Parasitoid pressure does not elicit defensive polyphenism in the green peach aphid

1. Aphids are capable of adapting to parasitoid pressure in the absence of sexual reproduction using epigenetically controlled polyphenism. Asexual lineages of aphids could thus become resistant to parasitoids over time and with repeated exposure, hampering biocontrol efforts that rely on parasitoid wasps. 2. Prior to this study, wing polyphenism and reproductive polyphenism had been reported as parasitoid‐adaptive responses in asexual lineages of at least two species of aphids, but it remained unclear whether parasitoid exposure could induce other defensive polyphenisms. Using a rigorous, replicated experimental evolution design, we aimed to induce adaptive polyphenisms in the green peach aphid, Myzus persicae Sulzer, through serial exposures of single asexual lines to the parasitoid Aphidius colemani Viereck over four consecutive generations. 3. We measured changes in parasitoid susceptibility, reproductive schedule, body size, and production of pink or alate offspring compared to aphid control lines not exposed to parasitoids. Despite this consistently strong selective parasitoid pressure, there was no evidence that asexual lineages of M. persicae adapt to parasitoid pressure using adaptive polyphenism. 4. Our data indicate that pseudo‐crowding stress – where aphids disturbed by parasitoids or predators wander within a colony, increasing aphid‐to‐aphid contact and creating the illusion of a more crowded environment – may be necessary for M. persicae to produce defensive polyphenisms in response to parasitoid attack.