Polymorphism in a Subterranean Aphid, Pemphigus bursarius. I. Factors Affecting the Development of Sexuparae

To determine what factors were responsible for the appearance of sexuparae in Pemphigus bursarius (L.) and at what stage these factors were effective, cultures were reared in long and short photoperiods. From these preliminary experiments it seemed that sexuparae appeared in response to crowding rather than to short photoperiod. When the aphids were reared on lettuce roots and lettuce seedlings in strictly controlled conditions of crowding, photoperiod, and temperature, crowding, modified by temperature, induced the development of sexuparae, while photoperiod was not an influential factor. Further experiments showed that, though there might be some prenatal influence, the critical period for morph determination was postnatal, and that nymphs were responsive to the influence of crowding until the second nymphal molt.