STUDIES ON THE EFFECT OF THRESHOLD TEMPERATURES, DEGREE-DAY ESTIMATES ON CERTAIN POPULATION GROWTH PARAMETERS OF THE COTTON APHID, APHIS GOSSYPII (GLOVER) INFESTING WATERMELON IN UPPER EGYPT

Developmental rates and times, thermal constants and popula‌tion growth parameters for cotton aphid, Aphis gossypii (Glover) infest‌ing watermelon were estimated in relation to certain field conditions dur‌ing winter, spring and summer. Using both days and degree-days time scales, lower and higher temperature extremes in winter and late summer were found to delay development and shorten adult longevity as well as exhibiting a marked effect in natality and mortality schedules. As a consequence, the gross reproductive rates (GRR), net repro-ductive rates (Ro) and finite rate of population increase (X) were re‌duced, whereas the population doubling time (D) was lengthened as compared with the same parameters under the favourable conditions. The biophysical model predictions based on the regression equa‌tions describing the relationship between temperature and developmen‌tal rates provided estimates of lower and upper temperature thresholds to be 6.9 and 33.5°C for the generation with degree-day accumulations of 96.3. Population growth parameters varied among temperature re‌gimes when times were expressed in days. The proportion of immatures decreased while that of adults increased with increasing temperature.