Life table and population parameters of Nasonovia ribisnigri (Homoptera: aphididae) at different constant temperatures

Age-specific life tables of the lettuce aphid, Nasonovia ribisnigri (Mosley), feeding on lettuce were determined at different constant temperatures under controlled conditions. Our results showed that the proportion of alates increased with increasing temperatures, remaining below 7% at 16 degrees C and increasing to 40-57% at a temperature above 20 degrees C. The longest developmental time of apterous aphids was obtained at 8 degrees C (31.5 d), and the shortest was at 26 degrees C (6.2 d), whereas the developmental time of alates was always 0.7-1.1 d longer than for apterous. Most aphids needed four instars to reach adult stage, but at 8, 26, and 28 degrees C, many individuals passed through five or six molts. Age-specific survivorship, (lx) was always above 90% at the temperature range of 16-24 degrees C. Mortality rate (qx) was rather low but constant at 8 degrees C. However, mortality was high at 28 degrees C and occurred mainly in the last nymphal instars and adult stage. Unexpectedly, no nymphs were produced by the adult morpbs at 28 degrees C, but effective fecundity was high at 8 degrees C. Fecundity for alates was always lower than for the apterous aphids at the same temperature. The largest intrinsic rate of natural increase (r(m)), and the mean relative growth rate (RGR) occurred at 24 degrees C, for both apterous and alate morpbs, and the lowest at 8 degrees C. Our results show that lettuce aphid is better adapted to survive and reproduce at low (8 degrees C) than at high (28 degrees C) temperatures, and its best performance occurred at 20-24 degrees C.