The cotton aphid has developed into a key pest of cotton in California. The common pattern of seasonal dynamics of cotton aphid populations has changed repeatedly over the last 15 years. The cotton aphid has evolved from a non-pest to an early-season and late-season pest to a pest throughout the growing season and particularly during the mid-season. The silverleaf whitefly has emerged as an extremely serious cotton pest in many of the arid production areas. This pest was first found in the San Joaquin Valley in 1992 and has caused some late-season problems in cotton. Several characteristics of these pests make them regional pests and therefore difficult to manage on an individual field basis. We summarize seven characteristics of the cotton aphid and silverleaf whitefly which make them regional pests. We will emphasize the cotton aphid since we have the most research experience with this pest in California, and thus far, this species has resulted in the greatest economic loss in SJV cotton. Biological factors which may lead to aphid outbreaks will be discussed.