AbstractResistance to insecticides used to control pests is an issue of increasing concern for agriculture. The grain aphid,Sitobion avenae, is a pest of cereals and grasses worldwide, and one of growing concern due to the evolution of resistance to certain insecticides. Resistance confers benefits to insects by enabling them to survive exposure to insecticide compounds; however, the mutations conferring resistance may also penalise the insect in pesticide‐free environments due to fitness costs associated with the new phenotype. Here we tested the hypothesis of a reproductive penalty linked to the knockdown resistance mutation (kdr) to pyrethroid insecticides. The mutation occurs predominantly in a single SA3 clone. To date, only heterozygous‐resistant forms (kdr‐SR) have been detected in populations in Ireland and the UK, and this suggests that a fitness penalty may preclude the formation of both male and female heterozygous‐resistant sexual forms. By designing an experiment which included a resistant and a non‐resistant clone, we were able to simulate reduced daylight and temperature conditions which, in nature, trigger sexual reproduction and therefore study the responses of each clone. This allowed us to detect the switch from asexual females to sexual females and males and report on the conditions associated with the production of sexual forms. The results showed that both aphid clones were able to produce sexual forms with no difference in the onset of sexual reproduction, although reproductive strategies differed between clones. The later onset of male forms in the SA3 clone may decrease the likelihood of mating interactions to create fully resistant (kdr‐RR) genotypes and this may constitute a fitness penalty due to pyrethroid resistance.