1. Parthenogenetic adults of the sycamore aphid Drepanosiphum platanoidis exhibited reproductive diapause for 4–6 weeks in the summer.
2. The diapausing aphids had small gonads (accounting for just 13% of the total aphid protein content) bearing small (< 0.5 mm long) and developmentally immature embryos. There was no evidence of embryo resorption.
3. The diapausing aphids had significantly depressed essential amino acid content and elevated glutamine content, relative to reproductively active D. platanoidis.
4. The reproductive characteristics and amino acid titres of the diapausing aphids resemble those of aphids lacking functional symbiotic bacteria. Uncoupling of maternal and embryo growth and suppression of bacterial function are proposed as key elements in the diapause of D. platanoidis.