Thesis (M.Sc.)--Chulalongkorn University, 2011

The potential of Siam weed serving as aphid reservoir for aphid dispersal to crops was investigated during August 2010-September 2011 in Wiangsa District, Nan Province. Siam weeds and cabbages were sampled for species diversity and abundance of aphids and other arthropods, such as tended-ants and natural enemies, in forest, commercial cabbage fields, and a field experiment of cultivating mixture of Siam weed and cabbage over two growing seasons (November 2010-February 2011 and March-June 2011). Moreover, aphid dispersal between Siam weed and cabbage was investigated by conducting a caged experiment. From the field sampling of aphids and other arthropods, Aphis gossypii and Aphis spiraecola were both found on Siam weed, while A. gossypii was the only one species found in cabbage. Aphis gossypii was the most abundant aphid in both Siam weed (86.93, Siam weed sampling) and cabbage, and had highest population density during the dry season (October-March) which coincided with the growing season of cabbage and other Brassica in Nan Province. Camponotus rufoglaucus was the most abundant tended-ant (relative abundance=39.8%) with preference for honeydew and had the strongest relationship to A. gossypii while Odontoponera denticulata was the tended-ant with predatory role. However, the most important predator of aphids was spiders with strong negative relationship to the aphid population in Siam weed. Most aphids and other arthropods observed in the field experiment had similar diversity and rank abundance as the field sampling results, except Lipaphis erysimi which was the individuals m⁻²dominant aphid on cabbage instead of A. gossypii. The interaction of one aphid species, A. gossypii, and two tended-ants, C. rufoglaucus and O. denticulata, was conducted in a cage experiment to measure the population growth rate and dispersal rate of aphids, in Siam weed and cabbage. C. rufoglaucus treatment exhibited the positive effect on aphid abundance and aphid dispersal rate significantly while O. denticulata treatment exhibited the negative effect on aphid abundance and aphid dispersal rate confirming its predatory role on aphids and its ability to delay aphid population to reach carrying capacity. However, the nonsignificant difference of mean aphid abundance and dispersal rate between C. rufoglaucus treatment and non-ant treatment indicated that A. gossypii was the facultative myrmecophiles. In conclusion, this study confirmed the potential of Siam weed serving as aphid and beneficial arthropod reservoir as well as the different role of tended ants in the population growth and dispersal of aphids between weeds and crops. Therefore, consideration of both negative and positive effects in ecological roles of Siam weed and aphids were required in formulating the suitable management for Siam weed and aphids in agroecosystem.