CROWDING AS PRIMARY FACTOR IN PRODUCTION OF AGAMIC ALATE FORM OF THERIOAPHIS MACULATA (HOMOPTERA - APHIDIDAE)

The influence of various environmental factors on the production of the agamic alate form of the spotted alfalfa aphid, Therioaphis maculata (Buckton), has been investigated in the laboratory. Results indicate that crowding is the only factor inducing the development of winged forms, whereas parentage and nutritional state are secondary modifiers. The stimulus is entirely postnatal and is confined to the first instar. Time of inception and duration of crowding are both of importance and appear to reinforce one another in determining alate percentages. There is a specific length of time during which initiation of the stimulus is effective, since animals more than 18 hr of age are no longer receptive. If parents are prevented from depositing, so that larvae are held in utero beyond their normal birth hour, the duration o stimulus initiation time is correspondingly shortened. The crowding stimulus does not involve excretory products, pheromones, or induced changes in the host plants. It appears to require physical contact between live animals.