The Relationship between Fecundity and Adult Weight in Myzus persicae

(1) The number of large embryos (LEN) in individual apterous virginoparous Myzus persicae (Sulz.) raised on sugar beet and potatoes depended on adult weight. (2) Linear regressions of LEN on adult weight showed differences between aphids reared on different host varieties and in different experimental environments. (3) The negative exponential curve provided a significantly better fit than the straight line to these data and gave valid estimates of LEN when adult weights were very small. (4) A common negative exponential curve could generally be fitted to aphids from different environments. Where significant differences in curves did occur, they were largely accounted for by a simple multiplying factor. (5) It is concluded that the dependence of fecundity on weight in virginoparous aphids is a negative exponential of form y = a {1 exp[ (loge 2)(x X0)/(X50 Xo)]}, where x0 is the weight threshold for reproduction, x50 is the weight for median fecundity and a is a fertility factor reflecting host plant status and other, as yet unknown, environmental, and possibly genetic state variables.