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A year and a half has elapsed since the last newsletter. Many things have changed and it is the purpose of this issue to update workers with Bemisia about these changes and to disseminate any information that all of us might find useful.
Bemisia continues to cause extensive damage, both as a direct feeder and as a producer of honeydew. However, as time passed, its prominence as a virus vector has increased and more crops have become susceptible to more types of Bemisia - transmitted viruses. At the same time, research on virus transmission has made great strides by elucidating the pathways of the viruses in the whitefly and the modes of its transmission, and by enlisting the help of molecular biology to investigate the characteristics of the virus-vector relationships and to manipulate host-plant resistance.Some of the above, together with other studies, have added to our knowledge of basic biology of the insect.
New knowledge on the internal and external morphology of Bemisia is now available. Flight behavior and field dispersal studies have continued, and significant advances have been made toward development of an artificial rearing method for whitefly immatures. Bemisia may be the only major insect pest worldwide that has no artificial rearing system. Development of such a system would not only reduce costs, it would greatly facilitate research progress such as the investigation of the biochemical basis for host-plant suitability, and allow natural enemy production to be much more economical.
Novel studies have demonstrated the detrimental effect of antibiotics on endosymbionts. The importance of host plant species on the intensity of Bemisia infestations had been previously demonstrated, but recent studies have shown that the host plant is also very relevant to the third trophic level. The results have opened the door for the development of new approaches toward enhancing natural enemy efficacy.
The impact of natural enemy activity has also been studied, and advances have been made in the difficult field of estimating the effects of predators. New technologies utilizing both visual techniques and monoclonal antibodies have pushed the science to new levels.
The importation and study of whitefly parasitoids in the U.S. continues to produce interesting results. This is the first time that techniques of molecular biology are employed so extensively in the identification of natural enemies for biological control of any insect pest. Two groups of parasitoids that are typified by difficulties in specific determination, and for which these techniques are proving most useful, are the uniparental Encarsia formosa and the genus Eretmocerus.
The scientific world remains divided as to the taxonomic status of Bemisia, with many of the American scientists adopting the new species status of Bemisia argentifolii, and others, including those in Great Britain, are still maintaining that no clear specific boundaries can be drawn and that we are dealing with a continuum of multiple biotypes.
Bemisia has changed its status in many locations. Unfortunately, no good documentation as to these changes exists worldwide. A few highlights of the pest status in 1995 include the establishment of the pest in Australia, severe infestations of both vegetables and cotton in the southern regions of the USA, another severe season of whitefly and virus problems in Pakistan, and a relatively mild season in Israel. It is also interesting to note that although Bemisia occurred on almost every cotton plant that was examined in the Wuhan region of China (Hubei Province), the infestation was very low, usually reaching less than 10 pupae per plant. Some of the whiteflies are parasitized by Encarsia transvena? Thus, the Bemisia in this region that grows over 400,000 hectares of cotton, seems to be in a state that is similar to the pre-1985 condition in America, and pre-1976 in the Middle East. (D. G.)
Submitted by: Ian Denholm and Matthew Cahill, IACR. Rothamsted, Harpenden, Hertfordshire. United Kingdom
Use of insecticides against Bemisia has increased dramatically in recent years. Despite intensive research into non-chemical control tactics, this trend seems likely to continue for the time being, placing chemicals under ever-increasing threat from the development of insecticide resistance. The ability of Bemisia to evolve resistance is already well established from monitoring programmes, and from severe control difficulties encountered in many countries in Europe, the Americas, the Middle East and the Indian Subcontinent. In all cases, the result has been to reduce the availability of effective chemicals, placing even greater pressure on remaining or newer products. Work to document, analyze and combat the spread of resistance has consequently attained high priority in several whitefly laboratories, especially in the USA, UK, Israel and Pakistan. For obvious reasons, most of these programmes are restricted to analyzing indigenous populations in support of established or intended resistance management strategies. Research at Rothamsted has been more internationally-based; living in a climate less favorable for Bemisia survival we are well placed to import strains from around the world for a comparative study of cross-resistance patterns and resistance mechanisms.
Collectively, these programmes provide a valuable insight into the current status of resistance to conventional and novel insecticides. At present, resistance is most advanced to the pyrethroids and organophosphates (OPs), compounds with excellent intrinsic activity against Bemisia but whose repeated use, against both whiteflies and coexisting pests, has led in some cases to resistance factors of 100-fold or more. Cross-resistance within each of these classes is generally very pervasive; opportunities for switching between structurally-related chemicals are consequently limited. Although there is still no categorical evidence for a single mechanism conferring resistance to both pyrethroids and OPs, this is quite feasible through enhanced detoxification by esterases or enzyme systems. The prevalence of resistance to both these groups implies that control strategies based on combining or alternating pyrethroids and OPs may offer little respite, unless in potentialising effects or synergistic interactions. Synergism of pyrethroids (especially fenpropathrin) by acephate has been exploited to good effect on cotton in the USA, although over-reliance on these combinations, far from alleviating resistance, has promoted the selection of a new, unsynergisable resistance mechanism.
Based on laboratory and field data there is no doubt that the levels of resistance to pyrethroids and OPs now being recorded in Bemisia can substantially impair the field efficacy of these chemicals. For aldicarb (a carbamate), amitraz (a formamidine), endosulfan (an organochlorine) and nicotine, the significance of variations in tolerance documented in laboratory bioassays is less clear-cut. We would welcome observations by other workers as to the current field performance of these products, particularly from sites where they have a history of long or frequent usage.
Novel products achieving increasing importance for whitefly control include buprofezin ('Applaud'), pyriproxyfen ('Tiger'), imidacloprid ('Confidor', 'Provado' or 'Admire'), diafenthiuron ('Polo' or 'Pegasus') and pymetrozine ('Chess' or 'Plenum'). Being structurally and functionally distinct from established insecticide groups, and hence unaffected by cross-resistance, these collectively offer the greatest hope for regaining control of multiresistant populations and formulating sustainable resistance management strategies. So far, their potential has been realized to best effect in Israel, where all new products released onto cotton are restricted to a single application per season. One very worrying trend, however, is the temptation for growers, of protected crops in particular, to switch entirely to one or two of these products and apply them repeatedly. Localized occurrences of resistance to buprofezin in Dutch glasshouses, pyriproxyfen in Israeli greenhouses, and imidacloprid on vegetable crops in southern Spain are all attributable to successive treatments with a single product, in breach of recommendations by researchers and insecticide manufacturers. While this practice is to be deplored, it has, ironically, had two potentially beneficial consequences. One is to demonstrate categorically the capacity for Bemisia to resist these agents without incurring a major loss of biological fitness. The other has been to provide access to resistant populations at a sufficiently early stage for research into the expression and inheritance of underlying mechanisms to assist with preventing resistance from spreading further.
There is no universally-applicable approach to combating resistance in such a widespread and polyphagous pest. To be effective, management tactics must be tailored to existing knowledge of whitefly bionomics and movement, the local availability of insecticides, and prevailing socio-economic conditions. Aspects of monitoring and managing resistance in Bemisia have been addressed in several reviews, most recently by Rothamsted staff in the forthcoming book 'Bemisia 1995' edited by Dan Gerling and Dick Mayer (see below), where supporting references for most statements in this article are provided. As for all cases of resistance, the first response to a perceived control failure should be to enlist technical support from local researchers, extension staff or pesticide company representatives. These individuals may in turn need to seek specialist advice, especially if planning to implement monitoring tests for particular products. We are happy to offer assistance in this respect and to recommend colleagues with appropriate expertise. We also encourage use of the Bemisia Newsletter as a forum for exchanging information on suspected or confirmed cases of resistance, so that attempts to combat this threat can be based as far as possible on shared data and experiences.
Recent Publications on Bemisia From IACR-Rathamsted, UK:
Submitted by: A. A. Kirk, USDA, ARS, EBCL, Montpellier, France
Between 1992-1996 more than one hundred shipments of natural enemies of Bemisia were sent to the quarantine at the APHIS/PPQ, Mission Biological Control Center, TX, (MBCC) by scientists from the USDA, ARS European Biological Control Laboratory and USA and overseas collaborators.
Twenty five countries in Africa, South America, the Mediterranean, Middle East, Indian sub-continent and SE Asia have been explored once or several times for Bemisia natural enemies. More than 98% of parasitoids have been reared from Bemisia tabaci spp. complex and Trialeurodes vaporariorum infesting 50 plant hosts. Plant host species were: 17 crops (mainly Cucurbitaceae/Solanaceae), 13 ornamentals (no dominant family), 20 weeds (mainly Compositae/Solonaceae). Collaborators typed Bemisia populations from these diverse collections using esterase electromorphs.
The following parasitoids were collected: 14 Encarsia spp. of the 19 spp. recorded worldwide plus 7 new spp.; 1 confirmed Eretmocerus sp. of the 5 previously recorded species, plus 15 new spp; and 1 Amitus sp.
The parasitoids at Mission were identified using a RAPD diagnostic method. Distinct strains exhibit biological parameters adapted to temperature and humidity e.g. (the Nile strain of Encarsia formosa is more efficient at higher temperatures and lower humidity (Nile delta conditions) than the E. formosa strain from northern Greece (Continental conditions). Eretmocerus mundus from southern Spain was more tolerant of selected pesticides than an Eretmocerus sp. from the USA (Texas) and 2 Encarsia spp. Eretmocerus sp. from Pakistan parasitized a higher percentage of Bemisia nymphs on cotton than any other parasitoid evaluated.
More than 6 million Eretmocerus and Encarsia spp. from 24 cultures with 14 distinct DNA patterns have been released in 6 states. Three species have been chosen by the MBCC as key exotic natural enemies in the Rio Grande Valley Demonstration of Biological Control Based IPM program in collaboration with USDA, ARS, Weslaco, Texas: Eretmocerus sp. from southern Spain, Eretmocerus sp. from Pakistan and Encarsia nr. pergandiella from Brazil. Recoveries of these species have been made in 3 states. Release permits have been issued for twelve parasitoid species, while 25 species/geographic strains are currently in the quarantine at MBBC and 5 in Hawaii.
Four predator species have been collected and are being evaluated in the U.S. and France; Serangium parcesetosum from India; Serangium n. sp. from Malaysia; Clitostethus arcuatusfrom Spain and the drosophilid Acletocxenus formosus from Crete.
In addition, several hundred isolates of the fungal pathogen Paecilomyces fumosoroseus collected from Bemisia populations in India, Malaysia, Nepal, Pakistan and Taiwan are stored at the ARS laboratory in Ithaca, New York, and are under study in Montpellier and Weslaco, Texas.
Inter-Center Whitefly IPM Task Force Report
International Conference on the Biology and Molecular Epidemiology of Geminiviruses
This Conference, sponsored by the U. S. Department of Agriculture's, Foreign Agriculture Service/Research and Scientific Exchange Division, and the University of Arizona, was held on 3-8 June, 1995, at the Westward Look Resort, Tucson, Arizona, USA. The event was organized by Dr. J. K. Brown (University of Arizona). About 80 invited participants from 16 countries made presentations and posters, many on recent research results concerning whitefly vector-virus interactions. Both an Internet website and a "GeminiNet" network has been established to link geminivirus workers. For more information contact ILTAB/TSRI, Plant Division. La Jolla, CA, USA.
Biological Control Symposium in Mexico
The Symposium on Biological Control of the Sweetpotato Whitefly [Simposio Sobre Control Biológico de Mosquita Blanca] was held on 9 November 1995 in Tapachula, Chiapas, at the Annual Meeting of the Mexican Society of Biological Control. The Proceedings contain the following titles: (1) Situación actual de la mosquita blanca en México. Luis A. Aguirre Uribe (2) Los parasitoides en el control biológico de mosquita blanca (Homoptera: Aleyrodidae) en México, Hugo C. Arrendondo Bernal y M. A. Mellin Rosas, (3) Identificación de parasitoides de moscas blancas (Homoptera: Aleyrodidae), Juan Carlos Loyola Licea, (4) Chrysoperla carneacomo agente de control biológico de mosquita blanca en el Valle de Mexicali, B. C. César Cota Gómez, Armando Pulido Herrera y Leovaldo García Casas, (5) Development of entomopathogenic fungi for biological control of whiteflies in row crops in the Rio Grande Valley of Texas: Progress and Prospects, S. P. Wraight, R. I. Carruthers, S. T. Jaronski, C. A. Bradley, and coworkers, and (6) Contro microbial de mosquitas blancas con Paecilomyces spp. en México, Victor M. Hernández Velásquez, E. Garza González, y A. M. Berlanga Padilla.
Latin American Network for Whitefly and Geminivirus
Since 1992, a Latin American Network for Whitefly and Geminivirus Management has been operating. Currently, 17 countries are involved with a Technical Committee (National Task Force) in each one. One of the activities of the network is to hold annual workshops for exchanging information. The objectives of the whitefly workshops are: (1) to present research, management, and technology transfer results carried out in the last year, (2) to evaluate research results based on the original Regional Research Plan, prepared in 1992, (3) to define priority research to be conducted in the next few years, based on the research results obtained to date.
The workshop series was organized following the devastating damage caused by whiteflies in Central America. A group of scientists organized the first Central America Workshop on Whiteflies and Geminiviruses in Costa Rica in 1992. At this workshop, a Regional Research Plan was written based on regional needs. This Plan includes four areas of research: (1) Taxonomy, Biology, and Ecology; (2) Epidemiology and Diagnostics of Geminiviruses; (3) Management; and (4) Technology Transfer. A subsequent Central America Workshop on Whiteflies and Geminiviruses was held in Nicaragua in 1993; the third one in Guatemala and the fourth in Honduras. Because of the interest and participation of many other Latin American countries in 1994, these workshops were renamed the "Latin American Workshops on Whiteflies and Geminiviruses".
The latest workshop, IV Latin American Workshop on Whiteflies and Geminiviruses, was held October 16-18, 1995, at El Zamorano, Honduras. There were about 150 participants representing 14 countries in Latin America and the Caribbean, including technicians, students, researchers, and chemical company representatives. International participants included Judith Brown (USA), Frank Byrne (England), Dan Gerling (Israel), Rafael Rivera-Bustamante (Mexico), and César Cardona (Colombia). The event was organized by entomologist Rafael Caballero, formerly of Escuela Agrícola Panamericana, El Zamorano.
The V Workshop will take place in Acapulco, Mexico, from September 29 to October 4th. For more details on the workshop, contact: Victor Manuel Pinto. For general information contact the Regional Coordinator, Dr. Luko Hilje, Plant Protection Unit, CATIE. Turrialba, Costa Rica. For general information contact: Organizing Committee, V Whitefly Workshop Department of Parasitology, UACH. For additional information you can reach L. Hilje or José L. Martinez-Carrillo (Mexico).
So far, thirteen issues of the quarterly newsletter Mosca Blanca al Dia (Whitefly Update) have been published. L. Hilje is the editor. Hopefully, by the end of April it will be available by Internet.
First International Workshop on Bemisia spp., "An Assessment of the Biology and Management Strategies ofBemisia spp. from an International Perspective"
The First International Workshop on Bemisia spp., "An Assessment of the Biology and Management Strategies of Bemisia spp. from an International Perspective", took place at Shoresh, Israel on 3-7 October 1994. Nine topics dealt with basic biology, population dynamics, damage expression, viruses, plant resistance, international cooperation in research and control, biological control, chemical and physical controls, and integrated pest management. Abstracts of the presentations were published as a special issue (No. 8) of the "Newsletter" and in part, also in Phytoparasitica, vol. 22, 1994. A book edited by D. Gerling and R. T. Mayer containing chapters written by participants is being published by Intercept, Great Britain.
The Second Workshop will be conducted in the spring of 1988 in Puerto Rico, and will be a joint meeting with virologists. For more information please contact Dr. R. T. Mayer, USDA, ARS, Horticulture Research Laboratory. Orlando, FL, USA or Dr. D. Maxwell, Plant Pathology, Univ. Of Wisconsin, Madison, WI, USA.
IOBC-WPRS Symposium on Biological Control of Bemisia
The IOBC-West Palearctic Regional Section WG on Biological Control of Whiteflies sponsored a symposium on "Biological Control of Bemisia, From Research to Implementation" on 19 December, 1995 at the Annual Meeting of the Entomological Society of America, Las Vegas, Nevada, USA. Seven speakers presented recent results of action programs conducted by a variety of public institutions in the USA.
Fourth Annual Research and Action Plan Review
The multidisciplinary conference, "Fourth Annual Progress Review of the 5-Year Research and Action Plan for Development of Management and Control Methodology for Silverleaf Whitefly" was held 4-6 February, 1996, at the Sheraton Gunter Hotel, San Antonio, Texas. The Proceedings of this year's conference should be available by June, 1996.
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