Crowding inhibits pupation in Tribolium freemani: contact chemical and mechanical stimuli are involved

Pupation of Tribolium freemani larvae was strongly inhibited by crowding. When such larvae were transferred to regimes with various levels of food quantity (1, 2 and 3 g) and rearing density (1 to 5 larvae/g food), the percentage of pupation observed within 10 days after transfer increased as the rearing density decreased, regardless of the amount of food medium. Co‐rearing of single T. freemani larvae with Tenebrio molitor larvae or mechanical stimulation with a shaker or rotator also exerted a pupation‐inhibiting effect. Larvae confined individually in a wire cage held in vials containing a group of larvae underwent metamorphosis as did isolated ones. These results suggest that a tactile stimulus is important in manifestation of the crowding effect. Freshly killed larvae had some crowding effect on isolated larvae, but this effect was significantly reduced when the dead larvae had been washed with hexane. Hexane extracts of larvae delayed the time of pupation in isolated larvae in a dose‐dependent manner. Therefore, a substance(s) on the body surface of larvae may be involved in the crowding effect.