• Aphid;
  • host alternation;
  • Pemphigus;
  • Pemphiginae;
  • photoperiod;
  • phenotypic plasticity;
  • polymorphism;
  • polyphenism;
  • sexupara.

Abstract. Growth chamber experiments were conducted to determine what environmental factors induce the development of sexuparae in root-feeding colonies of the aphid Pemphigus betae Doane (Aphididae: Pemphiginae). Experimental colonies were derived from clones that had been reproducing for at least 15 months under constant laboratory conditions. At constant 20–23oC, sexuparae were almost never produced regardless of the level of crowding or photoperiod. In contrast, every culture of several clones produced sexuparae when subjected to temperatures alternating between 20 and 15oC during the light and dark phases respectively. This fluctuating temperature regime resulted in sexupara production regardless of photoperiod. All cultures also produced some sexuparae when cultures were kept in constant darkness at temperatures that were either alternated between 20 and 15oC or kept constant at 15oC. Constant 17oC resulted in sexupara production in most cultures. At 17oC, experimental manipulation of densities within cultures showed that crowding promotes sexupara production. Thus, within the range of conditions occurring at the time of sexupara induction in the field, photoperiod has no effect on sexupara production, temperature below a threshold between 17 and 20–23oC triggers sexupara production, and crowding promotes sexupara production but only if temperature is low enough.