An estimate of the energy requirements of the three species of aphids,Monellia caryella(Fitch),Monelliopsis pecanisBissell, andMelanocallis caryaefoliae(Davis), that feed on pecan,Carya illinoensis(Wangenh.) K. Koch, indicated that during a lifetime, an individual ofM. caryellaconsumes 8.4-fold more energy (301 J) than that ofM. pecanis(36.1 J) and ca. 7-fold that ofM. caryaefoliae(44.8 J). When expressed as an average-age individual typical of a season-long field population,M. caryellaextracts ca. 8-fold more energy thanM. pecanisorM. caryaefoliae. Most energy consumed by all three species was excreted as honeydew. Growth efficiency was low for all three species, but was highest forM. caryaefoliae(25%), followed byM. pecanis(19%) andM. caryella(5%). Regression models were derived to estimate the cumulative level of energy consumed by aphids of various ages for growth, respiration, excretion, and total energy removal. Determination of the season-long standing-aphid population (and aphid-days) in two different aphid management programs within a 70-yr-old ‘Stuart’ pecan orchard and influence of these populations on nut yield indicated that all three species are highly detrimental to nut production; extraction of 1 × 106J, via feeding byM. pecanisorM. caryella, reduced in-shell nut production by 58.7 g per 70-yr-old ‘Stuart’ tree. A season-long standing population of one individual ofM. pecanisper leaf of such trees reduced in-shell nut yield by 2.41 kg; 18.13 kg was lost because of the same level ofM. caryella.