Swede midge (Diptera: Cecidomyiidae) diapause initiation under stable conditions: not a family affair.
Larvae of the swede midge, Contarinia nasturtii (Kieffer) (Diptera: Cecidomyiidae), rely on environmental cues to initiate the diapause programme. This facultative diapause strategy, which provides opportunities for both bet hedging and multivoltinism, likely contributed to the successful colonisation and spread of this invasive pest in North America. Population diapause incidence is variable even under relatively stable conditions (especially at the critical day length), yet the mechanisms influencing the developmental trajectory of individuals under such conditions are unknown. Here we approach the non-environmental (i.e., heritable) controls of this variation by comparing diapause frequencies within and among full-sibling swede midge families reared in one of two stable environments. Under warm, long-day conditions > 99% of swede midges pupated directly, while 86% of swede midges entered diapause under cool, short-day conditions. In the latter condition, most families exhibited mixed developmental trajectories (ranging from 36% to 96% diapause). This developmental variation among siblings indicates that the diapause induction threshold does not follow simple Mendelian inheritance with complete dominance for a particular allele, but may follow incomplete dominance or a more complex heredity. Alternatively, within-family diapause variation may result from maternal bet hedging or factors such as maternal age or larval nutrition.