The role of crowding in the production of winged forms by two strains of the pea aphid, Acyrthosiphon pisum

Abstract

Apterous females of both strains of Acyrthosiphon pisum responded to being crowded by including some alate daughters in their progenies. Females of the pink strain produced many more alate offspring in response to this stimulus than did the green forms. The proportion of alatae in the progenies of crowded females of both strains was dependent upon both the duration of the treatment and the density of the aphids when crowded. Whereas two individuals of the pink strain when confined together for 24 hr induced a strong response in each other, aphids of the green strain produced few alatae unless confined in groups of 10. Control aphids starved for 24 hr in isolation gave no response. However, winged forms occasionally appeared, apparently randomly, in the offspring of unstarved but isolated pink females. Similar females of the green strain produced only apterous daughters. Larvae of both strains are irrevocably determined as alatae or apterae before their birth. No post-natal response to tactile stimulation or any other environmental factor could be demonstrated. Determination probably takes place shortly prior to birth. The maternal response of the green forms can be triggered by crowding adult females. This treatment, when applied to aphids of this strain prior to the larval-adult ecdyses, does not induce the production of alatae. However, fourth instar, and even third instar, apterae of the pink strain respond to a crowding stimulus by ultimately producing numerous alate daughters. The response to crowding can be ascribed to tactile stimulation only. Removal of antennae diminished the response to crowding considerably. Slow-motion film sequences have shown individuals of the pink strain to be particularly sensitive to encounters with other aphids. A 24-hr period of contact with the rough walls of a confining muslin bag induced in isolated pink aphids a response as intense as that of aphids subjected to any crowding treatment. Females of the green strain did not respond to this stimulus, and cine films show them to be much less sensitive to contact with other aphids than the pink forms.