Studies were conducted from 1974 through 1976 to determine the effects of temperature, leaflet maturity, and leaflet suitability as influenced by crowding and prior aphid feeding on the biology of individual blackmargined aphids, Monellia caryella (=costalis) (Fitch), feeding on pecans, and the relative importance of each of these factors in influencing aphid population dynamics. Nymphal mortality was greater on young than on mature leaflets and increased when temperatures varied above or below the optimum temperature of 23.9°C. Rate of development was not affected by leaflet maturity. Temperature had a significant effect on vivipara longevity and reproductive activity. Total fecundity was greatest at 23.9°C. More virginoparae produced progeny, lived longer, and were reproductive longer on mature than young leaflets. Fundatrices exhibited superior reproductive activity on young leaflets to virginoparae on either mature or young leaflets, especially at lower temperatures. Heat unit requirements for total nymphal development averaged 124.9°C-days above a lower developmental threshold of 8.2°C, using 32.2°C as the upper threshold. The rate of development of the blackmargined aphid was not affected by crowding but was decreased on leaflets conditioned by heavy prior aphid feeding. Nymphal mortality was increased by crowding and even more so on conditioned leaflets. Crowding decreased the proportion of virginoparae reproducing and their reproductive rate, but had no influence on longevity or reproductive period. The proportion of virginoparae producing progeny, longevity, reproductive period, and reproductive rates were significantly reduced for virginoparae reared on conditioned leaflets. When virginoparae were given a choice, 84 percent preferred leaflets which had not been conditioned. Studies of blackmargined aphid populations in southern Arizona pecan orchards confirmed laboratory and greenhouse findings that leaflet age, prior aphid feeding, and temperature were the major factors influencing aphid population patterns. The heat requirements for nymphal development under fluctuating orchard conditions were similar to those determined in controlled environments within ordinary temperature ranges. The relationship of vivipara longevity and reproductive rate to the daily rate of heat unit accumulation was determined.