Modern Agriculture and Pest Forcing 931 The Pesticide Connection

Estimates of crop losses due to all pests (animals, weeds, pathogens) is rather staggering when considered in the context of the powerful management tools that are available to fight back. Global losses of soybean, wheat and cotton are estimated to be 26-29%, but are even higher for maize, rice, and potatoes at 31, 37 and 40%, respectively.19 Such losses demonstrate not only the tremendous diversity of organisms that compete with humans for food resources, but the impracticality of devising management solutions for every species deriving nutrition and shelter from a crop. Differentiating between primary and secondary pests has been an essential point of distinction for allocating finite pest management resources. Targeting primary pests over and over again with pesticides, however, brought out another aspect of pest organisms - their resiliency. Resistance to pesticides has evolved across the pest spectrum and has exacerbated pest management costs, by increasing both the number of treatments and concentration of active ingredients necessary to manage resistant pest populations. Increasing pesticide use also led to gross imbalances in crop systems by depleting natural enemies of pest species, to such a degree that normally insignificant species would occasionally be released from natural control and increase to pestiferous levels. It was the careless treatment with pesticides of one pest problem that too often would lead to a second pest problem, followed by additional treatments that eventually resulted in a pesticide treadmill.20 Such scenarios became increasingly common in the 1950s and thereafter and became another motivating reason for Stern et al.16 to develop the integrated control approach and lessen dependency on pesticides. However, it was many years later before integrated pest management became the definitive term that embodied the principles within the integrated control concept, and still more years before it attained the institutional level that it now holds.21 In the meantime, the bad habits acquired early in the era of proliferating new pesticides proved hard to break as pesticides dominated pest management approaches.

Following the publication of Silent Spring, there was considerable examination by entomologists into the role that pesticides had played in crop failures and ecological crises worldwide. Cotton in particular came under scrutiny as pesticides were implicated in multiple destructive pest outbreaks. The pattern of pesticide-related pest problems in cotton led Smith22 to identify five phases of cotton production that had been observed on different continents, all ending disastrously. The pattern described began with low input cotton production during "the subsistence phase'', but eventually proceeded to "the exploitative phase'' through intensification measures, such as the addition of fertilizers or development of irrigation systems for constant water delivery. As Smith22 saw it, the investment of capital during the exploitative phase required protection from pests that would otherwise diminish yield and cut into profits. Thus, the intensive use of pesticides to the virtual exclusion of all other control methods brought about the third phase that he called "the crisis phase''. It was the crisis phase that was critical to the predominant thinking that eventually developed concerning the primary role of pesticides in pest outbreaks in agriculture. Once the situation had reached the crisis phase, pest problems would continue to mount due to a combination of pesticide resistance, pest resurgence, and induction of secondary pests to major pest status that altogether would bring on "the disaster phase''. At this point, pest problems would be so rampant that cotton would no longer be profitable to grow, leading in some cases to its abandonment as purportedly occurred in some regions.20,23,24 An alternative to abandonment was to reform pest management practices through adoption of IPM methods that emphasized biological and cultural controls to return to a more natural balance. This would initiate the final part of the five-part sequence known as "the recovery phase''.

The pattern of pesticide-induced disasters depicted by Smith22 and the supporting examples he presented were reiterated numerous times by various authors.25 29 These crisis-in-cotton examples came to represent the paradigm for pest control failures in general where pesticides were involved. It was not necessary that a major crop failure occur, only that unsatisfactory control leading to economic damage be realized that, more often than not, would elicit an explanation that pesticides were to blame for the resistance they had caused in the target population, or that natural enemies that had been eliminated had led to a resurgence of the target pest or an increase in secondary pests. While well-documented examples exist for each one of these phenomena, specific evidence in the vast majority of cases was rarely presented. For example, speculation made about the increased pest status of the whitefly Bemisia tabaci stated "Once whiteflies are established in an area, we believe that the disruptive influence of pesticides, coupled with resistance, is the largest factor responsible for the crisis situations often associated with population outbreaks''.30 Prior to this statement, there had been recent outbreaks of B. tabaci in the Sudan Gezira that had been attributed to pesticides either because of severe disruption of natural enemies that eliminated natural regulation of B. tabaci,29 or resistance to organophosphates which arose in B. tabaci and hormoligosis to DDT residues occurred.31 While both of these publications acknowledged that pesticides were behind the outbreaks of B. tabaci, they did not agree how pesticides exerted their disruptive influence. An earlier report about outbreaks of B. tabaci in the Imperial Valley of California32 also implicated pesticides, in this case an upset of the natural balance caused by the recently commercialized pyrethroids. So there was general support for the brief summary about B. tabaci outbreaks referred to earlier concerning the "disruptive influence of pesticides'',30 but a lack of certainty and confirmatory data concerning the specific ways that pesticides had induced these outbreaks. The same is true of other pest species for which investigative studies were rarely done to identify underlying mechanisms leading to outbreaks. However, the ubiquitous use of pesticides during the second half of the 20th century and the scorn they received following Silent Spring made them an easy target for blame whenever pest populations were not adequately controlled.

Despite their unsavory reputation, pesticides continue to play a major role in pest management and are one of the critical considerations, perhaps impediments, to adopting greener IPM programs. Accurately assessing the role pesticides have played in managing pests, or perhaps exacerbating pests in some instances, is crucial to designing better IPM programs that will not have to rely as heavily on pesticides as in the past. Some would argue that reliance on pesticides in the first place was misguided and that dependency grew over time due to the careless way they were implemented into pest management programs.33 An alternative view (see Figure 9.1b) is that intensification measures that were the hallmark of the Green Revolution contributed to the

Resource Improvement

• Intensification

• More suitable crops

• More favorable agronomic practices

Resource Improvement

• Intensification

• More suitable crops

• More favorable agronomic practices

Figure 9.1 Two scenarios describing increased pest problems following resource improvement associated with agricultural development. The predominant view (A) expressed by Smith22 and others suggests that pesticides used to protect crops cause resistance to pesticides and resurgences in pests resulting in outbreaks. An alternative view (B) suggests that resident pest populations respond with growth to improved resources brought on by intensification measures, and increased pest pressure is met by stepped-up pest management responses including greater pesticide use.

Figure 9.1 Two scenarios describing increased pest problems following resource improvement associated with agricultural development. The predominant view (A) expressed by Smith22 and others suggests that pesticides used to protect crops cause resistance to pesticides and resurgences in pests resulting in outbreaks. An alternative view (B) suggests that resident pest populations respond with growth to improved resources brought on by intensification measures, and increased pest pressure is met by stepped-up pest management responses including greater pesticide use.

forcing of pest populations by creating a more enriched environment for the pest species. That is, the natural increases in pest populations responding to superior environments led to greater pest pressure and ultimately forced heavier pesticide use. This is a sharp distinction from the predominant view (see Figure 9.1A) that pesticide abuses reduced natural enemies to ineffectual densities, and caused resistance and resurgences of pest populations leading to economic damage and in some cases field failures.

9.3.2 Agricultural Intensification

Looking further back than the 1950s, we can find well-documented cases of B. tabaci outbreaks prior to the advent of synthetic organic pesticides. Nymphal densities as high as 150 cm-2 were reported in cotton during the early 1940s in the Sudan Gezira.34 Even higher densities were reported from cotton grown in India during the late 1920s and early 1930s35 following large-scale increases in area planted to so-called "American" cotton varieties newly introduced to India at that time. Destructive infestations of B. tabaci in vegetables were also described in Palestine during the 1930s.36 What these early records indicate is that B. tabaci was already a serious pest of agriculture before synthetic pesticides were developed. There were clearly circumstances involving various crops and locations in which B. tabaci populations developed to high densities that would be considered outbreak levels by today's standards. The changeover to American cotton that occurred in parts of the Punjab region of India involved not just a shift to a new variety, but actually to a different species of cotton. Cotton production in India at the beginning of the 20th century depended exclusively on locally adapted varieties belonging to Gossypium arboreum L. Between 1911-13, ca. 6000 ha were planted to G. hirsutum, called American cotton at that time in India, but now more commonly known as "upland cotton". Another 111 694 ha of G. hirsutum were planted between 1917-18,37 greatly expanding the scale of American cotton that turned out to be a more susceptible cotton species to B. tabaci?5 It was soon after this transformation that whitefly numbers began to increase and cotton crop failures were reported.37 However, there was considerable uncertainty about the cause of the cotton crop failures, since there was no precedent for a minute insect such as B. tabaci causing destruction without ever chewing holes in leaves. It was the thorough research of Husain and Trehan35 that finally resolved the causal nature of the crop failures and by doing so identified a destructive new pest species.

The change to a higher yielding and more desirable cotton fiber that took place in India represented a form of agricultural intensification, not unlike the higher yielding plant varieties introduced at the start of the Green Revolution. Improved quality and yield were the goals in both cases, but with some uncertainty in terms of how respective pest complexes might respond. Two lines of evidence, i.e. the improved performance of B. tabaci on G. hirsutum versus the indigenous G. arboreum as determined in controlled studies,35 and the rise in B. tabaci infestations reaching outbreak status that coincided with the wide-scale adoption of G. hirsutum in India, together suggest that the alteration in cotton agriculture itself may have been largely responsible for the increased pest problem in Indian cotton during the 1920s. In the Sudan Gezira, there was a 2.3-fold increase in cotton acreage that occurred between 1956 and 1967, going from 103 062 to 232 545 ha in just 11 years. During the same period, the area planted to groundnuts increased 106-fold, from 403 to 42 698 ha. This coincided with the time period of the late 1950s to mid 1960s that Eveleens29 claimed was the time frame when B. tabaci attained the status of a serious pest. The additional acreage for both crops greatly expanded the resource base, but also improved the quality of that base since cotton and groundnuts are favored crops of B. tabaci. With the example from the Imperial Valley of California ca. 1980, there also was a major expansion of cotton acreage that had taken place over the previous 12-15 years that peaked in 1977 at 58 275 ha. Cotton acreage remained high in the Imperial Valley through 1980, the year of a major outbreak of B. tabaci.32 Although it was the role of pesticides only that was implicated in the outbreaks of B. tabaci in Sudan29,31 and California,32 the simultaneous expansion of favored crops cannot be ignored given the historical examples of B. tabaci outbreaks prior to the era of synthetic organic insecticides.

In addition to the two aspects of agricultural intensification identified from the historical examples for B. tabaci, i.e. (1) the expansion of favorable crop acreage in Sudan and California, and (2) the replacement of an indigenous cotton species with an exotic, more susceptible cotton species in India that allowed higher colonization rates by B. tabaci, still other forms of intensification occurred in the Sudan during the putative emergence of B. tabaci as a primary pest.31 One involved a shift in cultural practices made in the 1960s with the adoption of a longer season, more vegetatively vigorous cotton variety. The sowing date for cotton in the Gezira Scheme had originally been at the end of the rainy season so as to avoid a rain-dispersed bacterial blight.38 The new variety 'Barakat' was resistant to the causal bacterium, Xanthomonas mal-vacearum (Downson), and so could be sowed at an earlier time, thereby extending the cotton season and enabling additional generations of B. tabaci to develop. Another critical development was the expansion of the Gezira Scheme through construction of a new irrigation project. Lying between the White and Blue Nile rivers, the Sudan Gezira region was supplied with irrigation water beginning in the 1920s after construction of a dam on the Blue Nile at Sennar. The success of the Gezira Cotton Scheme from the 1930s-50s provided impetus for further expansion of the irrigation infrastructure in the Gezira region. A project called the Managil Extension opened up another 336 000 ha of irrigated cropland in the 1960s and virtually doubled the cultivated area. The traditional fallowing of one-half of the cropland area had been gradually reduced in the established areas of the Gezira, with the same limited fallowing observed for the new areas under the Managil Extension.39 A push to diversify the agriculture by adding new crops, while also increasing areas planted to traditional crops, greatly intensified agriculture in the expanded and fully irrigated Gezira Scheme through the 1960s-70s.

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