cotton-insect control in the sudan gezira: analysis of a crisis

CROP PROTECTION (1983) 2 (3), 273-287

Cotton-insect control in the Sudan Gezira: analysis of a crisis

K, G EVELEENS*

Food and Agriculture Organization of the United Nations, 00100 Rome, Italy

ABSTRACT. In the Sudan Gezira, cotton growing is in a state of crisis. Important among the underlying factors are escalating costs of crop spraying in conjunction with increasing infestations of the cotton whitefly, Bemisia tabaci Genn. It is argued that the whitefly has risen to the status of a major pest as a result of certain features of current control strategies, based on intensive aerial spraying. The significance is discussed of the widespread adoption during the period 1972-80 of so-called 'package deal' arrangements. Under this scheme, the Gezira management delegated responsibility for all aspects of insect control to commercial companies. The crop-protection predicament is considered in the larger context of the entire complex of factors which limit cotton productivity, relating not only to crop husbandry in general, but also to socio-economic and public health problems. Development of a more economic and ecologically sound insect-control strategy by itself would not solve the crisis. Nevertheless, without relief from the current crushing financial burden of crop spraying, the means for urgently needed remedial action in the various spheres involved are severely curtailed.

I n t r o d u c t i o n

Cotton, wherever grown, usually receives more insecticide than other agricultural crops and the drawback of over-reliance on chemical insecticides in pest control is most strikingly manifest in the cotton agroecosystem. Therefore, it is largely the cotton entomologist's accumulated experience of failures in pest control resulting from the combined effects of pest resistance, pest resurgence and the outbreak of secondary pests, which has led to the concept of a characteristic sequence of five phases of crop growing in relation to pest control: (1) subsistence; (2) exploitation; (3) crisis; (4) disaster; (5) integrated control (Smith, 1969). Recent reviews on this subject (Brader, 1976, 1979; Bottrell and Adkisson, 1977; Van den Bosch, 1978) show that elements of this pattern have been identified in cotton growing in various countries, albeit with considerable variations in the actual development of the

* Present address: Malang Research Institute for Food Crops, Jalan Wilis 10, Malang, East Java, Indonesia. 0261-2194/83/03/0273-15503.00 © 1983 Butterworth & Co (Publishers) Ltd

274 Cotton-insect control crisis in the Sudan Gezira

sequence, due to differences in particulars of pest situations, cropping conditions, and responses of farmers and involved institutions to increasing crop-protection problems.

Among the cotton-growing areas which have entered a crisis phase in pest management is the Sudan Gezira. This important agricultural region has been the mainstay of the economy of the Sudan for many years and the implications of its declining productivity are grave. A complex of interacting factors is involved, relating not only to control of insect pests, but also to various socio,economic aspects. This report represents an attempt to identify the causes of the acute aggravation of problems relative to insect pests of cotton and to appraise the significance of these problems in the context of the larger crisis in the Gezira Scheme.

Cotton growing and the crop-protection problem in the Gezira

The Gezira Scheme and its administration

The Gezira is an alluvial plain, situated in the fork of the Blue and White Nile, south of the confluence of these rivers at Khartoum. Its location is in the climatological zone of semi-desert with average annual rainfall ranging from 250 mm in the north to 450 mm in the south. Water for large-scale irrigated agriculture is available from the Sennar dam on the Blue Nile near the southern end of the Scheme. The land has a slight but regular slope from south to north and irrigation is by gravity flow through a network of canals. At present, approximately 840 000 hectares of land are under cultivation. Each season (July-April), about a quarter of this area is sown with cotton, in a rotational sequence involving various food crops (wheat, groundnuts, sorghum, vegetables) and, in part of the Scheme, a fallow.

Organizationally, the Gezira Scheme is one large farm (in fact, the largest in the world) with the land owned by the government, cultivated by almost 100 000 tenant farmers and centrally administered by the Sudan Gezira Board. The size of a tenancy of cotton is 4-2 ha. In this production relationship, the tenants are responsible for most aspects of crop husbandry, but not for crop protection: responsibility for all actions in this area is with the Sudan Gezira Board through its Crop Protection Department. Recommendations on choice of pesticides for crop spraying and modes of application are issued by the Agricultural Research Corporation of the Ministry of Agriculture.

Cotton crop production and protection, past and present

Archaeological findings indicate that cotton was grown in the Sudan as early as 50 BC (Bacon, ~948). Traditionally, the Gezira was a centre for home production of cotton fibres, based on old Varieties such as the perennial 'Sennar Tree Cotton' (Gossypium arboreum race soudanense). Completion of the dam on the Blue Nile in 1925, with subsequent rapid expansion of irrigated cotton-growing into large-scale commercial enterprise, marked the transition from the subsistence to the exploitation phase. From that time on, varieties planted were predominantly of the long-staple (G. barbadense) type.

Shortly after the initiation of large-scale irrigated agriculture in the Gezira, a severe outbreak of leaf curl virus, transmitted by the cotton whitefly, Bemisia tabaci Genn., threatened to halt the expansion of cotton growing. This challenge was

K. G. EVELEENS 275

successfully met by a combination of cultural and sanitary practices (elimination of ratoon cotton, important as a source of seasonal carry-over of virus) and varietal resistance. The development of management strategies for leaf curl and other major diseases and pests during the first two decades after the start of the Scheme has been reviewed by Crowther (1948).

An important insect which appeared to be hard to suppress by such non-chemical means of control was the cotton jassid, Empoasca lybica de Berg. Severe depredations by this insect constituted the principal reason for the rapid spread in use of organosynthetic insecticides, once these became available at the end of the Second World War, In the season 1945-46 D D T was applied on 600 ha, and the area treated increased rapidly in the following years.

As shown by Snow and Taylor (1952), this treatment significantly increased crop production. The same authors found that maximum benefit from the spraying was obtained by application early in the growing season and that the positive response to the insecticide application was greater in cotton which had received nitrogenous fertilizer than in non-fertilized cotton. Consequently, an early-season spraying with D D T and application of N-fertilizer had been adopted as standard practice in all of the Gezira by the 1954-55 growing season.

Until about 1960, pest control by chemicals was mostly limited to this single early-season spray, first applied by tractor-mounted equipment and later by aircraft. In the 1960-61 season, jassid attacks were extremely heavy. Satisfactory control was not achieved any more with either D D T or endrin, by then also applied in the Gezira. At the same time, the cotton whitefly was also becoming more and more injurious. Whereas the main significance of this insect in the early years of the Scheme had been in its transmission of leaf curl disease, the principal concern was now feeding damage and honeydew contamination resulting from high infestations.

As D D T was also ineffective against whitefly, alternative insecticides were tried. An organophosphate, dimethoate, appeared to be suitable for suppression of both jassid and whitefly, and was widely adopted. A complication arose, however, with the sudden eruption around 1963 of severe infestations of the American bollworm, Heliothis armigera-(Hiibn.), against which dimethoate was ineffective. This development prompted the introduction of insecticidal mixtures such as D D T - dimethoate, with a range of action capable of controlling the jassid/bollworm/ whitefly complex. Although from that time on the jassid was effectively suppressed, the overall pest situation, particularly in respect of the whitefly, continued to get worse.

This brought about an increase in the average number of sprays per season, from one in 1960-61 to eight in 1980-81. The resulting increase in cost of spraying was further compounded by the steep rise in prices of agricultural chemicals after the oil crisis of 1973. These changes in the Gezira over the last 25 years in use of insecticides and in expenditure (purchase of insecticides and cost of application) are presented in Figure 1. The accelerated increase in outlay for crop spraying of recent years is striking: it amounts to more than 600~o between the seasons of 1972-73 and 1980-81 (£SUD 10"74/ha and £ S U D 77.02/ha respectively, with at thepresent rate of exchange £ S U D 1-00 equalling US $1-11).

The cotton area and average yields over the period 1945-50 to 1975-81 are reproduced in Figure 2. The cotton acreage underwent a sustained increase from the inception of the Scheme to a maximum in the 1973-74 and 1974-75 seasons, when slightly more than 250 000 ha were under cotton. In subsequent years the area


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FIGURE 1. Seasonal averages of numbers of sprays (e - - - e) and average expenditure for crop spraying per hectare in Sudanese pounds (purchase of insecticides and cost of applications, + +) in selected seasons at 4-year intervals. Figures for package-deal areas (see text) not included. Source: Sudan Gezira Board, Barakat.

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Yields and areas grown with cotton, averaged for 5-year periods from 1945-75 and for the 6-year period 1975-81. Yield data relate to long-staple, G. barbadense, varieties only. Figures for medium-staple, G. hirsutum, varieties grown in some parts of the Gezira from 1976 on, have not been included in the average yield determination for the latest period. After data provided by the Sudan Gezira Board.

K. G. EVELLENS 277

declined, as marginal land was taken out of production due to lack of sufficient crop returns from the increasingly costly inputs. At the same time, yields dropped from an average of 1653 kg seed cotton per hectare for the period 1970-75, to 1020 kg/ha for the period 1975-81 (Figure 2).

Relating this development of cotton in the Gezira to the phases referred to in the introduction, it appears that the heyday of the exploitation phase occurred during the 1950s. At this time, the use of fertilizer and pesticides at a relatively low cost showed the highest crop returns from cotton-growing in the history of the Scheme (Pollard, 1981). The crisis phase was heralded by the rapidly increasing frequency of spraying during the mid-1960s. The subsequent explosive increase in cost of crop protection, in conjunction with continuing aggravation of pest problems, declining yields, and abandonment of marginal areas, are all signs of impending disaster in cotton growing (Falcon and Smith, 1973).

Status of principal cotton pests

The two main target pests for intensified crop spraying in recent years are the American bollworm (H. armigera) and the cotton whitefly (B. tabaci). A characteristic common to both these insects is that they reached the status of major pests at a relatively late stage in the development of the Scheme. Until about 20 years ago, both species were occasional, rather minor pests, less important than the cotton jassid (E. lybica). An understanding of the current crop-protection predicament first requ~ires identification of the main factors underlying these changes in pest spectrum.

Empoasca lybica, which in the early years of chemical crop protection was the most damaging cotton insect, has of late dwindled into relative insignificance. It is effectively controlled by most of the insecticides registered for use on cotton in the Sudan.

Heliothis armigera underwent a fairly sudden change from a relatively minor into a major pest in the Gezira, approximately20 years ago. Possible factors underlying this development according to Balla (1981) include: (1) accelerated expansion of areas under cotton and other host plants (groundnut and sorghum) at that time; (2) advancement of the sowing date for cotton; (3) start of cultivation of early-flowering Gossypium hirsutum in addition to the traditional G. barbadense varieties; (4) adverse effects on natural biological control agents from intensified chemical spraying. The incidence of H. armigera is highest on plants during the developmental stage of crop formation, i.e. the phases of squaring, flowering and early boll development; afterwards, populations naturally decline.

Bemisia tabaci up to the mid-1950s was characteristically an occasional early- and mid-season pest, with incidence varying widely from year to year. Since then it has come to reach the status of the most destructive insect, infesting cotton in an

annually recurrent, all-season outbreak condition. The impact on crop production is considerable, not only in terms of quantity of yield of seed cotton but also of quality of lint, because of excessive contamination with honeydew secretion (Gamed, 1970). Factors implicated in this development, as summarized by Abdel Rahman and Eveleens (1980), are: (1) the intensification and diversification of cropping; (2) changes in husbandry practices, particularly the increased use of nitrogenous fertilizer; (3) outbreak-inducing effects of D D T and other broad-spectrum insecticides.

Soon after the initiation of large-scale spraying with D D T against jassid it was


found that whitefly populations increased and the attainment of peak numbers was delayed (Joyce, 1955; Joyce and Roberts, 1959). The underlying cause of this phenomenon has been the subject of considerable debate. An early hypothesis that D D T would exercise a stimulating effect on egg hatching (van der Laan, 1961) was later experimentally refuted (El Bashir, 1974). Other explanations include the removal of competition by jassids (Joyce, 1955) and adverse effects on parasites and predators (Gameel, 1969). More recently, investigations into another species of whitefly (the banded-wing whitefly, Trialeurodes abutilonea (Haldeman)) infesting cotton in the United States, have shown that the induction of whitefly outbreaks by D D T and other broad-spectrum insecticides is through effects on natural enemies (Clower and Watwe, 1973; Watwe and Clower, 1976).

The change in cotton whitefly in the Gezira can be seen by comparing Figures 3 and 4. Figure 3 is based on data presented in a report on the 1932-33 growing season, when cotton was not yet sprayed. This season was characterized by unusually heavy whitefly attacks, which were reported to result from an abundant growth of weeds which acted as food plants for early population build-up, after exceptionally heavy rainfall before and during the cotton-planting time (Cowland, 1933), After peak incidence in October, natural mortality factors quickly brought about a population decline to insignificant levels. Through the following 25-30 years this pattern of whitefly as an occasional, early- to mid-season pest, essentially remained unchanged, as demonstrated by recorded population figures from untreated cotton (Joyce and R0berts, 1959; Razoux Schultz, Jackson and Faulkner, 1967). A subsequent report of seasonal population trends in unsprayed plots in the period 1964-68 (Gameel, 1970) indicated an increase in whitefly incidence late in the growing season, and the following years witnessed a continuing exacerbation of infestations to the present persistence at sustained high levels, through the late stage of the season into picking time. This is illustrated in Figure 4, which is based on counts in commercially grown, sprayed cotton in the Gezira. In spite of eight sprays with various insecticides, including dimethoate and quinalphos as reportedly effective compounds against whitefly, the numbers of whitefly steadily increased through January. A better understanding of this emergence of whitefly as the most harmful

I000-

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Months

FIGURE 3. Seasonal population curve of cotton whitefly in an outbreak during the early years of the Gezira Scheme, prior to the use of insecticides. After data from Cowland (1933) for the season 1932-33.

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FIGURE 4.

K. G. EVELEENS

Spray applications

t t t t t t t

J

Oct Nov Dec Jan Feb Mar

Months

Seasonal population curve of cotton whitefly in an area, sprayed eight times in a standard treatment regime involving the following materials (figures in parentheses refer to amounts of active ingredient in kg/ha): 1. DDT 25%/dimethoate 32% (1.1/0.45); 2. dimethoate 32% (0-45); 3. quinalphos 25% (0"6); 4. endosulfan 35%/dimethoate 32% (1.0/0.45); 5. dimethoate 32% (0.45); 6. quinalphos 25°/0 (0-6); 7. and 8. dimethoate 32% (0.45). Season 1979-80.

279

pest, and the least amenable to current means of control, requires a closer look at the evolving pattern of crop-protection strategies during the same period.

Crop-protection strategies

Control of the three major pests is exclusively based on aerial spraying of insecticides. During the 1970s, in the Gezira and elsewhere, two general approaches were adopted, known respectively as the 'conventional method' and the 'package deal'. The designation 'conventional' is used here for those situations in which the Gezira Board assumes responsibility for policy formulation and execution of crop-protection measures. Purchase of chemicals and contracting of aerial application proceed by tender. Decisions on timing of application and choice of insecticides are made on the basis of scouting for pests which is carried out under the supervision of so-called group entomologists, each responsible for an area of about 15000 ha of cotton. Sprays are applied by aircraft, either in low volume (approximately 20 f/ha) or in ultra-low volume (approximately 5 f/ha).

As mentioned earlier, the conventional approach began about 30 years ago, when the yield-increasing effect of a single application of D D T against cotton jassid was demonstrated convincingly (Snow and Taylor, 1952). Subsequent emergence of additional pest problems prompted an increase in the number of sprays and the use of mixtures of insecticides rather than a single one. D D T remained, however, the most used insecticide in the conventional method up to 1981, when its further use in cotton spraying was prohibited.

The term 'package deal' refers to a contractual agreement between the management of a cotton-growing scheme and an agrochemical company, whereby the latter assumes responsibility for all operations for control of insect pests, and


guarantees a certain minimum yield. Payment for these services is in the form of a fixed rate per unit area. The package deal came into being as a response to the emergence of H. armigera as a major pest from 1963 onward. On the basis of the results of a co-operative research project to analyse the factors underlying the development of infestations and distributional patterns of this pest on cotton (Haggis, 1981), a control strategy was formulated which centred around the following tenets (Joyce, 1973, 1976, 1981):

1. American bollworm is the key pest of cotton in the Gezira; 2. Waves of oviposition occur synchronously over substantial areas in relation to

seasonal weather changes; 3. Incipient oviposition calls for area-wide spraying to prevent destructive

outbreaks.

In this conceptual framework, strategy implementation consisted of execution of 'synoptic surveys' for early detection of infestations, and subsequent synchronized control operations over entire areas considered to be at hazard. The capability to maximize daily coverage in aerial spraying operations was important for this last objective and was achieved by adopting application at ULV rates with increased swath width. In addition, a system was introduced of sequential application of two sprays at so-called split dosage: that is, two sprays, both at half the recommended rate against American bollworm, were applied at an interval of 2-3 days; this period corresponds to the incubation time of the eggs. The insecticide used most throughout the operation has been monocrotophos in a proprietary ULV formulation developed by the company which initiated the system.

The package-deal strategy was advocated as offering an ecologically more attractive alternative to the conventional approach by virtue of higher target specificity of sprays and the substitution of an organophosphate, non-persistent compound (monocrotophos) for D D T and other persistent materials being used in the conventional system. It was implied that this approach would be more suited to avoid the pitfalls of pest resistance and the emergence of new pests.* At first, such claims seemed to be justified by higher yields in areas under the package deal (see below). After the adoption of the system as a commercial practice in the 1972-73 season, the areas allotted to package-deal contractors increased to a maximum in the 1978-79 season, when almost half of the Gezira, and substantial portions of other schemes, were covered.

More recently, however, problems arose due to the rapidly increasing severity of whitefly resurgencies, and by 1981 the situation had deteriorated to the point that package-deal arrangements were terminated altogether. The yearly acreages under the package arrangement with the initiating company, and the differences in yield with areas under conventional crop-protection management, are shown in Figure 5. It can be seen that, in the final years, yields in the package-deal areas were, if anything, lower than those in the conventional areas.

At the end of the 1980-81 season, in which much of the Sudan's irrigated cotton suffered from heavy, largely uncontrollable whitefly outbreaks, a ministerial decree was issued with considerable consequence for cotton-pest control. Firstly, all use of

* This was argued by R.J.V. Joyce at a Symposium on Crop Pest Management in Khartoum (February 1978) in a presentation entitled 'Pest control by chemicals in the Sudan Gezira: an exercise in applied ecology'.

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K.G. EVELEENS 281

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FIGURE 5. Areas of the Gezira Scheme under package-deal contract with the initiating company and percentages of yield increase or decrease compared with cotton under conventional crop-protection management. The latter figures apply to long-staple (G. barbadense) cotton only and have been calculated with the formula adopted by the Sudan Gezira Board: y = 100 (T/T--A/_A), in which T=average group yield in the area under package deal, T=average group yield in the same area in a standard 10-year period preceding the initiation of the package deal (1961-62 to 1970-71); A =average group yield in the entire Gezira; ¢J=average group yield in the entire Gezira for the standard 10-year period 1961-62 to 1970-71. Compiled' from statistics made available by the Sudan Gezira Board.

D D T and DDT-containing mixtures in cotton spraying was prohibited; secondly, no package-deal contracts were to be allowed any more.

Both decisions entailed important changes from established practices and reflected increasing dissatisfaction with prevailing pest-control strategies. The intrinsic weakness of the conventional approach with respect to whitefly was its emphasis on broad-spectrum insecticides with D D T as the principal compound. This practice remained, in essence, xmchanged until 1981, in spite of early evidence of outbreak-inducing effects by D D T on whitefly in cotton in the Sudan and, as


discussed in the preceding section, more recently also in cotton in the United States. As to the package deal, although this strategy was to some extent successful in its original objective of providing better control of American bollworm, an effective response to the more intractable whitefly problem was precluded because of inherent limitations in orientation: it was geared to a particular pest (H. armigera), to be controlled with a particular insecticide (monocrotophos), using a particular strategy (area-wide, sequential ULV sprayings). Acknowledgement of a whitefly problem came more as an afterthought than as an integral part of the concept. This was indicated by the continuing adherence to the practice of repetitive applications of monocrotophos for several years after its relative efficacy against whitefly in the early years of the package deal had been lost, probably due to the development of resistance (FAO, 1980). The demise of the package deal was thus precipitated by the tendency in such arrangements to put the cart of marketing interests before the horse of crop protection.

With the shortcomings of control strategies vis-a-vis the whitefly forcibly exposed by the course of events, this pest gained national notoriety. The resulting inclination to view effective whitefly control as the most pressing need for irrigated cotton sometimes hampered due consideration of other elements in the complex of factors underlying the problems in the Gezira. A proper perspective on this matter first requires a closer look at the relative significance of the crop-protection crisis in the wider context of other major constraints in the functioning of the Scheme.

Crop protection and the larger crisis of the Gezira

For a long time the Gezira stood as a highly successful model of the way in which the rich potential of an area could be turned to the benefit of an entire nation. In the words of a noted conservationist: 'Early this century the possibilities of gearing economic progress to social advancement from low standards of life were remarkably demonstrated here by the Gezira project. As a planned contribution to economic and social welfare this project has rarely again been matched. . . ' (Nicholson, 1970). Such euphoria has, of late, been dissipated in the face of mounting problems in the Scheme. Titles of recent analyses of the situation reflect the resulting disillusion- ment: The Gezira, an Illusion of Development (Barnett, 1977) and The Gezira: a Study of Failure (Pollard, 1981). As causes of the present malaise, various factors are being listed, ranging from cultural impediments (incompatibility of traditional living patterns with the imposed production structure, stemming from the colonial period) to socio-economic problems and various constraints in the areas of crop husbandry and public health. Important in the last category is the formidable problem posed by the increasing incidence of schistosomiasis, resulting from the introduction of year-round irrigation with the intensification of cropping. According to the latest available figures, the prevalence in the Gezira of schistosomiasis has risen to over 50~/o, with no less than 80% of school-age children and adult males infected (Cheesmond and Fenwick, 1981). Both Schistosoma haematobium and S. mansoni occur, but the latter, more debilitating species is now far more common than the former. By comparison, 30 years ago infection percentages of 9~o for both S. mansoni and S. haematobium were reported (Greany, 1952).

Against this background, how important is the pest-control emergency? The limitations of considering pest-control aspects-in isolation are demonstrated by an analysis where, on the basis of extrapolation of cost/benefit figures of crop protection

K. G. EYELEENS 283

up to about ten years ago, it was concluded that the existing chemical crop- protection strategy in the Gezira was economically sound and would remain so for a long time to come (Bartsch, 1978). This view appeared over-optimistic. For one thing, the accelerated price increases of agricultural chemicals following the oil crisis in 1973 made all previous cost estimations obsolete. Moreover, and perhaps more important, the prerequisite for a realistic appraisal of the given situation is consideration of modes of interaction between factors in the various categories involved. For example, the dwindling margins of profit due to stagnating returns and increasing cost of production adversely affect tenant's motivation and interest, resulting in acceleration of the drift of rural people into cities, senescence in remaining populations, and increase in absentee tenantships. All this leads to more and more failures in the proper execution of those elementary husbandry practices without which cotton growing is not economically viable. The same applies, mutatis mutandis, for the correlation between the tenant's performance and the public health problem posed by the increased incidence of schistosomiasis. Altogether, the system is trapped in a vicious circle of decreasing returns and a complex of multiple constraints, interacting in a mutually reinforcing fashion.

It is in the light of this complex of positive feedbacks, underlying the current process of decline, that the matter of relative significance of the crop-protection crisis is to be considered. Unquestionably, the present outlay for insecticide spraying constitutes a crushing burden on the Scheme. The average annual cost of cotton-insect control per tenancy of 4.2 ha has now surpassed the amount of £SUD300.00 (see Figure 1), which far exceeds the average net return from cotton-growing for the •tenants, even the more successful ones (Turner, 1980). It goes without saying that an expenditure of this magnitude is a major drain on the resources of the Scheme needed for effective remedial action in the various problem areas. A precondition for execution of such corrective measures is a re-orientation away from the practice of applying insecticides at ever-increasing cost without achieving notable improvements in insect control. The drawbacks of the current situation, which bears all the hallmarks of a 'pesticide syndrome' as described by Doutt and Smith (197 i), call for a concerted effort to explore alternative strategies.

Ret rospec t and prospect

With the benefit of hindsight, one may argue that the emergence of massive late-season infestations of whitefly as the most formidable problem in protection of the cotton crop was a consequence of the main orientation of pest management since the area-wide adoption of chemical control three decades ago. The prevailing trend during the subsequent years was one of intensification of early- and mid-season crop spraying by aircraft of broad-spectrum, non-selective mixtures of insecticides. This practice weakened the operation of natural mortality factors which, in the past, used to limit whitefly to an occasional early- and mid-season pest (cf. Figure 3). Among these mortality factors, some species of hymenopterous parasites belonging to the family Aphelinidae are of major significance. Investigations of the incidence of these parasites in unsprayed cotton indicated a delayed density-dependent pattern of population increase (Gameel, 1969): that is, at first they are relatively scarce and lagging behind the rapid build-up of whitefly in early season; later on, however, rates of parasitization increase to levels which suggest effective contribution to natural mortality. The regime of frequent treatments with broad-spectrum mixtures of


insecticides militates against realization of this potential for natural biological control. In such heavily sprayed cotton, whitefly populations, relatively free from suppression by natural biological control agents, rapidly increase to injurious levels at more advanced developmental stages of the cotton plant. At the same time, however, aerial spraying against whitefly becomes increasingly ineffective with the maturation of the cotton. Contact action is hampered because the virtually closed upper canopy catches most of the spray droplets, whereas the target, i.e. whitefly at the underside of the foliage at the lower plant levels, is largely sheltered. Systemic action is adversely affected by the decline in translocation in advanced stages of plant maturity and senescence.

The aforementioned ministerial decision of 1981, prohibiting further use of D D T and terminating all package-deal arrangements, removed from the scene of cotton-crop protection major elements implicated in the whitefly predicament, but an effective alternative approach still remained to be developed. The immediate result of the decision was a shift in the spectrum of chemicals applied in the 1981-82 season. Substitutes for D D T were endosulfan and some newly registered pyrethroid compounds. In addition, for control of whitefly, increasing use was made of soil applications with the systemic carbamate aldicarb (Temik 15G), in spite of difficulties in implementing the necessary safety precautions for humans and livestock, which are needed for this extremely hazardous compound (see Matthews and Clayphon, 1973).

Although whitefly infestations in the 1981-82 season turned out to be somewhat less severe than in preceding years, one may question whether the strategy of mere substitution of one set of chemicals for another will provide lasting relief. Case histories of sequences of cotton-growing phases in relation to pest control, referred to in the introductory section, indicated that, if crisis situations could be overcome, this was achieved by implementation of more profound changes in approach. For example, in cotton in north-east Mexico, continuing reliance on chemical control only, in the face of deepening crisis caused by aggravation of pest problems, led to economic disaster and collapse of cotton growing. At the same time, in the adjacent cotton area of southern Texas, suffering from a similar complex of pests, such collapse was averted by implementation of the results of a concerted research effort, aimed at a more rational use of chemicals and incorporation of non-chemical elements in the cotton-pest management regime (Van den Bosch, 1978).

Whereas successful use of integrated pest control in cotton has been documented in several instances, one notices at the same time that the potential of integrated pest control is often slow to be turned into effective application. On a global scale, the discrepancy between promise and performance is most pronounced in developing countries, because of a variety of constraints. These are partly of technical and socio-economic nature (Brader, 1981) and partly result from institutional limitations in scientific expertise, budget and infrastructure, leading to insufficient execution of essential activities at the various levels of development and implementation of integrated pest control: research, education, training and extension (Beingolea, 1981).

Strengthening these aspects is particularly important for development of a sustained yield agroecosystem in a cash crop like cotton. In the Sudan and elsewhere, increasing pressure towards maximization of crop returns to generate badly needed export revenue has set the stage for adoption of those strategies which promise immediate yield increases. Forceful promotion of chemical insecticides as the most

K. G. EVELEENS 285

effective means to accomplish such yield increases has resulted in a shift in orientation of crop-protection research from the balanced, multidisciplinary approach in the early decades of the Scheme (cf. Crowther, 1948) to a lopsided system, over-emphasizing aspects of chemical control. As a result, there was increasing engagement of agricultural entomologists in routine insecticide regist- ration trials, at the detriment of pursuit of other research activities relating to cotton-pest management (FAO, 1981). This development culminated in a prolifer- ation of package-deal arrangements between scheme managements and agrochemical companies, in which national crop-protection expertise was bypassed altogether. The collapse of the package deal served the purpose of providing a very forceful and instructive, albeit unwanted, demonstration of how such an overly narrow orientation in cotton-pest management becomes self-defeating. Moreover, it pointed to the existence of a fundamental imbalance. As argued by Zimdahl (1978), in public controversies concerning crop-protection strategies, distinction is to be maintained between the different functions of scientists, seeking scientific truth, and of advocates, striving for acceptance of certain opinions. Developments in the Sudan over the last 15-20 years have tended to mute the voices of scientists in the clamouring of advocates promoting specific chemically orientated approaches in pest control.

What is needed at this stage is a re-assertion of the scientist specializing in protection of the cotton crop to pick up the pieces and, with a realistic view of the problems of cotton-growing in the Gezira, to aim for the development of pest-management strategies utilizing not only chemical but also non-chemical means. With regard to the principal problem, the cotton whitefly, research in the following areas is considered to be essential (FAO, 1980): varietal resistance in cotton against whitefly; strengthening the potential for natural biological control by parasites through application of more selective insecticides for suppression of early-season American bollworm, in conjunction with a re-assessment of treatment thresholds for this pest; manipulation of the irrigation regime to minimize unnecessary late-season vegetative growth on which whitefly continues to breed. In addition, in view of the aforementioned relative ineffectiveness of late-season spraying by aircraft, the possibility is to be considered of substituting, under certain conditions, ground spraying for aerial application. The logistics of such a change would necessitate a departure from the established, centralized system of decision- making and management towards more local autonomy in formulation and execution of crop-protection policies. This would probably also require a break in the long tradition of non-involvement of tenants in crop-protection matters. Such tenants' engagement might actually be considered already overdue, in the light of current trends to remedy shortcomings in the functioning of the Gezira Scheme by providing more scope for differential rewarding of individual tenants according to yield performance.

In conclusion, crop protection in the Gezira at present is at the crossroads. Formerly established practices have been found to be inadequate and have been rejected. At the same time, effective alternatives still remain to be developed and altogether the situation substantiates the remark by Norgaard (1976) that the introduction of new knowledge in pest management tends to be more difficult than the introduction of new materials. But mere introduction of new materials no longer suffices: new knowledge is needed to provide a crop-protection system which is more permanent, economical and ecologically sound than the present one.

286 Cotton-insect controI crisis in the Sudan Gezira

A c k n o w l e d g e m e n t s

Fo r valuable c o m m e n t s on earlier drafts o f this paper I am m u c h obliged to D . G . Bottrell , L . Brader , W. Job in and L. Razoux Schultz. Assistance o f the Crop Pro tec t ion D e p a r t m e n t o f the Sudan Gezira Board in provid ing various statistics on co t ton -c rop p roduc t ion and protec t ion is gratefully acknowledged. I am also indebted to the fol lowing s t a f fm e m be r s o f the Agricul tural Research Corpora t ion in Wad Medan i for s t imulat ing discussions on the subject mat te r o f this paper: As im A. Abdel R a h m a n , A h m e d N. Balla, E1 Tigani , M. E1 A m i n and O s m a n I. G a m e d .

Since causes o f the c rop-pro tec t ion crisis in the Gezira and measures for effective remedial action are still topics o f considerable debate and controversy, I m u s t emphas ize m y sole responsibi l i ty for opinions expressed, as well as for omissions and errors in presenta t ion o f facts, or in terpreta t ion o f events.

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Accepted 17 January 1983

cotton-insect control in the sudan gezira: analysis of a crisis

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