Crowding, Maternal Age, Age at Pupation, and Life History of Zophobas atratus (Coleoptera: Tenebrionidae)

Abstract

In Zophobas atratus (F.), crowding of larvae inhibits pupation, whereas isolation of crowded larvae triggers pupation, allowing duration of the larval stage to be manipulated experimentally. Effects of larval and adult crowding, birth order (maternal age), and duration of larval stage (larval age at pupation) on larval growth, adult reproduction, and adult longevity were determined in a factorial experiment spanning 2 1/2 generations. Larval growth decreased with density and increased with birth order. As larvae grew, they became repelled by other larvae and dispersed for pupation: the proportion pupating successfully first rose with age, then fell as death at metamorphosis became more frequent. Experimental isolation of larvae produced cohorts of adults that had spent 4, 7, and 10 mo as larvae. Body weight of these adults increased with larval age, decreased with larval density, and was higher for males. These F1 adults were allowed to breed at high and low density until they died. Lifetime egg and larval production generally decreased with birth order and larval age and increased with adult density. Male (but not female) longevity was strongly reduced by adult crowding. Both male and female longevity declined with birth order and larval age. The experimental factors acted on lifetime reproduction largely through their effects on longevity, but there remained a direct negative effect of birth order and larval age. Body weight and reproduction were uncorrelated because females of early birth order and early pupation were lowest in body weight but had the highest reproductive output. Females produced their highest-quality offspring early in life, but the effect was delayed until their offspring reproduced. These effects are discussed in light of their possible importance in this beetle's life history and population dynamics.