The foraging behavior and grazing intensities of Utbetbeisa puLrbeLLoides (Lepidoptera: Arctiidae) larvae were examined on its host plant, Argusia argentea, a prostrate shrub commonly found on coral cays of the Great Barrier Reef. Larval abundance was greatest during summer (January-February), just after the major flush of Argusia leaves. The larvae were host specific, consuming approximately 2.9 cm2 leaf area/day (annually 2-5% Argusia leaf production of One Tree Island), and exhibited significant preferences for windward, shade leaves. Their diurnal activity w-as comprised of 53 percent feeding, 8 percent moving, and 39 percent resting. Displacement experiments showed greatest nonrandom tendency to relocate the host planr when navigating westward, but nonetheless involved a large time and energy investment. The turnover of Argusia leaf biomass through the grazing pathway is not high; presumably predators, physical environment. or factors other than food supply limit the Utetheisa populations on coral cays. ALTHOUGH HERBIVOROUS LARVAE ARE COMMONLY ASSOCIATED WITH FOREST COMMUNITIES, the dispersal and feeding behavior of individuals are difficult to determine in this habitat due to the logistic problems associated with the structural complexity and inaccessibility of the canopy. Because of these problems encountered in the field, most studies on feeding behavior of insect herbivores have been confined to the laboratory (Cates and Orians 1975, Scriber and Feeny 1979). Most defoliation studies rely on both controlled conditions and simulated grazing methods (e.g., Lee and Bazzaz 1980, Lowman 1982a). However, the isolation of individuals from their natural surroundings may influence feeding and development. The coral islands of the Great Barrier Reef provide isolated patches of vegetation that are rather simple both structurally and floristically. In particular, One Tree Island (23'30's latitude, 152'08'E longitude), has 2 1 permanent plant species (Heatwole et al. 1981) including approximately 128 shrubs of Argusia argentea, a prostrate, continuously-leafing shrub whose distribution extends throughout the Indo-Pacific from East Africa to the East Indies. On One Tree, Argusia argentea is the host plant to a monophagous Lepidopteran larva of the moth Utetheisa pulcbelloides (Arctiidae). The caterpillars are abundant, albeit with some seasonality, with cohorts staggered throughout the year. Argusia establishes on open beaches, usually just above the high tide line although sometimes extending into the island interior. This tendency to grow on exposed island edges may be a consequence of its oceanic seed dispersal (Guppy 1891). In fact, salt water exposure appears to be a prerequisite to germination (Lesko and Walter 1969).