Morph determination in the bird cherry‐oat aphid, Rhopalosiphum padi L

SUMMARY Both crowding and poor nutrition induce the appearance of emigrants in Rhopalosiphum padi L. No emigrants developed when the aphid was reared in isolation for six successive generations on actively growing leaves of bird cherry. However, emigrants developed as soon as the leaves ceased to grow or when the aphids became crowded. Crowding of mothers and postnatal crowding of the nymphs of apterous exules both influenced the induction of alate exules. The highest proportion of alate exules developed when both mother and offspring were crowded. Short day-length induced the appearance of gynoparae and males. For 50% production of gynoparae and males, over the range of temperature 10–18dC, a 1.75 h reduction in day-length is required for every 4d increase in temperature. At 18 dC, even at day-lengths as short as 6 h, exposure for three generations is required before all offspring become gynoparae or males. Low temperature and short day-length was ineffective in inducing the development of gynoparae or males in the first three generations developing from the fundatrix.