Larval gregariousness and neonate establishment of the eucalypt-feeding beetle Chrysophtharta agricola (Coleoptera: Chrysomelidae: Paropsini)

The selective advantage offered to individuals living within groups may relate to natural enemy defence, but in leaf feeding insects may also relate to overcoming plant defences, especially with respect to feeding establishment. We conducted a series of experiments focusing on neonate larval survival, examining the effect of group size and leaf age on the survival of a eucalypt-feeding beetle, Chrysophtharta agricola, which formed groups of up to 43 larvae on the foliage of Eucalyptus nitens in the field. In the laboratory, in the absence of natural enemies, we found that initial density, leaf age and damage to the leaf margin significantly affected larval survival. Survival of solitary first-instar larvae on young foliage was around 80% whereas on older foliage it was around 11%. Prior damage to the leaf margin significantly increased survival on older leaves to around 61%. Initial larval density also affected survival, although mortality was always significantly higher on older leaves. On older leaves the larval group size above which mortality increased no further was over two-fold that on young leaves. Observations of group feeding behaviour at each instar showed that the majority of larvae (75.7%) were aligned facing away from the feeding site and that only around 7.5%, or just 1-2 larvae per group, fed at any one time. Feeding larvae chewed the leaf edge by straddling the leaf margin. Measurements of leaf margins showed that older leaves had significantly thicker leaf margins and 'thickness' ratios (leaf margin to leaf lamina proper). In the field, approximately 85% of all larvae occurred on the first two expanded leaf pairs, and larval mortality was highest between eclosion and establishment of the first instar. However, beetles apparently did not adjust clutch size according to leaf age.