Eucalyptus grandis is the preferred food of larvae of the cossid moth Xyleutes boisduvali. In the Coffs Harbour region of New South Wales, plantations of E. grandis established in the last 10 years for pulping have allowed a great increase in the population of cossids. Woodboring larvae, especially of X. boisduvali and of the cerambycid beetle Tryphocaria acanthocera, are eaten by the yellow-tailed black cockatoo (Calyptorhynchus funereus). Cerambycids live near the surface of the tree, but cossids live in the heartwood and excavation by the cockatoos may weaken the tree so that it snaps in strong wind. The life of the cossid is described in detail; it is a larva for 2 years, may reach 18 cm in length and was highly prized as food by aborigines. Feeding habits of free cockatoos and a captive are described. The bird searches for a frass hole and tests its size by biting. If the size of the hole indicates a fully grown larva, the bird pulls down a strip of bark on which to stand as it excavates. Cossids are eaten at any time of year but mainly in June and July when final instar larvae are most plentiful and the young birds leave the nest and accompany the parents to the plantations. Before the E. grandis plantations were established, the cockatoos often ate seeds of Punis spp. as they do in South Australia. They eat also larvae of a chalcidid gall-wasp and non-boring insects. Formerly the cockatoos controlled the borer population and some thinning of the trees did no harm, but modern plantations are spaced for optimum growth and up to 40% of trees may be lost by cockatoo damage. Proposed methods of control are to reduce cossid infestation by preventing stress to very young trees, using selective weedkillers and fertilizer, and to encourage undergrowth of wood perennials. Because of the flight characteristics of cockatoos they do not forage among undergrowth and are kept away from the lower parts of the trees, where the cossids are. ADDITIONAL ABSTRACT: Wood-boring insect larvae are eaten by the yellow-tailed black cockatoo (Calyptorhynchus funereus). In the Coffs Harbour region of New South Wales, the birds extract larvae of the cossid moth Xyleutes boisduvali Roths. from the trunks of Eucalyptus grandis, a species extensively grown in that area for pulpwood production. In gouging out the larvae, the birds weaken the trees, which may be snapped off by strong winds. Losses of up to 40% of the trees in a plantation have been recorded.The biology and behaviour of both the insect and the bird are discussed. Physiological stress, particularly competition with grasses, predisposes young trees to infestation by the cossid. The birds systematically search for trees containing cossid galleries, and excavate at least 50% of the larvae. Tree losses can be reduced by silvicultural methods that reduce stress on young trees, and that encourage the development of a vigorous understorey of woody perennials. The flight characteristics of the cockatoo are such that the birds will not forage for cossids in plantations where undergrowth impedes their access to the lower portions of the tree trunks where the insects predominantly occur.