Thesis (M.Sc.)--Chulalongkorn University, 2014

Host-parasitoid interactions and diversity of parasitic wasps were investigated in three Brassica agricultural areas at Lainan Subdistrict, Wiang Sa District, Nan Province, started from October 2012 – November 2013. During the Brassica cultivated seasons, thirteen species of beneficial parasitoids were recorded from rearing nine lepidopteran pests which collected from five cultivars of Brassica crops. Spodoptera litura was the major lepidopteran pest, with the highest number of individuals collected in October 2013 (779 individuals). Its abundance was positively correlated to varieties of Brassica cultivars. The abundance of parasitoids, Microplitis sp. was positively correlated to the abundance of S. litura with high parasitism rates in March, October and November 2013 (25%, 18.9% and 18%, respectively). Diversity and total number of lepidopteran pests affected the varieties and total number of parasitoids in hosts and parasitoids rearing methods, while the Brassica cultivation area was positively correlated to diversity of beneficial parasitoids in trap samplings, with no significant difference of the beneficial parasitoids abundance between the cultivated seasons. Four trap sampling methods collected 24 taxa of the parasitic wasps which parasitized the lepidopteran hosts. Malaise traps had the highest effectiveness for collecting parasitic wasps. The most abundance group of parasitoids found in the trap sampling was Microgastrinae with no significant difference in their number between the cultivated seasons. Varieties of Brassica cultivars and Brassica cultivation area were positively correlated to the diversity of parasitic wasps.