Insect herbivory on mangrove leaves in north Queensland

Leaves from canopy and understory species were sampled within mangrove forests from an estuarine site at Tully Heads, two tidal embayment sites in Missionary Bay (Hinchinbrook Island) and Chunda Bay near Townsville, during October and November 1984. A total of 25 species were sampled.

Each tree or shrub was sampled by removing leafy shoots or rosettes of leaves until between 50 and 100 leaves were collected. For shorter trees and shrubs, leaves were removed randomly from all parts of the canopy. Larger trees were sampled only to a height accessible by climbing, except for those at the Missionary Bay sites, where a raised walkway allowed greater access to the canopy. Leaves were placed in plastic bags and stored at 4°C until processed.

The number of trees of each species sampled within a site was dependent on the abundance of each species. Up to 15 trees per site were sampled for common species or species observed to exhibit highly variable levels of herbivory and 3 or fewer trees were sampled for rare species. Samples were taken from seedlings, saplings and mature trees.

The percentage of leaf area missing was used to score insect damage. Estimates of leaf damage were made visually, with each leaf assigned to one of 12 damage categories (0% = undamaged, 1-5%, 6-10%, 11-20%, 21-30% ..... 91-100%. The accuracy of damage estimates was determined using samples of leaves of Rhizophora stylosa sampled at Chunda Bay and comparing observer scores with damage measured with a Lamda Instruments leaf area meter.

Experiments were also carried out at Chunda Bay to calculate appropriate correction factors for problems which may contribute to over- or underestimation of the proportion of leaf production removed by grazing insects. Holes of known size were punched in newly opened leaves of Rhizophora stylosa, Ceriops tagal var. australis and Avicennia marina and the hole expansion monitored as each leaf grew to mature size. Comparisons of rates of herbivory on juvenile and mature leaves were made by monitoring marked leaves. The frequency of total leaf loss was monitored by marking and monitoring 30 newly opened leaves of each of the three species for a period of three months.