author(s)
Nu Nu Yi and Takatoshi Ueno

abstract
The cotton aphid Aphis gossypii is a worldwide pest insect on a variety of agricultural crops. This pest is also a major pest of cotton in Asia, causing not only a decrease in the yield but also decreases in the fiber quality. Cotton aphid management with chemical control has become more difficult due to high resistance to many common insecticides. The aim of the present study was to determine the usefulness of the predatory ladybird beetle Menochilus sexmaculatus (Fabricius) for conservation biological control of cotton aphid. For this purpose, the effective predator/prey ratio was examined using larval and adult M. sexmaculatus in greenhouse conditions. The ladybirds (1-4) and 50 cotton aphids were released on each cotton plant, and the number of aphids were counted until they were eaten out. When the ratio was 1/50 with adult ladybirds, the time required until aphid populations had reached zero was 16 days. As the ratio increased, aphid populations decreased at a greater rate. Although the days taken were longer when test was made with 1^st instar of M. sexmaculatus, the density of cotton aphids reached zero until 20 days of initiation of the test, regardless of the ratios. In contrast, aphid density became double or nearly three times larger when no predator was present until 20 days. Predator/aphid ratio had a significant effect on aphid density, and higher ratio resulted in faster decrease of aphids. The present result suggests that M. sexmaculatus is an effective natural enemy of the cotton aphid since it can suppress the aphid population even when predator/aphid ratio is 1/50 ratio. Augmentative release of a number of M. sexmaculatus would provide satisfactory control for the pest. Conservation of this predator via IPM practices could also provide a satisfactory control of the cotton aphid.