Agricultural intensification and pest outbreaks: A reappraisal of events in the Sudan Gezira

Pest control crises in agricultural systems often have been attributed to e ither direct or indirect effects of pesticide use and less frequently to th e underlying cropping structure and environment. As an example of intensive , high-input agriculture, the Gezira Irrigation Scheme in Sudan and the pes t crises that occurred there in the late 1970s are drawn upon to examine 2 principal arguments that have frequently been invoked to explain causes of pest outbreaks in crops. Bath arguments considered pesticides to have been the principle inducers of the pest crises in the Sudan Gezira, but differed in their evaluation of the responsible mechanisms. One argument considered that pesticides indirectly led to the depletion of natural enemies and the concomitant rise in pest status of Bemisia tabaci (Gennadius), whereas the other argued that pesticides directly affected B. tabaci populations throu gh fertility stimulation and resistance to insecticides. A 3rd argument is submitted herein to suggest that rising pest problems occurred in conjuncti on with a program of agricultural intensification and diversification that took place in the Gezira from the late 1950s through the late 1970s. This p rocess began with crop intensification, was followed by favorable changes i n agronomic practices from the standpoint of a crop pest, and together led to increasing densities of pests responding to significant improvements in the environment. Finally, management reacted to the pest surge by increasin g the number of spray applications and types of insecticides used. Dramatic changes in the crop environment are viewed as having given rise to pest po pulations that were intrinsically capable of exploiting an expanded, improv ed resource base and that ultimately exceeded the inherent limitations of p esticides to effectively manage their populations.