gerling, A. C. Bellotti, and F.J. Morales 1992a. Esterase electrophoretic variation of Bemisia tabaci (Genn.) among host plants and locations in Israel. Jour. Appl. Entom. (In Press)
Wool, D., L. Calvert, L.M. Constantino, A.C. Bellotti, and D. Gerling 1992b. Differentiation of Bemisia tabaci (Genn.) populations in Columbia, South America Jour. Appl. Entom. (In Press).
Judith K. Brown. Department of Plant Sciences, University of Arizona, Tucson, AZ 85721
Biographic surveys of Bemisia tabaci Genn. populations during 1989 to the present indicate that the new "B" biotype is present in nearly every major agricultural locale throughout the Southwestern and Southeastern US and Hawaii, Northern Mexico, the Caribbean Basin, the Eastern Caribbean, Central America, and at least several countries in South America (Brazil, Venezuela). In addition, the "B" biotype has been documented in greenhouse production systems in the US, Europe and Japan, and in vegetable and cotton cropping systems in the Middle East. In these studies, biotypes were monitored using native gel electrophoresis of characteristic non-specific esterase markers associated with adult female Bemisia tabaci.
The "A" or indigenous Southwestern US population and the prototype "B" population maintained in the Tucson laboratory were used as standards in all assays. In addition to the characteristic "A" and "B" type esterase patterns found in some populations sampled, at least eighteen distinct esterase patterns were observed for Bemisia tabaci collected from other world regions. Biological characteristics have not presently been ascertained for the majority of the esterase-typed populations. Collaborative studies are now in progress to examine these populations on the DNA level (Dr. D. Frolich, UA, Tucson, Arizona), by morphometric analysis (Dr. M. Houck, Texas Tech, Lubbock, Texas), and for virus transmission capabilities (Dr. P. Markham, John Innes Institute, Norwich, UK). Intensive surveys of the Bemisia tabaci populations in the Southwestern US (SW US) and Northwestern Mexico by esterase typing were conducted from 1989-1992.
In the SW US in 1989, the "A" type was found exclusively in the field, while the "B" biotype was found in greenhouse grown ornamentals. In 1990, the "A" the "A" biotype still predominated in the field, but the "B" type was identified in field grown melons and in greenhouse crops. In 1991, the "A" type could be found in only several scattered, but remote areas of Arizona, whereas the "B" type predominated in all urban areas and rural agroecosystems. In 1992, the "B" biotype was the exclusive type identified from all sites sampled.
From 1989 to the present, all US populations received from locales outside of Arizona were of the "B" biotype. Bemisia tabaci collected from Sonora, Mexico sites, 1989, until the fall of 1991 were of either the "A" or "A1" esterase type. By November 1991, the "B" biotype was detected in greenhouses and adjacent field crops. At the present, the "B" biotype is present in Sonora in both field vegetable and greenhouse locales. As of the fall, 1992, only "A" types have been found in field sites in the state of Sinaloa. These results suggest that the "B" biotype, believed to have been introduced into the US and elsewhere on infested plant materials as early as 1985-96, has nearly or completely displaced the indigenous (putatively) populations.
Brown, J.K. and J. Bird. 1992. Plant Dis. 76:220-225.
Brown, J.K., H.S. Costa and F. Laemmlen. 1992. Plant Disease 76:426.
Costa, H.S. and J.K. Brown. 1991. Entomologia exp. et applic. 61:211-219.
P.G. Markham, I.D. Bedford, R.W. Briddon, and S. Liu. John Innes Institute, Colney Lane, Norwich, NR9 7UH UK. J. K. Brown, H. S. Costa, and R. C. Rosell. Dept. Plant Sciences, University of Arizona, Tucson AZ 85721 U.S.A.
We have been studying Bemisia tabaci at the John Innes Institute for several years as part of our objective to understand the molecular basis for vector specificity in the transmission of geminiviruses (1). To assess transmission efficiency of Bemisia tabaci worldwide, comparisons are being undertaken of 22 populations from different locations (North and Central America, Caribbean, Africa, Mediterranean, Middle East, Pakistan, Nepal and India) which are being maintained as breeding colonies. These colonies are being compared for their ability to transmit a selection of geminiviruses; where possible, viruses from the same country as the whiteflies are being used. One colony will transmit 18 geminiviruses, another only one and that from the same locality as the colony, but in general most colonies transmit most viruses (2). Identification of colonies was a major priority. The morphology of 4th instar/pupal stages has been studied by scanning electron microscopy: all colonies have features typical of the species, although variations in setae were shown within populations when reared on different host plants (3). Isoenzyme analysis of the populations has shown differences (4). DNA fingerprinting is complimenting the isoenzyme results. "Silver-leaf" assay (phytotoxicity in Cucurbita pepo and other species) identified the deleterious "B" biotype from many different global localities. The ability of colonies to adapt to new host plants appears to be the most important feature in the efficiency of transmission to alternate hosts. The "B" biotype has the largest host range and is most likely to transmit. Three "ornamental" plants infected with geminiviruses are all non-transmissible. All cloned viruses tested were non-transmissible. Analysis of the coat protein, including site directed metagenesis, is currently being used to determine vector specific epitopes. We have dedicated containment facilities to maintain the whiteflies, the viruses, and to undertake genetic manipulation. We would also be grateful to receive material from colleagues in areas not mentioned above for inclusion in the comparative test.
Briddon et al., l989. Geminivirus coat replacement alters insect specificity. Virology 177, 85-94.
Bedform et al., 1992. Bemisia tabaci - biotype characterization and the threat of this whitefly species to agriculture. Proc. British Crop Protection Conf., Brighton 1992. Pest and Diseases 3, 1235-1240.
Markham et al., 1992. John Innes Annual Report 1991, 24-25.
Brown et al., l992. Abstract American Phytopathological Soc. Annual Meeting, Portland, 1992.
Markham et al., l992. Abstract Proc. XIX Int Cong Entomology, Bejing p. 685a.
David Byrne. Department of Entomology, University of Arizona, Tucson, AZ. 85721 U.S.A.
By all accounts, the designated B" strain of Bemisia tabaci is now predominant in the southwestern U.S. There is some controversy as to whether or not this is the result of competitive displacement. Some believe that it is while others feel that resources for the polyphagous sweetpotato whitefly are not ever likely to be limited across the entire range, making displacement questionable. The presence of the two strains was first investigated in the southwest by Byrne and Miller (1990). They examined the carbohydrates and amino acids in the phloem sap of cotton and poinsettia (hosts associated with the two strains) and in the "honeydew" produced by the "A" and "B" strains feeding on these two plants. They could find no differences in the way the two strains processed these compounds. They did, however, find a carbohydrate, trehalulose (glucose and fructose with a 1-1 linkage) in honeydew that had not previously been associated with insects (Bates et al. l990). A more thorough investigation of whitefly honeydew is being conducted by D. Hendrix of USDA-Phoenix and W.B. Miller at Clemson University. More recently, N.J. Gawel and A.C. Bartless of the USDA-Phoenix have had remarkable success using the RAPD-PCR technique to compare current field populations ("B" strain) with insects collected several years ago ("A" strain). These comparisons eliminate problems associated with comparisons with laboratory colonies that are subject to "genetic drift". RAPD-PCR demonstrated differences in DNA structure between the two forms of Bemisia tabaci. All twenty of the RAPD primers tested readily distinguished between the "A" and "B" forms. DNA extracted from individual eggs and nymphs showed similar differences. Genetic similarity statistics indicate the forms of Bemisia tabaci were no more closely related to each other than to the bayberry whitefly or banded winged whitefly. Different found in DNA composition and differences noted by other researchers in pathogen transmission characteristics, host range, honeydew production, egg production and esterase patterns, suggest no clear reason to include these insects under the same specific designation.
Bates, R.B., D.N. Byrne, V.K. Kane, W.B. Miller and S.R. Taylor. 1990. N.m.r. characterization of trehalulose from the excrement of the sweetpotato whitefly, Bemisia tabaci. Carbohydrate Research 201:342-345.
Byrne, D.N. and W.B. Miller. l990. Carbohydrate and amino acid composition of phloem sap and honeydew produced by Bemisia tabaci. Journal of Insect Physiology 36:433-439.
Gawel, N.J. and A.C. Bartlett. 1992. Characterization of differences between whitefly species using RAPD-PCR, submitte
Thomas M. Perring. Department of Entomology, University of California, Riverside, CA 92521
Our laboratory is approaching the issue of relatedness between Bemisia tabaci type A and type B whiteflies with the working hypothesis that these are two distinct species: an idea suggested by several whitefly researchers. Using the definition of species proposed by Diehl and Bush (1984) as "natural populations that are reproductively isolated from one another and that follow distinct and independent evolutionary paths" we are conducting two studies (crossing experiments and mating behaviour studies) to address the reproductive isolation aspect and two studies (genetic distance analysis based on isozymes and polymerase chain reaction based differentiation) to elucidate the evolutionary paths followed by the two whitefly types. In our crossing experiments which are in progress, we have yet to observe any female offspring in cross-matings between A and B types, suggesting reproductive isolation between the two. As a follow-up to this work, we are conducting mating behaviour observations using video recording equipment. The two whitefly types will court each other, but we have not observed any copulation between type A and type B whiteflies at the present time. We have published our work on allozymes using isoelectric focusing techniques for distinguishing the strains of individual whiteflies (Perring et al., 1992). This research identified two enzymes, phosphoglucomutase, and phosphoglucose isomerase, which are particularly useful for distinguishing the two whitefly types. We also evaluated other enzymes, including esterases. For esterases, we found complex banding patterns in five tightly linked loci; we were unable to interpret these patterns consistently. These problems, in addition to the inducible nature of esterases, has steered us away from using this enzyme for strain determination. With our assays, we have sampled whiteflies from the US (Arizona, California, Florida, Georgia and Texas), Egypt, Mexico and Spain, from a variety of hosts and have found only the two types. Finally, we are using a polymerase chain reaction based assay known as single primer amplification to compare DNA fragments of the two whitefly types. We anticipate having the results of this work to report at the national ESA meetings in Baltimore in December.
Diehl, S.R. and G.L. Bush. l984. Ann. Rev. Entomol. 29, 471-504.
Perring, T.M., A. Cooper and D.J. Kazmer. 1992. J. Econ. Entomol. 85, 1278- 1284.
Hsing-Yeh Li, James E. Duffus and S. Cohen. USDA ARS Salinas Calif. U.S.A. and ARO, Volcani Center, Israel.
Preliminary crossing experiments between the "A" and "B" biotypes of Bemisia tabaci from the California desert using virgin males and females in short breeding periods produced no detectable hybrids. Limited analysis of field collected whiteflies from the desert region had indicated an almost complete shift in the population from the previously occurring "A" biotype to the newly introduced "B" biotype. The biotypes were known from previous work to differ in the suitability of various hosts for nymphal development. Thus it was of interest to determine in the laboratory how this shift in population took place. Equal numbers of males and females of both biotypes (25 A males and 25 A females, plus 25 B males and 25 B females) were enclosed in large muslin-covered cages. An esterase isozyme analysis on polyacrylamide gels was made of the caged mixed population at approximately monthly intervals. Parent survivors and subsequent generation adults on most hosts were a mixture of the "A" and "B" biotypes. However, as there were a substantial number of hybrids or segregates, it is possible that a breakdown of apparent reproductive barriers occurred during the prolonged mixing experiments. Populations maintained on sweetpotato and bean shifted quickly to the "A" biotype, whereas those maintained on broccoli and melon shifted to the "B" biotype. However, after five months, segregates or recombinations of the "A" and "B" biotypes as determined by isozyme patterns were present in all hosts. An isozyme analysis of Bemisia from California ("A" and "B"), Florida, Texas, Nigeria and Israel indicates that these populations have the range of isozymes representative of the variations or segregates of the "A" and "B" populations. However, the Israeli population had additional bands not present in populations from the other regions. Recent analyses of desert populations indicate a mixture of "A"s, "B"s and hybrids. Thus under the laboratory and natural field conditions these two populations do not remain distinct. It seems that host suitability plays a major role in the adaptability of whitefly biotypes to different regions. Manipulating hosts and/or biotypes through breeding may be useful in changing the predominant whitefly in a region.
Ray K. Yokomi, D. R. Jimenez, and J. P. Shapiro. USDA ARS, Orlando. Fl. U.S.A.
Florida populations of the sweetpotato whitefly (SPW) induced squash silverleaf (SSL) symptoms in several varieties of Cucurbita sp. These populations exhibited typical "B" biotype patterns of esterase activity in PAGE and extracts from the insect contained dsRNA bands at 4.0 and 4.4 x 106 daltons in PAGE. Biotype "B" esterase patterns were obtained from SPW populations from Arizona, Texas and Mississippi which induced SSL symptoms under Florida conditions. These insects also contained the same dsRNA bands as the Florida "B" SPW. SPW populations from California and Arizona showed the "A" esterase pattern and did not contain dsRNAs and did not induce SSL. We plan to continue periodic esterase banding pattern analysis of field collected SPW from Florida to obtain a handle on "genetic diversity". Natural enemies The increase in Bemisia tabaci associated problems is accompanied by numerous new studies of its enemy fauna. These include all phases of research, from foreign exploration through taxonomy and rearing, to colonization, practical testing and studies of biology. Most noteworthy are the efforts made by American institutions (USDA and Universities) towards world-wide exploration and testing of parasitoids and other natural enemies. Some of these have already been imported into the U.S. and are reared in quarantines or being released. In addition, there is work conducted on rearing and utilizing predators and fungal organisms in the control of Bemisia tabaci. Many of the studies are in their initial phases and, hopefully will be reported in later issues of 'Bemisia'. However, a recent contribution* permits us to present an updated version of the table presented in "Bemisia No.5:
*Reference: Polaszek, A., G. Evans, and F. D. Bennett, 1992. Encarsia parasitoids of Bemisia tabaci(Hymenoptera: Aphelinidae, Homoptera: Aleyrodidae) - a preliminary guide to identification. Bulletin of Entomological Research, in press.
1. Bemisia tabaci has been a prominent pest in India during most of the past century, and numerous scientific contributions about this pest have originated there. In 'Bemisia' No. 4 we reported the work on Bemisia tabaci transmitted viruses headed by Dr. Muniyappa. Since then, we were asked by Dr. S. N. Puri to report the work on biology and pest management carried out by his group.
Dr. S. N. Puri. Department of Entomology. Marathwada Agricultural University. Parbhani -431 402 (M.S.). India Dr. Puri and co-workers are conducting a study of Bemisia tabaci that includes: 1. Biology and life cycle - including life table studies, season incidence on cotton and on other host plants. 2.Effects of agrotechniques on whitefly incidence, including different cultivation techniques (spacing, irrigation and fertilizers) and insecticide treatments with conventional insecticides and with oils. 3. Breeding resistant varieties to Bemisia tabaci. 4.Natural enemies - a survey revealed the existence of 17 species of enemies including Encarsia transvena and Eretmocerus mundus, Chrysoperla carnea and Serengium parcesetosum. The biology, occurrence and effect of the enemies on cotton and other plants is examined and their susceptibility to insecticides is being tested.
Seventeen publications and notes have been issued between 1986 and 1991 on the subject. 2. Prof. J. C. van Lenteren (Agricultural University. Department of Entomology, Wageningen, The Netherlands) has reported in the latest issue of the newsletter 'Sting' that Bemisia tabaci is becoming a more severe pest and causing more concern in Europe. where it is a widespread greenhouse pest.
On the 28-30 October 1992 the 7th technical advisory committee of plant protection directors of the Caribbean, sponsored by IIBC-IICA and CSC, was held in Dominica (West Indies) . It dealt chiefly with Bemisia tabaci and Thrips palmi and was convened to inform the pest management directors of the Caribbean region of what is known to date about these pests their dangers and the various routes to decrease the damage and manage them. A large number of papers and a symposium about Bemisia tabaci will be presented in the annual meeting of The Entomological Society of America to be held in Baltimore Maryland Dec. 6-10. A first follow-up meeting of the 5-year National Research and Action Plan concerning the Sweetpotato Whitefly, that was put together in Houston Texas during the meetings held February 18-21 Feb. 1992, will be held in Arizona during February 1993. The first International Cotton research Conference will be held in Brisbane, Australia, during 13-17 February 1994. The meeting is sponsored by the International Cotton Advisory Committee. Its secretary general, Dr. R. Chaudhry informed us that emphasis will be placed upon cotton stickiness and Bemisia tabaci. Those interested may contact him at:
Dr. M. R. Chaudhry
An update to the annotated bibliography of Bemisia tabaci (IIBC 1986) has recently been compiled and edited by M.J.W. Cock. This useful publication contains 447 titles that include most of the material published between 1986-1992. As in the 1986 edition, a useful introduction is given, which includes subject summaries of most pertinent subjects.
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