The gradual increase in kernel (nut) size as the macadamia seed develops, appears to play a role in the decline of kernel susceptibility to feeding by insects of the suborder Heteroptera. In some cultivars the kernel becomes resistant to feeding before others due to differences in husk and shell thickness (e.g. feeding by Nezara spp.), but species with a long proboscis (e.g. Bathycoelia spp.) could still feed on the kernel after maximum fruit size was reached. The thickness of the husk and shell, and not the hardness of the shell, was the limiting factor in kernel feeding. Bathycoelia distincta Distant (Pentatomidae) and Pseudotheraptus wayi Brown (Coreidae) were the most damaging heteropteran species with respect to the size of the kernel lesions caused. A single feeding incident during early fruit development could cause large necrotic lesions, which totally malformed the kernel. Farnya sp. and Nezara sp. caused only small shallow lesions. During the fruit maturation period, only B. distincta was able to damage the kernel significantly due to its long proboscis. The extent of fruit damage was dependent on the stage of fruit development and the species concerned. A tolerance index was developed to express the relative tolerance of macadamia cultivars to the damage caused by kernel-feeding Pentatomidae at the later stages of fruit development. Parameters for the index were the number of pentatomids attacking macadamia, their proboscis length and the combined thickness of the husk and shell of the fruit.