Dispersal Behavior of the Larval Tenebrionid Beetle Zophobas rugipes

Abstract

In the laboratory, crowding inhibits pupation of larvae of Zophobas rugipes (Coleoptera: Tenebrionidae). Pupation-ready larvae must disperse away from high-density populations in order to find an undisturbed pupation site. In the laboratory, the pupation chamber is defended against intruding larvae, and the success of occupants in preventing intrusion increases with isolation up to 96 h. Thereafter the larvae begin to enter the pharate pupal stage. Over a challenge period of 5-10 h occupant success declines to a plateau but remains above the randomly expected value for 24 h, provided the occupants have been isolated at least 12 h. Dispersal and single occupancy of chambers are due to fighting between occupant and challenger. The major components of the behavior are "shoveling" with the head, opening mandibles, biting the opponent, and violent side-to-side thrashing of the body. All these components increase rapidly during the first 24 h of isolation. Among mature larvae, success in chamber defense is not affected by relative weight (650-750 mg vs. 850-950 mg) or relative age (11-12 mo vs. 14-15 mo). Success is decreased but still significantly nonrandom if occupants are removed from chambers and forced to redisperse along with the challengers. Success is greatly increased if larvae are selected for success in a prior experiment. The ability to defend a pupal chamber successfully thus appears to be an intrinsic property of the larvae. Immobilized larvae are unable to prevent intrusion into their chambers, indicating that single occupancy results from active defense. Pharate pupae and pupae have a limited capacity to exclude intruders from their chambers. Immobilized larvae are subject to heavy cannibalism by active larvae.