The effects of temperature on the life history traits of Rhopalosiphum maidis (Fitch) were determined at 6, 10, 15, 20, 25, 30, and 35Ϯ1°C on corn leaves, Zea mays (L.). At 35°C, only a few nymphs survived and completed development, but all failed to reproduce. Total immature stages from birth to adult decreased as temperature increased from 51.7 d at 6°C to 5.2 d at 30°C, but became 8.0 d at 35°C. Overall immature development required 119.05 degree-days above 4.44°C. Adult longevity was as high as 26.7 d at 15°C, but significantly declined at 30 and 35°C. The average adult fecundity was as high as 45 offspring at 15 and 20°C, but dropped to 1.8 at 6°C and 8.6 at 30°C. In general, as temperatures increased, age-specific survivorship (l x ) declined more quickly, but age-specific fecundity (m x ) peaked earlier. At 25°C, the age-specific net maternity (v x ϭl x m x ) was the highest in the early reproductive period, resulting in the highest intrinsic rates of increase (r m ϭ0.329). At 20-30°C, the values of r m were significantly higher than those at lower temperatures. The results indicate that corn leaf aphids probably are better adapted in population growth to a wider range of high temperatures in warm regions.

Variation in the life history traits of cotton aphids, Aphis gossypii Glover, reared on four widely distributed weeds, Ageratum houstonianum Mill., Bidens pilosa L. var. radiata Sch. Bip., Solanum nigrum L. and Spermacoce latifolia Aubl., were investigated. Cotton aphids were reared in the laboratory at 25°C. Each host plant had a distinct effect on aphid life history traits. Cotton aphids reared on S. nigrum had a significantly shorter developmental period, and age‐specific fecundity peaked early. In contrast, cotton aphids reared on S. latifolia had a long developmental period and low age‐specific fecundity. Cotton aphids that fed on B. pilosa and A. houstonianum displayed intermediate rates of growth and age‐specific fecundity. Because the curves of age‐specific fecundity (mx) and age‐specific net maternity (lx mx) on each host plant were close together, development time and the pattern of age‐specific fecundity were the major factors determining the population growth potential of the cotton aphid on each weed. As a result, the intrinsic rate of population growth for aphids reared on S. nigrum was significantly higher (rm=0.527 ± 0.011) than it was for aphids reared on S. latifolia (rm=0.194 ± 0.012).

The age-specific survival and fecundity of female adults of the aphidophagous parasitoid, Aphelinus gossypii Timberlake (Hym., Aphelinidae), were determined at a host density of 50 Aphis gossypii Glover (Hom., Aphididae) per leaf of Ageratum houstonianum Mill each day at 25degreesC. The age-specific mummy production, emergence rate and sex ratio of progeny were calculated. The implication of these results in terms of potential population growth of A. gossypii and related species is discussed. The age-specific survival curve (l(x)) of females exhibited a Type I pattern, which resulted in little difference between Sigmam(x) and Sigmal(x) m(x). The l(x) curve of males exhibited a Type II pattern, and their survival time was much shorter than that of females. Each mated female produced on average 598.9 +/- 64.0 aphid mummies, and preyed on 87.9 +/- 6.2 aphids. Most of the biological performance parameters were not different significantly between mated and virgin females. The highly female-biased sex ratio of offspring produced by mated females soon after their emergence resulted in a high fecundity rate (m(x)) during the early reproductive period. Therefore, although the sex ratio of progeny was male-biased during the entire reproductive period, the intrinsic rate of increase estimated using age-specific sex ratios was larger than that estimated using a constant sex ratio of 0.5. These results indicate that the high survival and the adaptation of producing mostly female offspring during the early reproductive period contribute much to the population increase potential of this parasitoid