The emergence of cotton aphid as a significant pest, and secondarily the buildup of silverleaf whitefly infestations, has challenged the cotton IPM system in California. The number of insecticide applications needed for cotton production has increased from an average of 2-3 to 4-6 or more per season in recent years in many areas. A portion of this increase has resulted directly from cotton aphid applications; these treatments with non-selective materials have disrupted the biological control of other arthropod pests such as spider mites and lepidopterous larvae. Additional applications have then been needed for these pests. In 1995 and 1996, cotton yield was compromised by outbreaks of one or more of these arthropod pests. In 1997 and 1998, management of these arthropod pests was apparently good; however, the increased cost of production is problematic. Although aphid management with insecticides is presently feasible, the increased costs of production are problematic. IPM strategies which mitigate cotton aphids before they explode in the cotton field are needed and may improve the profitability of cotton production in many parts of the SJV. High levels of nitrogen fertilization appear to promote increased cotton aphid reproduction and the buildup of high in-field aphid populations. Studies were conducted to quantify this interaction and to develop nitrogen strategies to optimize cotton production while still minimizing cotton aphid population development.