REPRODUCTIVE EFFORT AND MATERNAL AGE IN ITEROPAROUS INSECTS USING APHIDS AS A MODEL GROUP

1. Life-history theory predicts that as the residual reproductive value of an organism declines its current investment in reproduction should increase. This hypothesis has not been previously tested for insects. 2. The results of a laboratory study on the reproductive investment of the willow-carrot aphid, Cavariella aegopodii and the vetch aphid, Megoura viciae were compared with the predictions of the aphid optimal energy partitioning model. 3. The model's assumption that the fecundity function in aphids is triangular was supported. 4. As predicted by the model, the sizes of gonads decrease and those of the offspring increase with the age of the mother. 5. The large offspring born towards the end of a mother's life achieve a greater adult weight, mean relative growth rate and potential intrinsic rate of increase than the small offspring born early in a mother's life. The better performance of the last born is a consequence of their large birth size. 6. The size of the offspring varies inversely and the reproductive investment positively with residual reproductive value. The increase in offspring size towards the end of a mother's life is a consequence of the time lag between ovulation and birth and the cessation of ovulation well before a mother dies. The excess of energy produced by the soma in old mothers is used to accelerate the growth rate of the remaining offspring. This result can be extended to other groups that conform to the aphid model's assumptions.