Bemisia Tabaci (Gennadius)

Crop Pest And The Principal Whitefly Vector Of Plant Viruses

A.N. Basu

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9780429723582
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

This monograph provides information on the basic and applied research on the morphology, taxonomy, biology, ecology and behaviour of the whitefly, Bemisia tabaci (Gennadius). It is devoted to the epidemiology of the diseases transmitted by the whitefly and the approaches to control.

1
Introduction

Bemisia tabaci (Gennadius) stands out as the most important member of the family Aleyrodidae for its grave impact on tropical and subtropical agriculture. The whitefly causes direct damage by feeding, soils the plant and the produce with honeydew and, more alarmingly, inflicts severe crop losses by transmitting a fairly large number of viral diseases.

Eggs of B. tabaci are usually laid on the lower surface of leaves. After hatching, the minute first-instar nymphs crawl actively in search of suitable sites for settling down. In fact, mobility during the entire period of immature development is limited to exploratory crawling during the early part of the first nymphal stage. The second-, third- and fourth-instar nymphs look like scale insects with atrophied legs and antennae. The fourth-instar nymphs stop feeding after some time. Since the adult develops inside the fourth-instar nymph, the same is referred to as the pupa.

The number of eggs laid by a single female in captivity is usually over 50 and the maximum recorded number exceeds 300. The species is capable of laying unfertilised eggs which develop into males only. The minimum period of development from egg to adult during the warmer parts of the year has been recorded as 11-14 days while the same period during the winter months may range from about 6-12 weeks. The number of generations in a year depends on local conditions and generally ranges between 11 and 15.

Since its first description as Aleurodes tabaci from Greece in 1889, the species has been described in various names over the years. Like most whitefly species, B. tabaci is identified on the basis of pupal morphology, which shows a wide range of host-correlated variations. Even the host-correlated variations in pupal characters in one season may be quite different in another. The wide range of intraspecific variations has led to a long list of synonyms, of which Bemisia gossypiperda Misra and Lamba is most frequently encountered in the economic literature.

Although B. tabaci is recognised as an important pest, quantitative data on the damage or economic thresholds are strikingly scant. Sucking of plant sap by large populations of nymphs and adults can greatly reduce plant vigour. Chlorotic spots appear at feeding sites on leaf surface, followed by wilting and leaf shedding. Such damage to foliage at the early stages of growth, affects development of the reproductive structures and consequently the yield may be greatly reduced. Mound (1965) experimentally demonstrated that general weakening of the plants due to whitefly infestation could cause serious reduction of cotton yield, due in part to a decreased number of bolls and in part to a decline in weight of seed and lint per boll. Byrne et al. (1990) cited an estimated population of 80 million adults per hectare on defoliated cotton regrowth in the Imperial Valley of California (pers. comm.). The impact of such population levels on plant vigour is readily imaginable. However, direct damage due to feeding would not appear to have been a matter of much concern, as reflected by the general lack of attention to this aspect in the literature.

Heavy colonisation of B. tabaci can cause serious indirect damage to some crops due to honeydew excreted by all insect stages, particularly the late nymphal instars. Accumulation of honeydew on leaf or fruit surfaces encourages growth of sooty moulds, which affect yield both in quantitative and qualitative terms. Deposition of the sticky exudate on cotton lint can cause serious problems during the ginning and spinning processes. In fact, the latter is the most serious economic damage caused to cotton by B. tabaci (Horowitz et al., 1984).

Bemisia tabaci is highly polyphagous and has been recorded on more than 500 species of plants including numerous field crops, ornamentals and weeds. Oddly enough, there is far more information on B. tabaci with respect to cotton than any other crop. The apparent reasons for this lopsided attention in favour of cotton are its commercial value, large-scale cultivation and the formidable pest complex affecting the crop. The transition of B. tabaci from a minor or locally important pest to a 'superpest' of cotton in various parts of the world today, merits discussion.

Bemisia tabaci first proved to be a serious pest of cotton in northern India, parts of which are now in Pakistan, in the late 1920s and early 1930s (Misra and Lamba, 1929; Husain and Trehan, 1933). Subsequently, severe whitefly infestation of cotton was recorded in the Sudan and Iran (1950s), El Salvador (1961), Mexico (1962), Brazil (1968), Turkey (1974), Israel (1976), Thailand (1978), Arizona and California (1981), and Ethiopia (1984). In all these areas B. tabaci had initially been a sporadic pest before reaching epidemic proportions (Horowitz, 1986). In the Sudan, for instance, the major concern of growers was the transmission of cotton leaf curl disease by the whitefly (Eveleens, 1983). With the advent of synthetic organic pesticides, the severity and frequency of infestations of B. tabaci became such that it rose from a secondary to a primary pest of Sudanese cotton in the late 1970s. Again, the whitefly was present on desert cotton in California for 14 years before it became a major pest in 1981, following an unprecedented population explosion (Dowell, 1990).

Such abrupt changes in pest stature of B. tabaci in various parts of the world are difficult to explain. The plausible explanations point to human involvement as the main factor due to increased production and protection of the same at any cost. The production thrust has necessitated wiping out natural vegetation to bring more and more land under cultivation and intensive cultivation of crops in order to achieve maximum return from that land. This enabled the whitefly to flourish with consequent pressure on plant-protection measures. The urgency to protect the crops led to overreliance and overuse of organic insecticides with limited effect on the whitefly, but drastic effect on the non-target beneficial natural enemies of the target species. Studies have shown that indiscriminate use of insecticides may not only induce resistance problems, but sublethal exposure to certain insecticides, such as DDT, may also stimulate the reproductive potential of B. tabaci. This is just an overview of the situation and different sets of factors may be responsible for unmanageable outbreaks in different agroecosystems. The sudden violent population explosions of B. tabaci in various parts of the world in the recent past and gross differences in susceptibility of the same plant species to the whitefly in different locations, suggest the existence of different biotypes of the whitefly. Recent studies on the biochemical characteristics provide evidence of such a possibility and a clear picture is expected to emerge in the near future.

Another disturbing factor is that B. tabaci has expanded its range of distribution to new parts of the world, apparently due to human-assisted introduction. The species has already been reported as a glasshouse pest in Canada, some North American areas and several countries in Europe where it was previously unknown. With the notorious greenhouse whitefly, Trialeurodes vaporariorum, already problematical, B. tabaci is likely to add new dimensions to the problem, especially as a potent virus vector. The appearance of B. tabaci on greenhouse plants such as poinsettia, may be a matter of considerable concern to growers but data on the magnitude of infestation and damage is not yet adequate to allow detailed discussion of this matter at the moment.

Incidentally, Bemisia tabaci is the first whitefly species to be implicated as a vector, the disease being cotton leaf curl in the Sudan and Nigeria (Kirkpatrick, 1930; Golding, 1930). Besides leaf curl of cotton, the white-fly-borne diseases that caused considerable concern during the early period, include tobacco leaf curl and African cassava mosaic. The notoriety of the species as a vector has risen so steadily over the years that this has obscured the direct injury due to dense insect colonisation. Understandably, much lower levels of whitefly populations can cause much more damage by transmitting diseases. In fact, with the exception of cotton and a few other crops, agriculturalists are much more concerned about the role of the whitefly as a vector than as a pest, due to colossal losses caused to various economic crops such as cassava, tobacco, grain legumes, tomato, chillies, squash, melons and various cucurbits, lettuce and papaya. A few loss estimates in recent years provide some idea of the impact of whitefly-borne diseases on crop production. Sastry and Singh (1973) estimated 20-95% loss in tomato yield due to tomato leaf curl disease in India. Bock (1982) reported yield losses due to bean golden mosaic virus to vary from 40-100%, depending on age, variety and possibly also on strain of the virus. Lettuce infectious yellows virus occurred pandemically in 1981 in the south-western United States (Duffus et al., 1986), virtually affecting every major crop grown in the desert region. Yield reduction in lettuce ranged from 50 to 70% and in sugar beet from 20 to 30%. Yield losses in cassava, the most important food crop grown in the African continent, due to the African cassava mosaic virus are staggering. On the basis of available data, Fauquet and Fargette (1990) estimated that the total reduction of cassava yield in Africa in plants raised from diseased cuttings, is at least 50% or 50 million metric tons and may be equivalent to $2 billion (U.S.). Mung bean yellow mosaic virus (MYMV) is a major constraint to the cultivation of grain legumes in India, especially mung bean (Phaseolus aureus) and black gram (P. mungo). According to Varma et al. (1991), in epidemic years losses due to MYMV have exceeded $300 million in three major crops, namely, black gram, mung bean and soybean. Brown and Bird (1992) have pointed out the increased prevalence as well as expanded distribution of whitefly-borne viruses during the last decade and the devastating impact. Yield losses range from 20 to 100%, depending on the crop, season, vector prevalence and other factors.

Most whitefly-borne diseases are now known to be caused by gemini-viruses, a group established as a taxon in 1978. This fascinating group of plant viruses with geminate particles and circular; single-stranded DNA genome, eluded detection for many decades, the reasons for which are not difficult to understand. Many geminiviruses are not mechanically trans-missible, thus requiring manipulation exclusively through the whitefly vector. The phloem-limited or phloem-restricted nature of geminiviruses adds to the difficulty; purification attempts have yielded only small quantities of virions. Moreover, the virus particles are generally fragile. Whitefly-transmitted geminiviruses cause an array of diseases in various parts of the world, sometimes exhibiting similar symptoms but often revealing distinct host ranges, which is quite confusing. Most of the viruses have not been adequately characterised as to host range, transmission characteristic and serology. However, with the emergence of new technologies, exciting progress in our knowledge of the biological, biochemical and molecular nature of some whitefly-borne viruses has been achieved in recent years. Enhanced knowledge of the composition, genomic organisation and virusencoded polypeptides has certainly improved diagnostic techniques and removed earlier confusion regarding the previously unrecognised whitefly-borne geminiviruses. The efforts so far have managed to bring order out of chaos but much remains to be done to isolate and characterise the viruses involved in whitefly-transmitted diseases.

The high reproductive potential, mobility, polyphagy and growing prevalence of the whitefly and the diseases borne by it in more and more areas, account for the intense attention that B. tabaci has been receiving in recent years. The proceedings of a symposium on various aspects of B. tabaci appeared as a special volume of Agriculture, Ecosystems and Environment, Vol. 17, 152 pp. (1986). In the same year a Literature Survey of B. tabaci edited by Cock (1986) was brought out by the C.A.B. International Institute of Biological Control with an annotated bibliography. In view of the growing notoriety of the whitefly, a newsletter titled BEMISIA was launched by the working group on B. tabaci, led by Professor Dan Gerling of the Tel Aviv University, Israel and six issues of the same have already appeared. Bemisia tabaci has also figured prominently in the recent multiauthored book on whiteflies edited by Gerling (1990), dealing with broad aspects of the systematics, diversity, ecology, behaviour, management and control of whiteflies in general. Besides the above, research papers on diverse aspects of B. tabaci and related topics have been steadily accumulating, which are multidisciplinary in nature. The present venture is the result of a long-felt need for a monograph presenting the essence of diverse and scattered information for the benefit of students, researchers and others involved in whitefly-related problems. An attempt has been made to provide an up-to-date projection of the development in the fundamental and applied directions of studies on B. tabaci as a pest and vector of plant diseases.

References

Bock, K.R. (1982). Geminivirus diseases in tropical crops. Plant Disease 66: 266-270.

Brown, J.K. and Bird, J. (1992). Whitefly-transmitted geminiviruses and associated disorders in the Americas and the Caribbean Basin. Plant Disease 76 (3): 220-225.

Byrne, D.N., Bellows, T.S. and Parrella, M.P. (1990). Whiteflies in agricultural systems. In: Whiteflies: Their Bionomics, Pest Status and Management (D. Gerling, ed.). Intercept, Wimborne, England, pp. 227-261.

Dowell, R.V. (1990). Integrating biological control of whiteflies into crop management systems. In: Whiteflies: Their Bionomics, Pest Status and Management (D. Gerling, ed.). Intercept, Wimborne, England, pp. 315-335.

Duffus, J.E., Larsen, R.C. and Liu, H.Y. (1986). Lettuce infectious yellows virus—a new type of whitefly transmitted virus. Phytopathology 76: 97-100.

Eveleens, K.G. (1983). Cotton-insect control in the Sudan Gezira: analysis of a crisis. Crop Protection 2: 273-287.

Fauquet, C. and Fargette, D. (1990). African cassava mosaic virus: etiology, epidemiology, and control. Plant Disease 74 (6): 404-411.

Gerling, D. (ed.) (1990). Whiteflies: Their Bionomics, Pest Status and Management Intercept, Wimborne, England, 348 pp.

Golding, F.D. (1930). A vector of leaf curl of cotton in southern Nigeria. Empire Cotton Growing Review 7: 120-126.

Horowitz, A.R. (1986). Population dynamics of Bemisia tabaci (Gennadius) with special emphasis on cotton fields. Agriculture, Ecosystems and Environment 17: 37-47.

Horowitz, A.R., Podoler, H. and Gerling, D. (1984). Life-table analysis of the tobacco whitefly Bemisia tabaci (Gennadius) in cotton fields in Israel. Acta Oecol., Oecoi. Appiic. 5: 221-233.

Husain, M.A. and Trehan, K.N. (1933). Observations on the life history, bionomics and control of the whitefly of cotton (Bemisia gossypiperda M. & L.). Indian J. Agric. Sci. 3: 701-753.

Kirkpatrick, T.W. (1930). Leaf curl in cotton. Nature 125: 85-97.

Misra, C.S. and Lamba, K.S. (1929). The cotton whitefly (Bemisia gossypiperda, n. sp.) Bull. Agric. Res. Inst., Pusa 196, 7 pp.

Mound, L.A. (1965). Effect of whitefly (Bemisia tabaci) on cotton in the Sudan...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• PREFACE
• Contents
• 1. INTRODUCTION
• 2. MORPHOLOGY AND TAXONOMY
• 3. DISTRIBUTION
• 4. HOST PLANTS
• 5. BIONOMICS
• 6. SAMPLING TECHNIQUES FOR POPULATION STUDIES OF BEMISIA TABACI
• 7. POPULATION DYNAMICS
• 8. BEHAVIOURAL AND ECOLOGICAL ASPECTS OF HOST SELECTION AND SPATIAL DISTRIBUTION
• 9. PLANT RESISTANCE TO BEMISIA TABACI
• 10. NATURAL ENEMIES
• 11. CHEMICAL CONTROL
• 12. BEMISIA TABACI AND PLANT DISEASES
• 13. EPIDEMIOLOGY OF WHITEFLY-BORNE VIRUSES AND APPROACHES TO CONTROL